Female sheep, technically called ewes, are pregnant for 138 to 149 days. The average gestation period is 146 to 147 days. There are no visible signs of pregnancy until about six weeks before birth.
Pregnancy can be confirmed with an ultrasound around 35 to 60 days after breeding. A single ram is often used to impregnate an entire herd of sheep. The breeding season for most sheep is in autumn although certain types of sheep breed year round. Ewes reach puberty between 5 to 12 months of age, but they should not be bred until they reach 70 percent of their mature size.
Breed differences have an effect on the length of gestation, making it somewhat shorter or longer than the average. For instance, Finnsheep have shorter pregnancies than Rambouillet sheep.
Female sheep, called ewes, carrying more than one lamb at a time have shorter pregnancies than the average. Gestation periods are longer for ewes carrying male or heavy weight lambs, while female lambs result in shorter pregnancies. The first 30 days of gestation are the most critical, as this is when implantation takes place and most embryonic miscarriages occur.