The aim of this study was to evaluate the factors maternal age and number of fetuses (single or twin) on the gestation length of Ile-de-France sheep. The study was carried out with pure bred Ile de France ewes from the breeding flock that was raised in experimental base (EB) of Institute of Animal Science (IAS) – Kostinbrod, Bulgaria. The study covered 830 records of ewes for the period 2008-2018. The ewes were at age between 2 to 8 years and gave birth to singles (n = 408) or twins (n = 422). The age distribution of ewes was as followed: 2 years of age: n = 65; 3 years of age: n = 133; 4 years of age: n = 169; 5 years of age: n = 147; 6 years of age: n = 137; 7 years of age: n = 105; 8 years of age: n = 74. The age of ewes was influenced significantly on the length of gestation (Р≤0.01), whereas the number of fetuses and the interaction between the effects of age of ewes and number of fetuses did not. The average length of gestation for all cases was 150.59 days. It was observed a shorter duration of pregnancy in ewes at two years of age, compared to older sheep, as the differences were significant between 2 years of age and: 6 years of age (P≤0.01), 7 years of age (P≤0.01) and 8 years of age (P≤0.05). 3/4 or 77.21% of all pregnancies were between 149 and 153 days, with the largest share being at days 150 (18.55%) and 151 (18.19%).