Are you breeding to produce a stallion? Or maybe you’re planning a breeding around a mare’s “normal” gestation to have a foal on the ground at a certain time. Either way, check your mare line! New study results from German researchers suggest that maternal lineage plays a significant role not only in the sex of the fetus, but also in the length of gestation.

Scientists at the Graf Lehndorff Institute for Equine Science, in Neustadt, have determined that mitochondrial DNA—which is passed down to offspring exclusively from the mother—appears to influence fetal sex and gestation length in horses. The researchers also identified heritability of both foal sex and gestation length in the dam’s bloodline. This marks the first time such a connection has been made in any mammalian species.

“If looking for effects on heritability, it is always advantageous if the traits have a high variability, and that is absolutely the case with horses because they have such variable gestation lengths,” said Christine Aurich, DVM, PhD, head of the Graf Lehndorff Institute. “Also, the fact that we had a large number of mares and breeding seasons with very constant environmental conditions (because they all live at the national stud at Neustadt) gave us the opportunity to make such a discovery.”

The researchers investigated 786 breedings resulting in 640 live foals from 142 mares. Most of the mares could be grouped according to six maternal lineages, while 23 mares were classed in smaller, unrelated lines. Gestation length varied from 313 to 370 days, and researchers noted definite trends in length within each maternal line. Time to foaling was further influenced by the individual mare herself (her own timeline), mare age, breeding year, breeding month, and sex of the foal