The average equine pregnancy lasts between 335 and 345 days, or approximately 11 months; it can be divided loosely into three trimesters. If you have bred your mare to a stallion, you will need to work with your veterinarian throughout the pregnancy to ensure that your mare has proper prenatal care. The way you care for your mare during pregnancy affects the health of her foal.
Fertilization occurs in horses when a mare who is in estrus is inseminated naturally by a stallion or artificially with semen taken from a stallion. Afterward, blood tests that monitor equine chorionic gonadotropin, a glycoprotein pregnancy hormone, in the blood can determine whether she is pregnant, but not until 10 days has passed since ovulation.
A veterinarian may palpate the mare rectally to determine whether she is in foal, though rectal palpation is not always the most effective method of determining pregnancy.
The most reliable means of confirming pregnancy during the earliest stages is ultrasound, which can detect pregnancy as early as five days into gestation and reliably determines the exact stage of gestation up to the 45th day of pregnancy.
The First Trimester
Once pregnancy is confirmed by blood test or ultrasound, a healthy mare's pregnancy will require relatively minimal effort on the part of human caretakers. The first trimester starts on the day of conception and ends day 113. During this time, a significant amount of initial development occurs. The embryo can migrate around in the uterus until approximately day 16. The heartbeat becomes present around day 25. Recognizable internal organs and features such as the hooves and tail develop between day 20 and day 40. The endometrial cups form around day 36 to 38; they produce equine chorionic gonadotrophin through day 120.
The Second Trimester
The second trimester starts around day 114 and and lasts through day 225. During this time, the fetus will continue to gradually develop as the pregnancy progresses, though the fetus does not change significantly in size during the first six months of the pregnancy. Gender can be determined approximately five to six months into gestation.
The Third Trimester
The third trimester begins day 226 and ends when the foal is born. The fetus does most of its actual growing during the last three months of pregnancy. This is why the average mare does not look pregnant until month eight or later of the pregnancy. It is not necessary to perform additional ultrasounds after confirming the initial pregnancy, but you can if you desire to do so or if your veterinarian suspects a problem may have occurred with the foal.
Caring for Your Pregnant Mare
You can ride and exercise a healthy mare during pregnancy, but discuss appropriate work load with your veterinarian to clarify which activities will be safe for both mare and fetus.
Your mare's nutritional needs will not change until around her eighth month of pregnancy. Supplement her diet with extra feed; offer her unlimited access to grass or good-quality hay. Vitamin supplementation may be required depending on the type of feed and hay you provide, so discuss your mare's diet with your veterinarian.
Provide regular veterinary and farrier care throughout a pregnancy. Update vaccinations during the last 60 days of the pregnancy to give the foal immunity.
Pregnancy Problems and Foal Abortion
Mares can lose foals for a variety of reasons. Twin pregnancies, if not caught early and corrected by a veterinarian, will often end in abortion. Problems with the umbilical cord or placenta, or sepsis that occurs in the uterus, can cause a mare's pregnancy to abort.
Medical illnesses that can cause abortion in horses include equine rhinopneumonitis, equine viral arteritis, leptospirosis, Potomac horse fever and equine mycotic placentitis. Consumption of fescue hay contaminated with endophytes of the Acremonium genus can also cause abortion.
It can be difficult to predict when a mare will abort a foal, but signs of impending abortion can include premature udder development, vaginal discharge, premature lactation, edema and any signs of impending foaling that are occurring too early in the pregnancy.
Foals born before 325 days are considered premature; those born before 300 days typically do not survive.