Mare gestation averages 340 days, with a normal range of 320 to 370 days. Breed, season of conception, and individual variation all influence length; draft mares tend to carry slightly longer than light breeds.
Gestation is divided into three trimesters. The first (days 1-114) covers embryonic implantation and early organogenesis. The second (days 115-226) is the period of fetal growth; organ systems mature and the fetus becomes recognizably equine. The third (days 227 to term) is rapid weight gain — roughly 60 percent of foal birth weight accumulates in this final stage.
Foaling is typically fast. Stage one, cervical dilation and repositioning of the foal, lasts one to four hours and is often missed. Stage two, active delivery, averages 20 to 30 minutes; a healthy mare in lateral recumbency delivers with strong abdominal contractions. Delay beyond 30 minutes in stage two is an emergency. Stage three is passage of the fetal membranes, expected within three hours of birth; retained placenta past that threshold requires veterinary intervention.
A foal delivered after 300 days is considered full-term. Birth before 300 days is classified as abortion; birth between 300 and 320 days may yield a viable but premature foal requiring intensive care.