When a horse owner plans the breeding of a mare to a stallion, the health of the mare and the unborn foal depends upon the owner's ability to recognise the signs of pregnancy in order to provide the mare with proper nutrients and care. Because a mare typically shows no outward physical signs until several months of gestation have passed, breeders need veterinary assistance to determine the clinical signs and formally make a pregnancy diagnosis.
Some mares don't begin to show physical signs of pregnancy until the fifth or sixth month of gestation. By that time, the belly begins to drop and the lower flanks start to widen. In "Storey's Guide to Raising Horses," Heather Smith Thomas recommends looking at your mare from the rear as she is standing still or walking away. With pregnancy, one side of her belly may appear larger than the other and there will be a definite swinging motion. After about the eighth month of gestation, you may notice movement toward the mare's right flank; the foetus is kicking and moving.
Over the final two months of pregnancy, the udder slowly enlarges. It becomes taut and the teats begin to fill out. Some mares develop oedema -- fluid-filled inflammation -- of the tissues of the belly directly in front of the udder. The mammary vein along the underside of the mare's belly becomes distended and the mare's gait changes, becoming wider to accommodate the placement of the foetus.
Approximately one month prior to giving birth, the muscular area around the mare's vagina begins to relax. The tail head, the perineum and the croup -- the top of the buttocks -- become soft and pliable in preparation for the soon-to-be-emerging foal. Right before she gives birth, drops of colostrum coat the mare's teats and harden, giving the udder a waxy appearance.
Veterinarians clinically diagnose pregnancy in mares by performing an ultrasound examination of the mare's uterus usually by day 15 or 16 after breeding and searching for the embryonic sac, or vesicle. By inserting the ultrasound probe into the mare's rectum and focusing on the uterus below, the vet can visualise the embryo by day 22 and can detect a heartbeat by day 24, say Dr. Mordecai Siegal and colleagues in "UC Davis Book of Horses: A Complete Medical Reference Guide for Horses and Foals." By the pregnancy's latter stages, the foetus can be seen using the wand on the underside of the mare's abdomen.
A vaginal examination by a vet reveals that the uterus and cervix of a pregnant mare increase in tone due to the influx of the hormone progesterone. The examination also reveals that the vesicle containing the embryo can be physically palpated by the 18th day of gestation.
Hormonal tests for a pregnancy diagnosis include the measurement of estrone sulphate concentrations in the mare's blood. Derived from the placenta, this hormone also shows the viability of the foetus as estrone sulphate declines upon fetal death. Oestrogen drawn from the fetal sexual organs show up in the mare's urine during the latter half of gestation and can be measured to evaluate pregnancy.
Equine gestation -- the length of pregnancy -- lasts for approximately 11 months, or 340 days, with a range of 320 to 360 days. Mares bred in the winter and spring typically hold longer pregnancies than mares bred in the summer and fall because of the horse's instinctual need to foal in warmer weather, says Dr. Rick Parker in "Equine Science." Veterinarians consider foals born after less than 320 days premature; these foals may need medical assistance. A small percentage of pregnancies exceed 370 days and are usually seen in mares grazing on fescue pastures.