Equestrian Health

Gestation Period Of Horses - How Long are Horses Pregnant

The gestation period of horses... a condensed version.

This is the period of development in the uterus from conception until birth. Beginning with conception, the oocyte, or egg, stays in the oviduct of the breeding mare for 5 days before moving down into the uterus. The horse embryo is mobile in the uterus until about day 16 when fixation occurs and pregnancy is established.

Detection of horse pregnancy using ultrasound can be done as early as 12 days post ovulation. The heartbeat can be seen using ultrasound by day 28 in the gestation period of your horse, at which time the vesicle is roughly the size of a small chicken egg.

How Long are Horses Pregnant

The formation of endometrial cups occurs between days 36-38. The equine chorionic gonadotrophin that is produced by these endometrial cups peaks between days 60-80. After this peak the endometrial cups begin to decline and have disappeared by day 150 of the gestation period of horses.

Formation of the horse placenta begins on roughly day 50 of the gestation period of your horse, at which point the fetus is about the size of a grapefruit. Placental formation is usually complete by day 150.

Between days 59 and 68 of the gestation period of horses, if the fetus is perfectly positioned, and all is right with the world, there is a possibility of determining the gender using ultrasound.

By day 150, the horse fetus is comparable to a rabbit in size. Fetal growth will be quite vigorous in the last three months of gestation, accounting for over 50% of the total. At birth it is expected that baby horses will weigh about 10% of the weight of their mothers.

The approximate gestation period of horses is 340 days. The normal range is 320-370 days. This may seem to be a broad range, but horse pregnancy and foal birth are very individual occurrences and will happen when the time is right for each mare.

Figuring foal dates, or the approximation there of, will require you to know the last date that your breeding mare was exposed to the breeding stallion.

If following the heat cycle of mares with ultrasound, you should be able to name the day of ovulation, or be very close in estimation. The day of mare ovulation will be the day you start counting as the gestation period of your horse. Here are some examples:

Horse Gestation Table

Ovulation Date--March 2 Foal Date--February 5

Ovulation Date--March 12 Foal Date--February 15

Ovulation Date--March 22 Foal Date--February 25

Ovulation Date--April 1 Foal Date--March 7

Ovulation Date--April 11 Foal Date--March 17

Ovulation Date--April 21 Foal Date--March 27

Ovulation Date--May 1 Foal Date--April 6

Ovulation Date--May 11 Foal Date--April 16

Ovulation Date--May 21 Foal Date--April 26

Ovulation Date--May 31 Foal Date--May 6

If you have a recipient mare carrying the result of an equine embryo transfer, or oocyte transfer, the gestation period of these horses will be the same as described above.

How Long are Horses Pregnant

Often, when attending the same mares foaling over a period of several years, you will get an idea of the individual`s horse pregnancy patterns and be able to make an informed estimation. Keeping health records of your horse that includes mare behavior when foaling will be of great benefit to you and your pregnant mare.

If yours is a maiden mare then she has had no previous foaling experience upon which to base a (normal) gestation period for this horse.

As part of your foaling mare management, it is highly recommended that you schedule routine horse pregnancy tests at 30 day intervals through the first four months of the gestation period of your horse. This ensures discovery of twinning, poor fetal development, or loss of the fetus. These issues must be addressed prior to day 35 of the gestation period to avoid later disappointment. Some horse breeders will continue to check past this point if it is a high risk horse pregnancy, or just for their own piece of mind.

Should your pregnant mare begin to lactate or actually foal 30 or more days prior to her expected foal birth date you should contact your horse veterinarian immediately as this could be a serious horse health problem.

These are just some important reasons to understand the gestation period of horses.

Gestation Period Of Horses - How Long are Horses Pregnant