Pregnancy length in small animals is usually much shorter than in people. In dogs and cats, the length is approximately 9 weeks. Dogs will only come into heat twice a year and so can only have a maximum of two litters in this time period. From early Spring female cats become ‘induced’ to ovulate by a male cat, meaning they can have multiple pregnancies in a year, which is why the cat population can easily get out of control.
A female dog in season is usually easily identified, her vulva swells, she bleeds for a week after which she is very fertile and then she generally bleeds for a further week on the way out (3 weeks in total). When she is most fertile, she is receptive to a male and it is worth knowing that sperm can survive for up to a week inside a female before fertilising an egg which does make calculating a whelping (birthing) date tricky unless we test the progesterone level in the blood.
In a cat a season is usually obvious from behavioural changes, a queen may roll around on her back ‘screaming’ or be very affectionate. If you are considering breeding your pet, please get advice from your Veterinary practice as it isn’t always straightforward!