• Mares can show signs of pregnancy after 14 days.Horse image by Dusi from Fotolia.com
  
  Female horses, or mares, have an average gestation period of 342 days--or about 11 months. A mare becomes obviously pregnant during the third trimester at seven to eight months due to the enlarged abdomen and developed udders. It can be challenging, however, to identify a pregnant mare in the earlier stages of pregnancy.
  
  

Behavior

• According to Horse Breeders Magazine, some mares can show a change in behavior when they become pregnant. This can involve the mare becoming aggressive and showing domineering behavior due to discomfort and hormonal changes. The mare will also react differently to the stallion once she is pregnant and will stop welcoming male advances.
  
  

Estrus cycle

• If 21 days pass after the mare has been put in with the stallion and the mare has not begun her estrus cycle, this is a reliable sign that she is pregnant.
  
  

Urine

• One reliable sign that a mare is pregnant is the massive increase of estrone sulphate present in her urine. This can be measured by using a home test kit, although this should be attempted only after gaining a clear understanding of how to use the equipment. Home test kits can be obtained through a veterinarian.