- The female horse, or mare, is not fertile during all times of year. She only ovulates during the period of estrus, which begins after the vernal equinox, when days start to get longer. Ovulating in the animal kingdom is also known as being "in heat." The estrus cycle lasts approximately 20 days and repeats itself regularly during the summer months. Not all horses will begin estrus at the same time. It will depend on the mare's physiology. Some breeders will "trick" their horses into entering estrus early by using artificial stall lighting to simulate the lengthening day. This can give their foals a size advantage over others born in the same year.
- When the mare is in her estrus period, she will let the male horse, or stallion, know by urinating near him. He will smell the fluid and be able to detect the presence of needed hormones. Then he will nudge the mare, approaching her from behind. She will lift her tail and reveal her vaginal opening and he will mount her. During intercourse, sperm will travel from the stallion's penis to the mare's vagina and hopefully meet an egg that was released by one of the ovaries. If fertilization is successful, the fertilized egg will travel to the uterus within five to seven days.
- A horse's gestation period, the amount of time from fertilization to birth, is approximately 11 months. The fetus uses the entire time for significant development, with most growth taking place during the final 3 months. This makes the survival rate for prematurely born horses very small. Most horses will only have one foal at a time, but there is a possibility of twins.
- Mares usually give birth at night and prefer not to be disturbed during this time. Labor is short, at about half an hour in length. The mare will lick the foal to remove the placenta and then it will stand and nurse within the hour. The foal must stay with its mother for at least four months, and some will nurse for up to a year. It will take the foal four years to grow to its full size.
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