Pregnancy loss in the early days of gestation perplex veterinarians and owners perpetually; after taking every measure to protect the embryo visible at Day 15 after ovulation, mares sometimes come up empty. And while scientists have learned volumes about getting mares in foal, there’s plenty left to decipher when it comes to keeping them there in the early days of conceptus life.

Tom Stout, VetMB, PhD, of the Department of Equine Sciences at Utrecht University, in The Netherlands, observed, “We have achieved impressive improvements in per-cycle and per-season pregnancy rates in well-managed horse farms over the past 20 years. (However,) our knowledge of factors contributing to the loss of 15% of the pregnancies detected at Day 15 after ovulation is lagging.”

The majority of pregnancies, more than 60% in fact, are lost by Day 42 after ovulation.

Scientists know little about the reasons for early embryonic death (EED) and to what extent they can prevent it. Although they generally believe a “progesterone insufficiency” is the underlying cause of EED. At the 2012 American Association of Equine Practitioners’ Convention, held Dec. 1-5 in Anaheim, Calif., Stout indicated that progesterone insufficiency is not often (or always) the cause of EED.

“Although progesterone produced by the mare’s ovary is needed to maintain early pregnancy, there are other causes of EED that need to be considered,” said Stout.

The main three categories of EED causes include:

  1. Embryonic abnormalities (e.g., the veterinarian detects a chromosomally abnormal embryo or a conceptus that is smaller