Female cats do experience a menstrual, or estrus, cycle by the time they are 6 months old. Sterilizing a cat before it reaches the age of maturity and begins its estrus cycle is the only way to prevent pregnancy.
Cats go through the estrus cycle seasonally, meaning that they cycle based on environmental factors, such as temperature and daylight hours. Some cats may cycle for 9 months for 1 to 3 weeks at a time, while others will cycle all year.
Unlike dogs, cats self-ovulate after mating. This means that ovulation only occurs when a female mates with a tomcat, and it may take up to three or four mating sessions with different tomcats to induce ovulation.