Early and accurate diagnosis of pregnancy in the mare is critical in determining her reproductive viability during the breeding season. Once a mare has been determined to be pregnant, her embryonic growth and fetal development may be accurately monitored. When a bred mare turns up not pregnant, early recognition enables reevaluation and less loss of time. Accurate early diagnosis of pregnancy also carries several benefits as regards the equine practice, including evaluation of semen-fertilizing ability, decision making concerning flushing of embryo donors, easier and accurate evaluation of early embryonic loss, and evaluation of factors that affect embryo survival, particularly in assisted reproductive technologies. 1
As we learned in science class, in order for pregnancy to be established, a viable spermatozoa fertilizes a mature oocyte (egg) at the ampulla-isthmus junction of the oviduct.2 From there, the developing embryo is transported through the oviduct, where it enters the uterus by day 5 to 6 after ovulation. Pregnancy can be diagnosed by a variety of different methods, depending on the stage of gestation.
If a mare does not return to estrus (heat) by 21 days after ovulation, many owners may surmise that their mare is pregnant, but this is not always the case. There are a number of reasons a mare may have a longer span of time between one heat and another. These might include normal variability, the mare going into anestrus due to hormonal manipulation or lactation, secondary or diestral ovulations, early embryonic death or retained corpus lutea.1 Thus, a failure to return to estrus alone is not a reliable method to detect pregnancy.
A number of expected changes occur in the mare's reproductive system during pregnancy; before the clinical use of ultrasonography, these were heavily relied upon to determine pregnancy in the mare. For many years, palpation of the mare’s uterus via rectum was the main technique for pregnancy determination, the accuracy of this method depending on the stage of gestation, individual mare characteristics and skill of the veterinarian.1,2
A veterinarian can perform a transrectal palpation 16 to 19 days after the mare has been with a stallion or artificially inseminated, and may be performed at any time thereafter, unless pregnancy has been confirmed by other means. Simply put, transrectal palpation involves the veterinarian inserting his or her hand in the mare's rectum to check the uterus for indications of pregnancy.
In early pregnancy (day 10 to 20), the embryonic vesicle is not palpable and may be confused with endometrial cysts.1 Uterine and cervical tone is increased at this time however, compared to diestrus, which could give the clinician familiar with the individual horse enough information to confirm pregnancy. At approximately day 21, a small ventral fluctuation becomes palpable at the base of either uterine horn. At this juncture the vesicle will be between 30 to 40 mm in diameter.
By day 30, the fetal fluid bulge will have grown to between 40 to 50 mm in diameter.1 The nonpregnant uterine horn will become extremely firm, often making it hard to differentiate between the bulge of the pregnancy and the curvature of the nonpregnant horn.2 At this point, it is important not to confuse the enlarged and formerly pregnant horn with the fluid of the new pregnancy.1-3 By day 40, the uterus will have far less tone and the fetal fluid bulge will be about 65 mm in diameter.1
The fetal fluid will begin to infiltrate the nonpregnant uterine horn by day 50; by day 60, it expands into the uterine body itself. “Between 75 and 100 days, the weight of the enlarging fetus and the accompanying amniotic and allantoic fluids pull the uterus cranioventrally, allowing it to rest on the ventral body wall.”3 If transrectal palpation is performed at this time, it will reveal the fluid filled uterus and an elongated, firm cervix; the fetus may or may not be able to be detected. Between 90 and 120 days of gestation, the veterinarian should be able to detect the fetus and its movements via palpation.
Rectal palpation timeline3
| Time | Diagnosis |
| 15-18 days | Embryonic vesicle is 15-20 mm, palpate the uterine tone and not the embryo itself |
| 20 days | Embryonic vesicle is 30-40 mm, uterine bulge between the uterine body and horn |
| 30 days | Can feel fetal bulge 40-50 mm, can detect twins after 35 days |
| 40 days | Can feel fetal bulge 65 mm |
| 50 days | Can feel fetal bulge 80 mm, bulge extends into the uterine body |
| 60 days | Can feel fetal bulge 100-130 mm, uterus pulled into the abdomen |
| 3-4 months | Can palpate fetus |
Today, transrectal ultrasonography is the most accurate method for the confirmation of pregnancy and diagnosis of pregnancy status in the mare. Originally, this technique was practical in three situations: early pregnancy diagnosis, early confirmation of nonpregnant mares, and detection of more than one blastocyst (twins), but as ultrasonography has become ubiquitous and attendant skills have become more refined, it is often used to investigate uterine and ovarian health.3 Further applications are also being developed, such as monitoring embryonic and fetal development and growth, fetal viability, placental function, and fetal sexing.
Advances in the quality of the ultrasound equipment have given rise to the development of small, portable ultrasound machines that offer high clarity and definition. With these machines, pregnancy diagnosis can be made as early as 10 days after ovulation.2 To perform transrectal ultrasonography however, the clinician must first be proficient at rectal palpation. Examination of the entire reproductive tract is essential when ultrasonography is performed to confirm pregnancy because embryonic vesicles migrate to the tips of the uterine horns and to a position immediately cranial to the cervix.1-3