Horse Gestation Timeline: Equine Due Dates, Ultrasound Scans & Foaling Signs
Written by equine veterinary specialists and horse breeders. Complete month-by-month care guide from breeding cover to foal delivery.
1 How Long Are Horses Pregnant? (Equine Gestation Length)
The average gestation period of a mare is 340 days (roughly 11 months, or approximately 48 to 49 weeks). However, normal equine gestations exhibit remarkable natural variability, with safe deliveries occurring anywhere between 320 and 365 days.
Foals born before Day 320 are considered premature and typically require intensive equine hospital care due to incomplete ossification of knee and hock bones and underdeveloped lungs. Interestingly, colt (male) foals are carried on average 2 to 3 days longer than filly (female) foals, and mares bred early in spring (under short daylight hours) often carry 10 days longer than mares bred in June.
2 Critical Ultrasound Scans: Why Day 14–16 is Mandatory (Twin Reduction)
In horses, twin pregnancies are life-threatening to both the mare and foals. Over 90% of equine twins abort late in gestation or produce severely compromised foals.
3 Equine Vaccination Schedule: Months 5, 7, and 9 EHV-1 Protocol
The single most important medical protocol during equine pregnancy is preventing Equine Herpesvirus-1 (EHV-1), commonly known as Rhino. EHV-1 causes "abortion storms" where seemingly healthy mares abort fully formed foals late in pregnancy without warning.
- Month 5: Administer first killed EHV-1 vaccine (Pneumabort-K or Prodigy).
- Month 7: Administer second booster EHV-1 vaccine.
- Month 9: Administer third booster EHV-1 vaccine.
- Month 10 (4–6 Weeks Pre-Foaling): Administer core pre-foaling booster (Tetanus, West Nile, EEE/WEE, Rabies). This boosts the mare's antibody levels so her colostrum protects the newborn foal.
4 Predicting the Exact Hour: Udder Changes & Milk Calcium Strips
Mares almost always foal at night (between 10 PM and 4 AM) in total quiet. Watch for these physical transformations during the final 30 days:
1. Udder Enlargement ("Bagging Up"): 2 to 4 weeks prior to foaling, the mare's udder fills with fluid.
2. Relaxation of Pelvic Ligaments: 7 to 10 days out, the muscles on either side of the tailhead soften like jelly, and the vulva elongates.
3. Waxing of Teats: 24 to 48 hours prior to birth, small drops of amber, waxy colostrum accumulate at the teat orifices.
4. Milk Calcium Testing: When calcium levels in mammary secretions exceed 200 ppm (parts per million), over 98% of mares foal within 24 to 48 hours.
5 The Three Stages of Foaling & The "1-2-3 Rule" of Foal Health
Unlike humans or dogs, equine labor is explosive and rapid. Stage 2 labor (from water breaking to foal delivery) normally takes only 15 to 25 minutes. If Stage 2 labor lasts longer than 30 minutes, it is an extreme veterinary emergency.
- 1 Hour: Newborn foal should stand upright on its own feet within 1 hour of birth.
- 2 Hours: Foal must latch and nurse colostrum vigorously within 2 hours.
- 3 Hours: Mare must completely pass the placenta within 3 hours. (Retained placenta in mares causes fatal laminitis and toxic shock).
Frequently Asked Questions About Horse Pregnancy
Can a mare stay out on pasture during pregnancy? ▼
Yes, mares benefit from gentle pasture turnout during the first 7 to 8 months. However, beware of endophyte-infected tall fescue pasture, which causes fescue toxicosis, thickened placenta (red bag), and agalactia (no milk). Remove mares from fescue pastures 60 to 90 days before foaling.
What is a "Red Bag" delivery during foaling? ▼
A red bag delivery (premature placental separation) occurs when the velvety red chorioallantois emerges before the translucent white amniotic sac. The foal's oxygen supply is cut off. You must immediately rupture the red membrane with clean scissors and pull the foal out immediately to prevent suffocation.
Why should you use straw instead of wood shavings in a foaling stall? ▼
Newborn foals are born wet. Wood shavings and sawdust stick to their nostrils, eyes, and wet umbilical cord stump, causing respiratory aspiration and umbilical infection. Clean wheat straw provides clean, soft, non-stick bedding.
What is a "foal heat" breeding? ▼
Mares come into their first postpartum estrus (foal heat) just 7 to 10 days after foaling. If the delivery was clean and the uterus has involuted, mares can be successfully rebred on foal heat to maintain a yearly breeding cycle.