The gestation period of a mare is approximately 340 days (roughly 11 months), but can range safely from 320 to 365 days. Managing a pregnant mare requires careful monitoring, particularly in the first and final trimesters.

Trimester 1 (Days 1 to 110) * **Milestones:** Fertilization, implantation, and initial organ development. * **Nutrition:** Keep the mare on a standard maintenance diet. Overfeeding in the first trimester is a common cause of early embryonic loss. * **Vet Checks:** Confirm pregnancy via ultrasound on Day 14–16 (essential for detecting and pinching twins), and confirm fetal heartbeat on Day 28–30.

Trimester 2 (Days 111 to 220) * **Milestones:** Rapid fetal skeletal development. The fetus grows to about the size of a medium dog. * **Nutrition:** Slightly increase protein and mineral availability, particularly calcium and phosphorus. * **Management:** Maintain standard exercise. Light riding is perfectly safe for healthy mares during this period.

Trimester 3 (Days 221 to 340) * **Milestones:** Over 60% of the foal's birth weight is gained during the final 90 days. The lungs fully mature in the last few weeks. * **Nutrition:** Significantly increase caloric and protein intake. The fetus occupies a large space in the abdomen, restricting stomach space – feed smaller, nutrient-dense meals. * **Vaccinations:** Administer the Pneumabort-K (EHV-1) vaccine at months 5, 7, and 9 to prevent abortion, and give standard boosters (Tetanus, EEE, WEE, Rabies) 4 weeks before the due date to ensure high antibody levels in the colostrum.