Objective:To study the factors affecting length of gestation in foals from artificially inseminated Marwari mares for a period of ten consecutive breeding seasons.Methods: Gestation length was measured in 126 pregnancies from which viable foals have b. Time of ovulation was confirmed through ovarian ultrasonography in order to determine true gestation length (ovulation to till foaling) as different to previous studies, in which either the mating to foaling interval or from fertilization to till parturition was used for calculating the gestational length. All the mares were inseminated artificially with frozen thawed stallion semen at appropriate time points by monitoring the point near to ovulation. Pregnancy was diagnosed approximately 11-15 d after ovulation initially, and re-affirmed at 30 to 60 d of pregnancy.Results:The observed mean gestation length for all the foals was recorded to be 342.20±1.91 (with standard deviation 21.4) d, and ranged from 259-388 d. Various factors like parity of the mare, fetal gender, fetal birth weight, season of breeding (Artificial Insemination) and season of foaling were considered for their effect in causing variation on the gestation length in Marwari mares. According to the present study point of view and after compilation of the results, season of breeding and foal birth weight were found to show significant effect in causing true variation of the gestation length in Marwari horses. Other factors like sex of the foal and parity of the mare also had an effect on length of gestation, but whose effect was observed to be non-significant. Over the range of gestation lengths observed, season of foaling was not significantly associated either with gestational length of mare or foal birth weight.Conclusion: From the current study it can be concluded that the true average gestation length for Marwari foals b is 344.1±0.49 d. The range of gestation lengths was found to be 315-388 d all resulting in viable foals. The length of gestation is not significantly affected by age and parity of the mare. The only factors significantly affecting gestation length are foal gender and month of foaling (P <0.05).