Equine gestation facts and figures
Equine gestation is the period from conception to birth. It averages 340 days, but may range from 320 to 370 days, with mares being known to deliver a healthy foal after 399 days.
If a foal is born before 315 days, it is considered premature and if born prior to 300 days, it will usually be too young to survive.
When a mare fails to show heat at 18 to 22 days after service, it is often because of pregnancy. A skilled examiner can diagnose pregnancy using rectal palpation as early as 20 to 30 days into gestation, and transrectal palpation between days 40 to 50 is 95 percent accurate.
The mare immunological pregnancy test (MIP) is frequently used because it is accurate, inexpensive and convenient to use. The test is based on detecting elevated levels of equine chorionic gonadrotropin in the mare's serum between days 40 and 120 of gestation. The test is 95 percent accurate.
In addition, transrectal ultrasound is often used to determine pregnancy, especially if the mare has a history of twins or double ovulation. In this case it is used between days 12 and 15 in order to effectively manage pregnancy reduction to a single foal. The fetal heartbeat of the foal is visible on ultrasound by day 30.
Tests on plasma progesterone levels and milk progesterone levels in lactating mares are useful as early as the 16th day of gestation since they are sensitive to elevations and are 90 percent accurate.
Stages of mare gestation
Although the first 100 days of a mare's pregnancy produce few noticeable physical changes in the mare's body, she will not come back into heat or show interest in a stallion, once she is pregnant. During this time the mare's uterus is changing shape with the uterine horn becoming firm and tubular and the cervix becoming firm and contracted. The uterine wall becomes thinner at the site of the implanted embryo.
During this time, the mare's appetite increases and she may exhibit some moodiness. As the mare enters the mid stage of her pregnancy many veterinarians recommend a second pregnancy test because nearly one-third of mares "slip" or lose their pregnancy during the early days without the owner's knowledge.
At around 150 days gestation, the foal weighs approximately two pounds and the extremities are fully formed. By 180 days, the foal will weigh nearly ten pounds and will be growing whiskers and eyelids that are capable of blinking.
By 250 days gestation, the mare shows noticeable abdominal weight gain and the foal begins gaining as much as a pound a day.
The foals lungs are developing and its body is getting ready for life in the outside world as it approaches day 300 of gestation. At this time, the mare's udder distends slightly and begins producing a sticky yellow discharge that will turn to milk about two weeks prior to the foal's birth.
The mare's abdomen grows heavier and her vulva relaxes and lengthens as time for foaling draws closer. By day 315, the mare's owner should be prepared for foaling and the mare should be observed closely on a daily basis.
Preparing for birth
Within one week of foaling, the foal drops and settles lower in the mare's abdomen. Her hind end relaxes and appears lower in proximity to her tail. The croup softens and the mare often shows evidence that she wants to be by herself.
The mare may appear agitated and colicky and may walk restlessly. She may bite at her sides or lay down repeatedly and sweating may be obvious. This often means she is feeling contractions prior to giving birth.
Knowing the stages of gestation and being thoroughly prepared for foaling is the best insurance against any developing problems. Fortunately, most pregnancies and deliveries are without any complications.
Visit EquiMed's reproductive care health center for more information about horse pregnancy, delivery and mare care. An excellent follow-up to this article is Preparing for Parturition.