[header] How to tell if a cat is pregnant [title] Determine if your cat is fertile. [step] If your cat is fertile and has recently been in heat, it's possible that she may be pregnant. [substeps] Female domestic cats become sexually active as the days get longer and the weather gets warmer, typically between spring and fall. 
A female cat may start her estrus cycle (go into heat) once the weather becomes warmer and she has reached about 80% of her adult weight. This means a cat may go into heat as early as four months of age in unusual cases.