Pregnancy diagnosis Serum (clotted blood)
Rectal examination and rectal ultrasonographic examination are the most efficient and cost-effective
means of diagnosing pregnancy. However, in cases that are not amenable to examination per rectum,
either due to temperament or size, blood tests can provide a reliable substitute. In late pregnancy,
transcutaneous ultrasonographic examination or even foetal electrocardiographic examination
provide alternative means of identifying pregnancy and confirming foetal viability.
SERUM PROGESTERONE
From 18-20 days onwards, progesterone concentration in pregnant mares should remain above 6.3
nmol/L until around day 200 of gestation. However, both false positives and false negatives can occur
and measurement of progesterone may therefore be unreliable in diagnosis of pregnancy or
placentitis.
EQUINE CHORIONIC GONADOTROPHIN (eCG, PMSG)
Equine chorionic gonadotrophic (eCG; previously known as pregnant mare serum gonadotrophin) is
secreted in large quantities by specialised cells of the foetal membranes into lymphatic sinuses of
maternal endometrium during the first half of equine pregnancy. Equine chorionic gonadotrophin first
appears in maternal blood 37-40 days after ovulation and concentrations rise steeply thereafter to a
peak at around day 55-70, before declining steadily and disappearing completely from maternal serum
between days 100 -140. The decrease in maternal serum equals the degeneration and dehiscence of
endometrial cups from the endometrium.
A number of factors have been shown to influence the concentrations of eCG in mare’s serum
between 40 and 100 days gestation including mare size, mare parity, mare’s diet, paternity of
conceptus, foetal gender, twin pregnancy, degree of folding of the endometrium and the uterine
environment.
False negatives: may occur from samples collected before day 35 and after day 90.
False positives: If embryonic death occurs after day 35 it does not result in regression of endometrial
cups and eCG levels may remain high despite an absence of a viable foetus.
Given that detection of eCG in mare’s serum gives no indication of the continued viability and presence
of the foetus, follow up measurement of oestrone sulphate after day 100 is recommended when a
positive eCG result is obtained.
OESTRONE SULPHATE
During pregnancy, several oestrogens are produced in high concentrations by the equine
foetoplacental unit and are released into the maternal circulation. The foetal gonads of both male and
female fetuses secrete large quantities of androgens, some of which are converted by the placenta
into oestrone sulphate. Since oestrone sulphate is only produced by a combination of viable foetus
and placenta, it is the only pregnancy endocrine test that confirms that a foetus is alive.
Oestrone sulphate can be detected from day 60, peaks at around day 150, then slowly declines (see
Figure 1). Non-pregnant mares have oestrone sulphate concentrations <10 ng/ml, and a concentration of >10 ng/ml after 100 days of gestation is generally considered positive for pregnancy. More
precisely, most pregnant mares have a concentration greater than 50ng/mL by 3-4 months and greater
than 100ng/mL by 4-5 months.
Oestrone sulphate is not a helpful indicator of ascending placentitis or impending abortion, since it
remains substantially elevated until less than 24 hours before abortion occurs (Canisso et al 2016).
Canisso, I. F., Ball, B. A., Esteller-Vico, A., Williams, N. M., Squires, E. L. and Troedsson, M. H. (2016), Changes in maternal
androgens and oestrogens in mares with experimentally induced ascending placentitis. Equine Veterinary Journal. doi:
10.1111/evj.12556
Figure 1: Minimum, mean and maximum
oestrone sulphate concentrations in preg-