Mares frequently show no outward signs of pregnancy, other than missing an expected heat cycle. Some mares can even be racing fit well into pregnancy, up to seven to nine months. Most of the development of the foals occurs in the final three months of of the pregnancy. Some mares show significant weight gain, and some do not.
Mares can be examined with ultrasound as soon as 18 days after fertilization to confirm pregnancy. Blood tests can be used at about 120 days, and rectal palpitation can be used from approximately three months.