A horse's pregnancy lasts about from 320 to 345 days. While the actual term of pregnancy varies according to each individual mare, every horse goes through three stages of pregnancy, known as trimesters.
Mark Your Calendar
After your mare gets inseminated by a stallion, mark it on the calendar. Count out 330 days to find out the approximate day the new foal may be born. Have your foaling area ready for the baby's arrival by the 320th day.
The First 110 Days
Days 1 through 17: After insemination, your mare will stop being receptive to mating as her hormones change to accommodate motherhood. Her fertilized egg may take as long as a week to pass down the fallopian tube to the horse's uterus. Cell division starts immediately and the divided egg -- known as a blastocyst -- moves around in the horse's uterus until the 14th to 16th day of pregnancy. It then affixes itself to the uterine wall at the base of one of the two uterine horns. Pony embryos come to rest around Day 15 while larger horses may not fix until Day 17. The embryo has a yolk sac that provides for its nutrition while it's floating around the uterus.
Days 18 through 40: Your veterinarian can detect the foal's heartbeat by Day 21. The embryo grows about 3 to 4 millimeters each day. The yolk sac changes shape from spherical to triangular and gets absorbed into the developing umbilical cord, a process complete by Day 40. During this stage, there is little visual difference between a horse embryo and any other type of mammal.
Day 40 and beyond: The unborn foal becomes officially known as a fetus and takes on horselike qualities. By the 50th day, the fetus is about an inch long. The baby's internal organs and skeleton is now developed and it has a defined head, tail and legs. Your vet can determine the horse's sex by observing a protrusion that will become either the penis or clitoris. If it faces upwards toward the anus, the horse will be a female. The fetus ranges in size from 5 to 9 inches at the end of the first trimester.
Veterinary care: Your vet can do a rectal palpation or ultrasound between the 14th and 16th day to determine if your horse is pregnant and whether there's one or two fetuses. Routine ultrasounds every few weeks check on foal development as most spontaneous abortions occur during the first 50 days. After the 60th day, your vet will give the mare her first deworming during her pregnancy. If your mare has had pregnancy problems in the past, ask your veterinarian to check her progestin levels at 6 to 8 days and 25 to 35 days after she's bred. If they are low, he'll put her on oral altrenogest during the first trimester to regulate her hormones and prevent spontaneous abortion of the foal.
Nutrition: The fetus places little strain on the mother during her first trimester. Follow your usual feeding program for keeping your horse in good body condition. Too much food can cause obesity-related issues later in pregnancy.
Second Trimester: Days 111 to 220
The fetus weighs as much as 2 pounds by the 150th day and develops limbs and facial features. During the second trimester, the foal begins to develop hair on his lips, nostrils, eyelids and tail and he can blink his eyes. The fetus remains about the size of a cat during much of the second trimester although some foals may grow to 28 inches during this time. Your mare's appetite may increase and she may act moody.
Veterinary care: Your vet may do a second pregnancy test on your horse as she begins the second trimester to ensure the fetus wasn't spontaneously aborted. Your mare will have her second deworming at Day 150 and will receive an equine herpes virus (EHV-1) vaccination on Days 150 and 210.
Nutrition**:** The fetus doesn't represent a nutritional strain on his mother throughout most of the second trimester of pregnancy. Feed your mare as you normally would to maintain her at an ideal body condition. Consult your veterinarian before increasing food for your mare to avoid obesity-related complications.
Third Trimester - Days 220 to 330
Fetal Growth and Mare Developments
Most of the fetal growth takes place during the third trimester. Your mare will look larger in her abdominal region. Hair develops all over the foal's body, including the mane and tail. By the 11th month, the foal can weigh more than 130 pounds and be up to 58 inches long. Mammary glands on the horse develop four to six weeks prior to giving birth. The udder protrudes slightly and may exude a sticky yellow substance. As delivery approaches, the mare becomes restless and may bite at her sides in response to contractions.
Veterinary care: Your veterinarian will perform a final deworming during the third trimester. The mare will get a third EHV-1 vaccine at Day 270 as well as any other vaccines recommended by your veterinarian. Move the horse to her foaling area during this time so she can develop antibodies that she'll pass on to her foal.
Nutrition: Your vet will recommend specific additions to your mare's diet to prepare her for foaling at about Day 300.