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Pregnancy Diagnosis - Equine

Confirmation of pregnancy in the mare relies on the following tests:

Day 19-22: Serum progesterone

Progesterone may aid in confirming pregnancy if it is high at a time it should be low if the animal was not pregnant. This situation occurs when:

  • Samples are collected at a time of expected oestrus if the animal is not pregnant (e.g. day 19-22 or 40-44 post mating in cattle and horses).
  • Samples are collected outside the breeding season. High concentrations at this time indicates there is a functional corpus luteum and as non pregnant animals should be in seasonal anoestrus, the animal would most likely be pregnant. This is useful for seasonal breeders like sheep, deer, alpacas and llamas.

In most species, the serum progesterone concentration during pregnancy is not significantly different from the concentration during the mid-luteal stage of the oestrus cycle. Therefore, a single random high value is not confirmatory of pregnancy if the stage of the oestrus cycle is not known and the sample is taken during the breeding season. However, if the concentration is still high 10 days later, then the animal is most likely pregnant.

A single low serum progesterone concentration (<0.5 ng/ml) indicates the animal is not pregnant.

As well as pregnancy, other causes of persistent corpus lutea will produce high progesterone concentrations and must be considered, e.g. pyometra.

Day 40-100: Serum PMSG

PMSG is detectable in mares between days 40-120 of gestation when functional endometrial cups are present. Peak concentrations in most mares occur between days 60-80. Variation between mares means that a negative test result before 60 days or after 90 days from breeding does not rule out pregnancy. The test is a latex agglutination test, with results reported as positive or negative. A positive result generally indicates pregnancy but false positives do occur if there has been foetal death after the endometrial cups have formed (i.e. after day 40).

Day > 100-310: Serum oestrone sulphate

This is the most reliable means of pregnancy diagnosis from approximately 100 to 310 days. Increased concentrations of oestrone sulphate indicate a viable foetus and concentrations will drop immediately if foetal death occurs. Oestrone sulphate concentrations can decrease in late pregnancy and can give a negative result in mares in late pregnancy. A non-pregnant mare has a oestrone sulphate concentration of < 5 ng/ml. The concentration in a pregnant mare is typically > 20 ng/ml and generally around 100 ng/ml.

Progesterone in Mares Later in Pregnancy

Measurement of progesterone is very method dependent. Using antibody assays high concentrations of progesterone are detectable throughout pregnancy but with other methods, the hormone is not detectable in the last half of gestation. Measuring progesterone is therefore not considered to be valuable for determining deficiencies during gestation or predicting time of parturition.