Equine prenatal development is subjected to a great variety of effects. The aim of the study was to compare the effects of the month of conception and month of birth on gestation length in mares. Data from 321 pregnancies were used. The conception occurred between February and August. Mares that conceived early in the year had a longer gestation than mares that conceived later in the breeding season (p<0.001). Foals were born between January and July. The gestation period increased in mares that foaled from January to April (p<0.0011 and then decreased in later months. Mares canying male fetuses had a longer gestation (p<0.05) than those carrying female fetuses. Mare age, stud farm and year of foal birth had no significant influence on gestation length. In conclusion, the data suggest that the effects of the month of conception and month of foaling on gestation length are the reverse in the first third of the year.