The first two 'waves' of kittens have reached the adoptable age of 8 weeks or older by mid-summer. Female cats go into heat thought out the year here in Southern California. Do to weather factors, most cats will go into heat in January or February of each year. Heat cycles in the female cats are regulated by the weather. The weather affects cats at the same time, creating a surge in pregnant cats at the same time.
Female cats can, and often do, become pregnant while still nursing a litter of kittens.
Most cats go into heat three times a year, and with some weather patterns in Southern California, four times per year. A cat, unlike a dog, will keep repeating a heat cycle until she gets pregnant. Dogs go into heat twice a year, whether or not they get pregnant. In addition to affecting the heat cycles on a female cat, a change of weather pattern can stop a mating season instantly. Weather also affects kittens under 4 weeks of age.