For best conception rate and litter size, breed the jill 2 consecutive days of the second week she is in heat. The gestation period is 42 days. An experienced veterinarian or ferret owner can diagnose pregnancy about 2 weeks after breeding, by feeling for the developing kits in the jill's abdomen. Ultrasound may also be used, starting at about 12 days, but is expensive.
A jill will return to heat within 2 weeks of weaning a litter if the photoperiod is right. Some jills with small litters will come in heat while still nursing the kits. Under natural light conditions, jills can have two litters a year. If kept under artificial light 14 or more hours a day, they can have three litters a year. If bred on every heat, they will have smaller and smaller litters each time, eventually failing to conceive and giving themselves a rest between litters.