Patterns of conceptus development and of progesterone concentrations in maternal blood preceding spontaneous early pregnancy failure in mares.

Sixteen cases of spontaneous pregnancy loss (11 of singletons and five of pairs of twins) are described. The losses occurred between gestation Days 13 and 25 in 12 mares being monitored almost daily by transrectal ultrasonography (for measurement of conceptus growth) and blood sampling (for determination of maternal plasma progesterone concentrations as evidence of luteolysis) in experimental studies of early pregnancy. In 10 of the 16 cases the uterus was flushed and eight conceptuses were recovered for morphological assessment. Five of the 11 losses of singletons occurred before Day 16 and, with one exception, were preceded or accompanied by luteolysis. The remaining six singleton pregnancies failed after Day 16, with two cases evidencing luteolysis beforehand. Thus, overall, 6/11 singleton losses were associated with luteolysis while 5/11 were not. The five cases of simultaneous loss or degeneration of twin conceptuses all occurred on Day 19 or 20, preceded by luteolysis in only one case. These observations suggest that while the causes of spontaneous early pregnancy failure are multifactorial, luteolysis might contribute to the problem more often than has been previously contended.