Related:Canine Pregnancy FAQ
Female cats can start their estrus, also called heat cycles as young as four months of age. More commonly, cats enter estrus between 6 and 9 months of age.
Unlike dogs, with little or no vulvar discharge seen in cats. The primary signs of estrus in the cat are behavioral, and the rolling around and yowling are often confused with signs of pain. Because the signs of estrus may be confusing/hard to detect (for people, anyway), cats may become pregnant without anyone being aware that the cat was in heat.
Learn More: Feline Estrus Cycle
Female cats are induced ovulators, meaning they only ovulate (release an egg) when mated. This is one reason why they are very fertile. There may be more than one father for a litter if the cat is allowed to meet up with multiple males.
On average, cats are pregnant for 63 days, close to two months. The length of pregnancy in the cat may range from 58-70 days.
At 3 weeks into the pregnancy, the cat's nipples will "pink up" -- become pinker than the surrounding skin and become larger and firmer. This is one way to estimate the stage of pregnancy, your veterinarian will also be able to help stage/plan your cat's pregnancy and monitor any health issues.