Gestation Period in Draft Mares Relates to Fetal Gender

A study in Japan has found that draft mares carrying fillies had slightly shorter pregnancies than mares carrying colts.

After several years of collecting data on 65 draft mares that delivered 209 foals, researchers at the Obihiro University of Agriculture and Veterinary Medicine found that average pregnancies lasted about 334.9 days. Mares delivering colts had longer pregnancies, averaging 337.1 days, while those delivering fillies had shorter gestations that averaged 332.7 days.

Factors that had no bearing on pregnancy length in this group of mares included age of the mare, month of foaling, the number of previous pregnancies, and environmental factors like barometric pressure and number of hours of sunshine in the days immediately prior to foaling.