The first estrus typically occurs between 5 and 9 months of age, but age at onset may be highly variable (3.5 to 18 months). In a survey of breeders of four breeds (Norwegian Forest cat, Maine Coon, Persian, Bengal) in Italy, the average age at puberty was 12 ± 7 months.1 Puberty was earliest for the Norwegian Forest cat, Bengal, and Maine Coon (10 months) and was latest for the Persian (17 months). The time of the first estrus is influenced by several factors; for example, breed (shorthair breeds often reach puberty earlier than longhair breeds), season (which determines the length of daylight), and the queen’s body condition. Persians and related breeds may not have the first estrus until 18 months of age or older and may not be sexually mature until 2 to 3 years of age. The average body weight at puberty is 2.3 to 3.2 kg (5 to 7 pounds) or 80% of adult body weight. Shorthair breeds, such as the Siamese and Burmese, are more precocious and may reach puberty at a lower body weight.

In the northern hemisphere, increasing daylight length in January and February promotes the onset of estrous activity. In the survey of cat breeders in Italy, queens were reported in estrus every month of the year, but 67% of estrous cycles occurred between January and June.1 Regular estrous activity will continue until as late as October or November, depending on the geographic distance from the equator (and therefore the length of daylight) as the effect of seasonality diminishes or disappears near the equator. Most cats housed indoors in North America will experience winter anestrus because of the short length of daylight. However, the effect of daylight length on cats housed indoors is complicated as they are exposed to natural and artificial light, which sometimes leads to unpredictable ovarian cycles. In addition, queens housed together may have synchronized estrous cycles (the “dormitory effect”).

Longhair breeds seem to be more sensitive to the amount of daylight than shorthair breeds. Although many longhair queens (such as Persians) will not exhibit regular estrous cycles even during periods of long daylight, many shorthair queens (such as Siamese and related breeds) exhibit estrous cycles year-round, regardless of daylight length. Inadequate intensity or duration of light is an important cause of infrequent estrous cycles in cats housed indoors. Breeding catteries should provide 12 to 14 hours of daylight or artificial light per day to encourage regular estrous cycles.

The feline estrous cycle is divided into proestrus, estrus, interestrus, anestrus, and diestrus (Fig. 45.1). Proestrus is considerably more difficult to detect in the queen than in the bitch. This part of the estrous cycle may last only 1 or 2 days and the signs may be subtle. Some queens will rub their head and neck against convenient objects and display affectionate behavior. Occasionally, queens in proestrus have a slight mucoid vulvar discharge and pollakiuria. During proestrus, tom cats may be attracted to the queen, but the queen will not be receptive to breeding.

Estrus is defined as behavioral receptivity to mating. This stage may last from 2 to 19 days, with the average duration being 5.8 ± 3.3 days.2 A queen in estrus will crouch with her front legs pressed to the ground, her back in lordosis, and her tail turned to one side to present the vulva (Fig. 45.2). The queen may roll or thrash about on the floor. Queens in estrus often vocalize to attract the attention of males. They may be restless, have a poor appetite, and show increased affection to their caretakers. It is not uncommon for inexperienced owners to interpret estrus behavior as a sign of injury or illness.

The period between one estrus and the next in queens that have not ovulated is the interestrus. During this time, plasma estradiol is low (<15 pg/mL [<55.1 pmol/L]) and no sexual behaviors are seen. In research colonies, the duration of interestrus can range from 2 to 19 days but on average is 7 days. However, data for pedigreed breeds may be different. In the survey of cat breeders in Italy, the average interestrus interval was 39 ± 17 days, with no difference among breeds (Bengal, Maine Coon, Norwegian Forest cat, Persian).1

Anestrus is the absence of cycling activity that may occur naturally in periods of short daylight, but individual variation is common. During this time, progesterone and estrogen are at baseline concentrations (progesterone <1 ng/mL [<3.2 nmol/L], estrogen 8 to 12 pg/mL [29.4 to 44.0 pmol/L]).

The luteal phase (diestrus) of the queen’s estrous cycle is the period after ovulation when the dominant hormone is progesterone. Unlike the bitch, the queen does not experience a pre-ovulatory rise in progesterone. After ovulation, fertilization of oocytes occurs in the oviducts, and the embryos enter a uterine horn 4 to 5 days after ovulation. The embryos then space out along the uterine horns and may even migrate from one horn to another before implantation.³ Some embryos may be lost in this process. Implantation occurs about 12 to 13 days after breeding, and the implantation rate is estimated to be about 84%.³ The feline placenta is endotheliochorial in structure and zonary in shape. The average duration of gestation is 65 to 67 days (Table 45.1).^2,4,5

If the oocytes are not fertilized after ovulation, a pseudopregnancy will occur that lasts 40 to 50 days. Pseudopregnancy may also result if early embryonic loss occurs. Pseudopregnancy in cats is not usually associated with maternal behaviors or lactation.

Estrus may resume about 10 days after the end of the luteal phase, but nursing queens often experience a lactational anestrus that can last for up to 8 weeks after weaning. Most queens will return to estrus about 4 weeks after weaning kittens if this occurs during the breeding season. However, it is entirely possible for a queen to return to estrus while still nursing. The first estrus after a pregnancy may be shorter and less fertile. Estrus behavior during gestation has also been reported in the queen, although serum estradiol is not increased and no luteinizing hormone (LH) surge occurs, even if the queen allows copulation.⁶ Superfetation—kittens of different gestational ages in one litter—has never been proven to occur in the cat. The presence of poorly developed fetuses along with kittens of normal gestational age in a litter is most likely a problem of arrested development.

Although little data about follicle-stimulating hormone (FSH) concentrations or activity in the queen exist, it is believed to be similar to that in other species. The pituitary gland produces FSH which initiates development of ovarian follicles. Three to seven follicles develop and start producing estradiol-17β. As the follicular activity peaks, plasma estradiol levels increase and may vary widely but usually are >20 pg/mL (>73.4 pmol/L).7 Estradiol levels stay high for 3 or 4 days during estrus and then abruptly fall. The high estradiol levels produce two important effects: overt estrous behavior and priming of the gonadotropin surge necessary to cause ovulation. Estradiol concentrations rise again at about day 58 of gestation and decline just before parturition.

Ovulation requires the release of LH from the anterior pituitary gland. During intromission, the penis probably causes distention of the posterior vagina and induces release of GnRH from the medioventral hypothalamus via neuroendocrine reflexes. Sufficient stimulus is required to provoke the release of GnRH. A surge of LH occurs within minutes of copulation. With multiple copulations, the LH surge is higher in amplitude and lasts longer than when only one breeding occurs, thus increasing the chances that ovulation will occur.

Several days of estradiol priming are required before LH release sufficient to cause ovulation occurs. This is typically reached by the third or fourth day of estrus. There also appears to be a stimulus threshold individual to each queen that must be exceeded for adequate LH release to occur. Unlike the rabbit, where a single mating is sufficient to induce ovulation, queens vary considerably in the number of copulations required to induce sufficient LH release and ovulation although most queens will ovulate after four or more copulations.

Ovulation occurs 48 hours or more following the LH surge.⁸ All oocytes are released at the same time. The granulosa cells of the ovarian follicles undergo luteinization and begin to produce progesterone almost immediately. Progesterone concentrations rise within 24 hours and may reach highs of 60 to 90 ng/mL (190.8 to 286.2 nmol/L) by 20 to 25 days postovulation.^9,10 Throughout pregnancy, progesterone is maintained at high concentrations until the last few days of gestation, when the level falls to about 2 ng/mL (6.4 nmol/L) and to <1 ng/mL (<3.2 nmol/L) immediately following parturition.¹⁰ A minimum progesterone concentration of 1 ng/mL (3.2 nmol/L) appears to be necessary to sustain pregnancy in the queen. Although progesterone declines at term, unlike the bitch, baseline concentrations are not required for onset of parturition in the queen.

As for other induced ovulators, the corpus luteum (CL) may be the primary source of progesterone throughout pregnancy in the cat.⁹ Two other hormones are important in feline pregnancy. Relaxin is produced primarily by the placenta in carnivores and facilitates parturition by softening the connective tissue of the pelvis, softening the cervix, and relaxing uterine musculature. Relaxin concentrations increase as early as day 20 of gestation.⁶ Prolactin is produced by the anterior pituitary and has various effects, including regulation of lactation. Prolactin concentrations increase from about day 35 of gestation, plateau at about day 50, and then increase abruptly just before parturition.⁶ Prolactin appears to be necessary for maintenance of pregnancy by supporting the CL, as suppression of prolactin with a dopamine agonist results in abortion.

Pseudopregnancy may result if a mating is infertile. High progesterone concentrations are maintained by a centrally mediated blockage of GnRH secretion during both pregnancy and pseudopregnancy. This prevents the queen from returning to estrus until the luteal phase has ended. During pseudopregnancy, progesterone concentrations start to decline by day 25 to 30 and are <1 to 2 ng/mL (<3.2 to 6.4 nmol/L) by day 40 to 50.⁹ The feline CL may be preprogrammed to atrophy after 25 to 30 days unless luteotrophic factors are present. These luteotrophic factors may originate from the fetoplacental unit and/or from the pituitary. The two most likely luteotrophic factors in the queen are relaxin and prolactin.

Traditionally, queens are described as reflex-mediated induced ovulators. Ovulation should not occur unless mating or a similar stimulus induces it. However, pyometra and mucometra can occur in middle-aged, intact, virgin queens. Studies have found evidence that spontaneous ovulation not only occurs in cats but occurs with some frequency. A study of 44 female cats with uterine disease classified them based on ovarian status (active or cystic follicles versus luteal phase ovaries).11 Of the 44 queens, 35 had no recent exposure to male cats. However, 20 of these 35 queens had luteal phase ovaries, established by histologic examination. In another study, 20 domestic shorthair queens ranging from 2.5 to 11 years old were evaluated.¹² These cats were housed individually, but they could see and hear other cats, including males. Seven of the 20 queens had evidence for spontaneous ovulation, and some queens experienced it repeatedly. A study designed to approximate conditions in multicat homes and catteries group-housed 15 young, nulliparous, female cats.¹³ After 3 months, a male cat was housed in the same room but caged separately so there was no physical contact with the queens. Of the 15 queens, 87% showed evidence of at least one instance of ovulation and pseudopregnancy without mating during the 4.5 months of the study. As well, 67% of the queens had evidence of spontaneous ovulation during the 3 months before the male cat entered the room. Finally, in a study of 89 queens presented for ovariectomy (OE), approximately one-third showed evidence of spontaneous ovulation.¹⁴

These studies indicate that non-copulatory ovulation may be possible in response to a variety of tactile, visual, auditory, or olfactory cues in queens. Thus, it is more appropriate to consider the queen to be an induced and spontaneous ovulator. Unrecognized spontaneous ovulation and subsequent pseudopregnancy is one important cause for infrequent estrous cycles that must be ruled out in cases of suspected infertility.

Ovarian remnant syndrome (ORS) is the presence of functional ovarian tissue with signs of estrus in a queen that has had an ovariohysterectomy (OVH) or OE. Neoplasia in ovarian remnants, such as granulosa cell tumors, is a rare cause of ORS. Signs of estrus may occur weeks to many years after OVH/OE and include lordosis, vocalizing, rolling on the ground, and receptivity to intact males.19 Age at the time of surgery and breed of cat do not appear to influence the risk of ORS, although one report did not find any cases in queens spayed before 4 months of age.¹⁹ The most common causes of ORS are failure to remove all or part of an ovary at surgery and revascularization of ovarian tissue inadvertently left or dropped in the abdomen during OVH or OE.^20,21

There are various methods for confirming a diagnosis of ORS (Box 45.2). Once ORS is confirmed, the ovarian tissue should be surgically removed. Queens with ovarian remnants may be at increased risk of mammary and ovarian neoplasia. Many owners will not be tolerant of the estrus behavior. During exploratory laparotomy, a thorough search of the peritoneal cavity is necessary, starting at the most common location for remnants, the ovarian pedicles. Other common sites for ovarian remnants are the omentum and the peritoneal walls. Remnants may be unilateral or bilateral. Surgery is most rewarding if performed when the cat is in diestrus or has been induced to ovulate as the corpora lutea will be visible as yellow–orange structures against the red background of ovarian tissue. Laparoscopy has also been used successfully to find and excise ovarian tissue, although the surgeon should always be prepared to convert to laparotomy if necessary.^22,23 Excised tissue should be submitted for histopathology to confirm ovarian tissue has been removed.

It should be noted that occasionally parovarian nodules located proximal to the ovary (near or within the vasculature) are identified in queens. They are typically 2 to 3 mm in diameter and white to tan in color. They may be mistaken for ectopic or accessory ovaries, which have not been documented in the cat. Histologic studies confirm that most parovarian nodules are ectopic adrenocortical tissue and are not clinically significant.^24,25 The second most common histologic diagnosis for parovarian nodules is residual mesonephric structures.

An interesting case report involved an 11-year-old spayed female domestic shorthair cat evaluated for a 4-month history of weight loss, aggression, urine spraying, and behaviors suggestive of estrus.²⁶ Diagnostic investigation included an adrenocorticotropic hormone stimulation test and adrenal hormone panel that revealed elevated androstenedione and testosterone pre- and ­post-stimulation, mildly decreased cortisol pre- and post-stimulation, and decreased resting aldosterone. On ultrasound examination, a 6-mm hypoechoic nodule was detected in the left cranial abdomen. At exploratory laparotomy, a cystic, nodular mass was found in the region of the left ovary. Histopathology confirmed an ovarian remnant with an intact CL and non-neoplastic parovarian cysts. Clinical signs resolved within 2 weeks of surgery and follow-up testing 6 weeks later revealed normal androstenedione, testosterone, and cortisol concentrations.

Approximately 80% of feline mammary masses are neoplastic, most commonly adenocarcinomas. The remaining 20% are benign and are predominately mammary hyperplasia (MH; also called fibroepithelial hyperplasia or mammary fibroadenomatous hyperplasia). Mammary hyperplasia is most commonly seen in young cycling queens but may also be seen in pregnant queens and in male or female cats treated with progestins (e.g., megestrol acetate, medroxyprogesterone acetate). There is one published case report of a male cat with MH that had no history of progestin treatment.27 Typically, most or all mammary glands are affected. The hyperplasia can be severe, leading to tissue necrosis, ulceration, and infection (Fig. 45.3). It is often mistaken for neoplasia on gross appearance. Histologically, the lesions consist of benign, unencapsulated, fibroglandular proliferation. Progesterone receptors have been commonly found in MH samples, while estrogen receptors have been found in only about 50% of cases.²⁸ The etiology is suspected to be an exaggerated response to natural progesterone or synthetic progestins, but the disease is also rarely reported in surgically sterilized male and female cats with no history of progestin therapy. In spayed queens, ORS should also be ruled out.

The diagnosis is made by clinical signs, patient signalment, and history. Biopsy of affected tissue and histopathology will confirm the diagnosis of MH. However, surgical biopsy of markedly swollen mammary glands may create incisions that are difficult to heal due to wound tension. Treatment varies with the underlying cause. If the cat is being treated with an oral progestin, treatment should be stopped. Intact queens not intended for breeding should undergo OVH or OE, and a flank approach may be most appropriate (Fig. 45.4).

Drug therapy can be used as the sole treatment for MH, especially if the queen is intended for breeding, or as an adjunct to OVH/OE. The drug of choice is the competitive progesterone receptor antagonist aglepristone (10 to 15 mg/kg/day, SC, days 1, 2, and 7). One study monitored 14 queens with MH for 12 months following treatment with aglepristone.29 Remission of clinical signs occurred in an average of 4 weeks. cats that had been treated with long-acting medroxyprogesterone acetate required treatment for 5 weeks. Six of the queens were subsequently bred, and four delivered normal litters. Another study found that cats with MH that had received long-acting progestin therapy (e.g., medroxyprogesterone acetate) were more difficult to resolve and had a higher risk of persistent mammary growth after OVH/OE than cats that had not received progestins.³⁰ In those cases, the authors recommend concurrent treatment with aglepristone; more than one treatment may be necessary.

Other drug choices include dopamine agonists that reduce prolactin levels, such as cabergoline (5 µg/kg, PO, once daily for 5 to 7 days) or bromocriptine (250 µg, PO, 5 to 7 days). In most countries, these drugs are not licensed in the cat. Infections should be treated with broad-spectrum antibiotics and analgesics should be used. Occasionally, MH will resolve spontaneously, but it typically takes several weeks to several months to resolve even with treatment.

Luteinizing hormone is released from the anterior pituitary gland in response to copulation and stimulates ovulation and luteinization of mature ovarian follicles. In intact queens, serum LH is maintained at basal levels through negative feedback provided by ovarian estradiol secretion. Following OVH/OE, negative feedback is lost, and serum LH levels are persistently elevated.31 Patient-side and referral laboratory assays for LH are available. A single negative test is highly likely to indicate a sexually intact queen. A single positive test suggests a spayed queen, although false positives may occur if an episodic LH surge is sampled, or the queen is in estrus. One study evaluated a point-of-care LH assay in female cats that were intact and surgically sterilized.³² Test sensitivity was 90.8%, specificity was 92.3%, and accuracy was 91.7%. Ten intact queens were unexpectedly positive for LH; two of these cats were in estrus. The authors recommend repeat testing in 24 hours for cats with signs of estrus and positive results. In another study, LH tests were performed on 236 female cats presented for OVH in adoption centers in the United Kingdom.³³ In this group of cats, sensitivity was 69%, specificity was 100%, positive predictive value was 1.0, and negative predictive value was 0.77. The authors suggest the use of LH assays in shelters may provide a lower risk, faster, and less traumatic alternative to OVH/OE which may decrease costs and facilitate more rapid adoption.

Anti-Müllerian hormone (AMH; also known as Müllerian inhibitory substance) can also be used to determine reproductive status. This hormone is produced by the fetal testes and inhibits Müllerian (paramesonephric) duct development in males. After birth, AMH is produced by granulosa cells in ovaries and by Sertoli cells in testes. Studies have shown that a negative or below reference range value for AMH in serum is highly likely to indicate the cat has undergone OVH or OE.^34,35

Congenital anomalies of the female reproductive tract in cats are not common and are poorly described in the literature. Segmental uterine aplasia is the anomaly most often reported. In this condition, portions of the tubular genital tract (uterus, cervix, vagina) are poorly developed or absent. There are a variety of aplasia/hypoplasia abnormalities that likely depend on the timing of the disruption in development. Disruptions early in development may lead to abnormalities that start in the proximal part of the tract and halt normal development of the remainder. Disruption later in development is likely to affect a smaller portion of the tract. In one case report, multiple reproductive tract abnormalities were documented in an 8-year-old queen presented for routine OVH.36 The cat had bilaterally hypoplastic uterine horns that were mildly distended with fluid and aplasia of the cervix and vagina that resulted in a blind dilation of the cranial vagina that had formed a cystic structure. The urethra and ureters were normal.

Segmental aplasia or hypoplasia of a uterine horn (also called uterus unicornis) is encountered occasionally and can present difficulties for veterinarians when found incidentally during OVH/OE. In a report on uterine abnormalities in cats and dogs presented for OVH, 49 of 53,258 cats (0.09%) had a suspected congenital uterine abnormality compared with 15 of 32,660 (0.05%) of female dogs.³⁷ The most common types in cats were unicornuate uterus and segmental agenesis or hypoplasia of one uterine horn. In affected cats, ipsilateral renal agenesis was common. Other studies have documented cases where one uterine horn is missing or reduced to a thread-like remnant, and the ipsilateral kidney is absent.^38–41 However, both ovaries are typically present, and the surgeon must ensure the ipsilateral ovary is found and removed during OVH/OE (Fig. 45.5). Failure to remove the ipsilateral ovary is likely to result in ORS. When one normal uterine horn and ovary are present, the queen may have normal estrous cycles and may even become pregnant.³⁸ However, segmental aplasia may also cause failure to conceive associated with fluid accumulation in the uterine lumen, depending on the location of the occlusion.^39–41 Segmental uterine aplasia/hypoplasia with ipsilateral renal agenesis may be more common in the Ragdoll breed than other cats.^42,43 In an ultrasound study designed to assess the prevalence of renal abnormalities in Ragdoll cats in Belgium, 2 of 244 (0.8%) had only one visible kidney.⁴⁴

The urogenital tract arises from interactions between two cell populations in the developing embryo. The urinary tract and tubular uterine structures develop from cells in the intermediate mesoderm and the ovaries develop from the gonadal ridge. This common ancestry explains why renal and uterine abnormalities often occur together while the ovaries are not affected. In humans, a similar condition called Mayer–Rokitansky–Küster–Hauser syndrome type 2 is inherited as an autosomal dominant mutation with incomplete penetrance and variable expressivity.45 To date, inheritance patterns or specific mutations have not been identified in cats.

There are several methods to diagnose pregnancy in queens (Box 45.3). The failure of a queen to come back into estrus after breeding is one of the most obvious signs of pregnancy, but pseudopregnancy will produce the same effect. However, queens experiencing a pseudopregnancy will return to estrus in about 40 to 50 days. One of the first physical indications of pregnancy is “pinking” of the nipples, which occurs around day 15 to 18 after ovulation. This change in the nipples, which become noticeably pinker and easier to see as the hair around them recedes somewhat and the nipples increase in size, is most obvious in maiden queens. With experience, it can be recognized in queens that have had several litters.

Radiography may be used to detect pregnancy once fetal bones begin to mineralize (Table 45.2). Until this time, only uterine enlargement may be detected, which could be consistent with pregnancy or uterine disease (e.g., pyometra). Radiography is useful for determining the number of fetuses by counting the number of skulls present, but this can only be done in late pregnancy (Fig. 45.6).

Ultrasonography can detect pregnancy earlier than radiography. The gestational sac, a spherical anechoic structure slightly compressed at the pole, can be detected at 11 to 14 days and the embryo at 15 to 17 days post breeding. From day 30, it is possible to identify fetal organs. Details on the time of ultrasound appearance of various fetal and extra-fetal structures in the cat have been published (Table 45.3).^46,47

A benefit of ultrasonography is the ability to determine fetal viability by detecting a beating heart (as early as 16 days) and fetal movement (as early as 32 days). Unlike the dog, fetal heart rate remains stable during gestation in the cat. Fetal sex determination is possible at about days 38 to 43 post breeding.⁴⁸ However, ultrasonography may not be as good as radiography for determining the number of fetuses. Ultrasonography views each fetus individually, and movement of the queen or the uterus makes identification of individual fetuses confusing.

Many sonographers prefer to shave the hair coat to improve image quality. If the hair coat is not shaved, alcohol or another wetting agent can be used in addition to the acoustic coupling gel to decrease the amount of air between the transducer and the skin. The queen ideally should have a full bladder at the time of examination to move the bowel out of the way and move the uterine body out of the pelvic canal so that it is more readily imaged. It also helps to fast the queen for 12 hours before the ultrasound examination so that intestinal gas is less likely to obscure views, especially in early pregnancy.

Traditionally, there has been no blood test available in the cat to detect pregnancy. cats do not produce a placental hormone similar to human chorionic gonadotropin (hCG), which is the basis for some human pregnancy tests. However, the hormone relaxin is produced primarily by the placenta and is therefore a useful marker for pregnancy. Relaxin levels increase in pregnancy but not in pseudopregnancy. Test kits (e.g., Witness Relaxin, Zoetis) are available in some countries for pregnancy detection in cats and dogs. In a study designed to evaluate the Witness Relaxin test, pregnant, nonpregnant, and ovariohysterectomized queens were tested.⁴⁹ The test was able to detect pregnancy as early day 20. From day 29 of pregnancy, test sensitivity was 100% and specificity was 96%. Three queens had false positive tests; two of these queens had large ovarian cysts, which suggests another source of relaxin.

The mean length of pregnancy in the queen is 65 to 67 days, but it can be variable. It is influenced by breed (the longest gestations are in the Siamese and Oriental breeds) and litter size (larger litters are associated with shorter gestations).1,4,5,50 Pregnancy length can also be influenced by the queen’s body weight (positive correlation) and age (negative correlation).⁵¹ Normal pregnancies lasting <60 days or >70 days are uncommon and may be associated with high neonatal mortality. During their breeding life, most queens will establish a predictable pattern for length of gestation. If the breeding date is unknown, it may be helpful to have an alternate method of estimating gestational age in some circumstances, such as preparing for parturition, scheduling a cesarean section (CS), and managing kittens at risk for neonatal isoerythrolysis. Two useful methods are assessment of maternal serum progesterone and imaging.

As mentioned earlier, maternal progesterone levels decrease near the end of gestation. In one study, levels declined from 4 to 5 ng/mL in late gestation to 2.99 ± 1.29 ng/mL 1 day before parturition and were about 2.2 ng/mL on the day of parturition.¹⁰ In another study, progesterone declined to 3.18 ± 1.68 ng/mL on the day of parturition.⁵²

Guidelines for estimating gestation age using radiography to detect fetal skeletal mineralization have been developed for cats. There is a predictable sequence of bone mineralization in the feline, similar to that in the canine, but beginning about 1 week earlier. Prediction of the date of parturition within 3 days was possible for 75% of 32 cats (and within 7 days in all cats) using a schedule for bone mineralization developed in one study (Table 45.2).⁵³ However, not all bones are reliable for prediction of parturition. Mineralization of the humerus and femur occurs over the narrowest range, while the ulna, fibula, and pelvic bones have more variable mineralization times. The fibula, calcaneus, and phalanges may not become visibly mineralized before parturition.

Prediction of gestational age and date of parturition can also be estimated using ultrasound measurements of the gestational sac (up to day 25) and the fetus (e.g., biparietal diameter, body diameter, crown–rump length), and by evaluating fetal organ development (Table 45.3). Use of a 7.5 to 12.5 mHz probe is recommended for this purpose. Crown–rump length is a simple assessment that can be used after day 38 but is difficult to perform near term when the fetuses are in a flexed position (Table 45.4). Formulas have been developed based on measurement of fetal structures (Fig. 45.7, Box 45.4).^10,46,52,54 In one study, the date of parturition was estimated ±2 days about 75% of the time.⁵⁴ A detailed review of gestational aging and prediction of parturition date in the bitch and queen has been published and should be consulted for more information.⁵⁵ Most of the research on parturition prediction using ultrasound has been performed with non-pedigreed cats. In a study of 23 pregnant Maine Coons aged 1 to 6 years and weighing 4.5 to 7.6 kg (9.9 to 16.7 pounds), serial ultrasound examinations with fetal measurements were performed and a breed-specific formula for predicting parturition date was developed.⁵⁶

With all methods, accuracy can decrease near parturition. In addition, the aforementioned factors influencing gestation length are difficult to incorporate into assessments. One study suggested that in the last 5 days of gestation, combining fetal head measurement by ultrasound with maternal serum progesterone level is the best approach.⁵²

Fetal sex can be evaluated with ultrasound between days 38 and 43 using a midsagittal view under the tail base.⁵⁵ In the male, the bulge of the scrotum dorsally and the prepuce ventrally will be seen. In the female, a single triangular bulge representing the vulva is seen distant to the tail base.

The nutritional requirements for reproducing queens are different from adult maintenance needs. Lactation is the most demanding phase of reproduction so queens should be in good body condition to meet the demands of nursing kittens. Unlike most species, the queen gains weight linearly from conception to parturition.57 Energy intake also increases linearly. Mean weight gain for queens during pregnancy is approximately 40% of prebreeding weight.⁵⁷ At parturition, only 40% of the weight gained during pregnancy is lost; the remaining weight is consumed by the demands of lactation.⁵⁷ High-quality diets designed for growth or reproduction are appropriate for the pregnant queen and obesity should be avoided.

During pregnancy, the queen should not be exposed to new cats or to sick cats. There is no need to restrict activity, although most queens become less active and eat smaller meals more frequently during the last trimester because of rapid abdominal enlargement. “Morning sickness” has not been documented in the queen, nor have diet cravings. During the last 2 weeks of gestation, the queen should be isolated from other cats and provided with a safe, quiet maternity area. Stress should be avoided because it may have detrimental effects on parturition and on maternal behavior. A birthing box should be provided that is lined with absorbent material that can be laundered (e.g., towels or blankets) or that is disposable (e.g., disposable diapers or pads). Some queens will change birthing sites as do feral queens, especially if they have access to the entire home or cattery.

The decision to use medications in a pregnant or lactating cat must be carefully considered, weighing potential benefits against risks. Most medications have not been tested in pregnant or lactating queens so information may be scant about the safety of a given drug. More information is available in Chapter 4: Guidelines and Precautions for Drug Therapy in cats.

The time from the start of active labor to the birth of the first kitten is usually less than 60 minutes. A queen that is in active labor for more than 2 hours without delivering a kitten may need veterinary attention. Once delivery begins, kittens are generally born every 30 to 60 minutes, although they may be delivered more rapidly. In one survey of research colony cats, the average delivery time for the entire litter was 16 hours (range, 4 to 42 hours).2 In a large survey of pedigreed queens, the time from delivery of the first kitten to delivery of the last kitten was less than 6 hours in the majority of cases, but it was more than 24 hours in 1.6% of queens.⁴ In another survey of 197 queens, the median time between kittens was 30 min (range, 2 to 343 min) and 95% were born within 100 min after birth of the preceding kitten.⁵

In extended deliveries, the queen may nurse the ­kittens that are already born, giving the appearance that delivery is finished. Queens may interrupt parturition if something disturbing occurs in the environment. In general, the queen in labor should be monitored but interfered with as little as possible.

Kittens are typically born within the amniotic sac, and the queen will bite through the amniotic membranes and the umbilical cord and lick the kitten to stimulate breathing. Since stage 2 and 3 labor happen concurrently, delivery of kittens is interspersed with delivery of placentas. The queen may or may not ingest the placentas; there is no evidence that it is necessary.

If kittens are born in rapid succession, the queen may not be able to clear membranes from each kitten or sever the umbilical cords promptly. This may also be a problem for inexperienced queens delivering the first litter. Occasionally, kittens may be found dead inside the amniotic sac, or several kittens may become entwined by their umbilical cords as they crawl around the birthing box (Fig. 45.8). Entrapment of an umbilical cord around a distal limb may result in significant injury. Gentle, calm intervention by the caregiver is necessary to ensure survival and prevention of injury in these situations. The amniotic membranes should be removed, and each kitten should be carefully cleaned and dried. The umbilical cord may be clamped, ligated, and transected about 1 inch from the body wall. Kittens should be kept warm and safe until the queen can attend to them.

Postpartum discharge (lochia) is typically scant in the queen. Because the queen cleans the vulva frequently, it may not even be noticed. Ultrasonographically, uterine involution is virtually complete by 28 days postpartum, much earlier than in the bitch.58 The queen should be monitored for signs of abnormal vulvar or mammary discharge, fever, anorexia, or neglect of the kittens. Indications for caregivers to seek veterinary assistance in the postpartum period are found in Box 45.5. It is normal to be unable to express milk from the queen’s mammary glands provided the kittens are gaining weight.

Dystocia (from the Greek “dys” meaning difficult or abnormal and “tokos” meaning birth) is defined as a painful, slow, or difficult delivery. It is not always easy to differentiate normal parturition from dystocia in the queen because prolonged time between the births of kittens can be normal. Most commonly, birth of the kittens is difficult from the start in a dystocia, but it is possible for some kittens to be delivered without incident before difficulties are encountered. Indications for caregivers to seek veterinary assistance during parturition are found in Box 45.6.