SOTAL - Advances in Ultrasonography of the Feline Reproductive Tract
World Small Animal Veterinary Association Congress Proceedings, 2017
L. Gatel; J. Saunders
Medical Imaging of Domestic Animals and Small Animal Orthopedics, Faculty of Veterinary Medicine, Ghent University, Merelbeke, Belgium

Normal Appearance

1. Uterus

The uterus is a thin tubular organ from 1 to 6,7 mm in diameter at the level of the horn and 1,6–7,2 mm at the level of the uterus body. The diameter of the uterus correlates with the weight of the queen, the season and probably also with age, previous pregnancies and contraceptive treatment. Histologically, the uterus is composed of 3 layers: the outside serosa, the myometrium and the inner endometrium. The serosa is seen as a thin hyperechoic line surrounding the horn that can be differentiated from the surrounding mesenteric fat. The apparent inner layer (undistinguished myometrium and endometrium) is iso- to hypoechoic to the surrounding fat. The endometrium and the myometrium can be observed during pro-oestrus, oestrus and dioestrus or pregnancy thanks to the combination of hormones (oestrogens and progesterone) and thickening of the uterus. The endometrium and the myometrium can be isoechoic to each other, with a thin hypoechoic band to distinguish them, or the endometrium can be hypoechoic compared to the myometrium. The lumen should be empty and is visualized as a thin hypoechoic line (due to a reflection artefact).

2. Ovaries

The ovaries are small in size, from 6,8 to 15 mm in length, ovoid in shape and they produce some edge shadow artefact. In anoestrus, a distinction between the cortex and the medulla is possible, with the medulla being hyperechoic compared to the cortex. With a frequency transducer higher than 15 MHz, a primary follicle can be observed as small hypo- to anechoic foci of less than 1 mm in the cortex. Queens are seasonally polyoestrous, reflex ovulators. Without vaginal stimulations, the cycle can be anovulatory, even if some spontaneous ovulations have been reported. During an anovulatory cycle, the follicles grow during pro-oestrus and oestrus and decrease in size in inter-oestrus. When the queen shows oestrus behaviour, the follicles are about 2,3 mm, and they can grow until 3.5–4,1 mm (follicular grow = 0,2 mm/day) . No corpus luteum should be observed.

If the cycle is ovulatory, the ovulation will start 23 to 28 hours after the stimulation and continue on at least 10 hours. The corpus hemorrhagicum can be recognized thanks to a more cellular content (= more echoic) compared to the follicle, and easily detected when the previous localization of the follicle is known. Compared to other species, detection of the corpus luteum is difficult in cats. Ultrasound of the ovaries during a cycle should be repeated several times and is the easiest, most precise and least invasive modality to detect ovulation time.

Abnormal Conditions

1. Uterus

1.a. Cystic Endometrial Hyperplasia

Cystic endometrial hyperplasia (CEH) is a common histological finding in nulliparous queens over 3 years old and in other queens over 5, without correlation with the number of parturitions. On ultrasound, a variable number of anechoic cysts of different sizes can be observed in the endometrium. The severity of the ultrasound findings correlates with the severity of the disease. Most of the time a CEH is associated with enlarged uterine diameter. Administration of synthetic progesterone for contraception leads to the development of advanced endometrial hyperplasia. CEH is more frequent in indoor than in outdoor cats. When a CEH is observed, there is high chance of a corpus luteum on the ovaries or an ovarian cyst.

1.b. Pyometra, Mucometra and Hydrometra

The 3 conditions have a similar appearance on ultrasound: the diameter of the uterus is increased and the lumen is distended by anechoic to echoic fluid. The amount of fluid located in the lumen depends on the cervix: with a close cervix, there is a larger amount of fluid. For differentiation between the 3 processes, it is important to look for any secondary changes, such as peri-uterus effusion, steatitis, abdominal lymphadenopathy or pneumo-abdomen. Pyometra is observed in old queens around 8 years old or in young queens after administration of contraceptive pills (progesterone). CEH-pyometra complex can be observed with cystic lesions in the endometrium, fluid in the lumen and an enlarged uterus.

1.c. Endometritis and Metritis

These disorders are underestimated due to their difficult diagnosis. In dogs, some changes in the thickness and echogenicity of the endometrium can be observed, but the examination could just as well be completely normal.

1.d. Neoplasia

Neoplasia of the uterus are rare (0,29% of neoplasia in cats). Adenocarcinomas seem to be more frequent than leiomyomas and breeding queens seem over­represented. When an adenocarcinoma is suspected, an FeLV test should be performed as well as a metastatic check-up. Some adenocarcinomas can be difficult to diagnose, as they can mimic pyometra.

1.e. Vaginitis and Foreign Objects

Vaginitis can be observed on US with a perineal approach. Mostly vaginitis is observed in prepubescent animals. In an adult a foreign object, such as grass awns, should be excluded.

2. The Ovaries

2.a. Cyst: Ovarian and Para-Ovarian

Cysts are round, anechoic structures observed in the ovarian region and can vary in size. They can be para­ovarian. In this case, they are not associated with any reproductive disorder but their size could eventually become dramatic, causing mass effects on other organs and discomfort.

Ovarian cysts can be located in the cortex or the medulla. Only those located in the cortex can eventually secrete hormones and thus have an impact on the reproduction. Most of the time, the diagnostic of an ovarian cyst requires the combination of clinical findings, hormonal dosage and ultrasonographic detection of the cystic lesion in the cortex. There are 2 situations where the cyst can be confirmed with only ultrasound: if it’s larger than 5 mm during the oestrus cycle or larger than 2 mm in anoestrus.

2.b. Remnant Ovarian Syndrome

Ovarian tissue can be observed in sterilized females at the usual localisation of the ovaries. It is highly recommended to perform the ultrasound when the queen has an oestrus behaviour. In addition, the uterus will show up enlarged with a layering of the endometrium and myometrium.

2.c. Neoplasia

Ovarian tumors are rare. They can be uni- or bilateral. Epithelial tumors are more frequent than germinal or stromal tumors. Their appearance on ultrasound is very variable.


1. Normal Pregnancy

Ultrasonography is a non-invasive technique that allows an accurate diagnosis of pregnancy and allows serial evaluation of the developing embryo/foetus and extra­foetal structures. The early diagnostic can be done about 11 days after the first mating. At that time, for an experimented radiologist, a 1,2 ± 0,1 mm small anechoic cavity can be observed in the lumen of the uterus (body or horns) which corresponds to the embryonic vesicle. Due to high risk of resorption during that period, additional exams should be performed to confirm the persistence of the vesicle. The heartbeat can be observed between 15–17 days after the first mating. Other structures can be recognised at different times of the embryonic and foetal development.

Table 1. Date of apparition of the most commonly used structures or organs (based on Zambelli et al. 2002, Topie et al. 2010)

Number of days after mating



Embryonic vesicle (1,2±0,1 mm)


Heart beat


Hyperechoic liver


8 shape of the embryo


Detection of the stomach


Lung are hyperechoic to the liver


Eyes and kidneys are detected


Intestinal loops and layering

In high risk pregnancies (previous dystocia, single kitten, pelvic fractures, old queens…), the apparition date of the various organs can be used as a landmark to estimate the gestation stage. Embryometry and fetometry are other possibilities. Several formulas have been proposed over the last 20 years, but have been poorly tested in breeding cats, and recent studies pointed out insufficient accuracy (about 50% at 2 days). This can be explained by the fact that several factors may influence the parturition time, such as litter size, weight, age and breed. Therefore, the most recent articles about the subject propose to combine both methods to improve accuracy. Ultrasound close to the parturition is important to assess the viability of the foeti and to detect potential foetal distress (cardiac frequency less than 220 bpm). It should be associated with foetometry and measurement of the progesterone (a drop can be observed).

Table 2. Formulas to predict the number of days of gestation (GL) or the date of parturition (DP) in days currently available in the literature

Measured structured

Usable formulas

External diameter of the gestational chamber (GC in cm)

GL = 1,602 GC - 12,13

Internal diameter of the gestational chamber (GC in cm)

GL = 1,368 GC - 11,566

Crown rump length of the embryo (CR in cm)

CR = 0,01 GL’ - 0,28 GL + 2,69 (between 19 and 40 days after mating)

Crown rump length of the embryo (CR in cm)

GL = 2,0087 CR - 31,43

Body diameter (BD in cm)

DP = 43,5 - 10,9 SD

Biparietal diameter (BPD in cm)

BPD = 0,05 GL - 0,64 (33 days after mating)

Biparietal diameter (BPD in cm)

DP = 61,2 - 24,6 BPD

Biparietal diameter (BPD in cm)

GL = 61,2 - 24,6 BPD

Biparietal diameter (BPD in cm)

BPD = 23,39 + 0,47 GL

Femur length (FL in cm)

FL = 0,050 DG - 0,79 (40 days after mating)

Femur length (FL in cm) (W = weight in kg, LS = litter size, A = age in years)

DP = 37,864 - 19,3 FL+ 1,227 W- 0,615 LS- 0,832 A

The uterine involution can be observed during 28 days after the parturition. It consists in a slow decrease of the uterine dimeter and the endometrium and myometrium to come back to normal anoestrus especially at the level of the insertion of the placenta. If fluid is observed 1 week after the parturition in the lumen of the uterus, a metritis should be suspected.

2. Abnormal Pregnancy

Ultrasound is also a very useful tool to detect embryonic resorption or foetal abortion. There are several causes to the abortion as infection (mostly viral), traumatic, neoplastic, toxic, congenital abnormality of the foetus... The confirmation is not easy to do. During both resorption and abortion, ultrasound can show loss of movement of the embryo/foetus, absence of heart beat, hyperechoic foci in the gestational chamber and thickening of the uterus wall. Recently, the ultrasound appearance of the drug induced resorption and abortion using Aglepristone was described. The ultrasound findings are similar to the one described in pathologic conditions.


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Speaker Information
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L. Gatel
Medical Imaging of Domestic Animals and Small Animal Orthopedics
Faculty of Veterinary Medicine
Ghent University
Merelbeke, Belgium

J. Saunders
Medical Imaging of Domestic Animals and Small Animal Orthopedics
Faculty of Veterinary Medicine
Ghent University
Merelbeke, Belgium

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