Though they must be in heat to get pregnant, cats have ample opportunity because their heat cycles come and go through most of the calendar year. According to Dr. Arnold Plotnick, an unspayed cat can have between 50 and 150 kittens in a lifetime.
Multiple Heat Cycles
Tabby is polyestrous, meaning she has heat cycles throughout the year, influenced by season, day length and temperature. The feline heat cycle average 14 to 21 days between the beginning of one cycle to the next. Your female cat is an "induced ovulator," which means she has to be in heat to mate; she has to actively mate with a tom cat to ovulate and become pregnant. She'll cycle in and out of heat until she mates or until environmental factors, such as shorter days, cause her cycle to stop on its own.
Proestrus, the First Heat Stage
The first of the four stages of heat for Tabby is proestrus, when she's restless, extra-affectionate and eating more. Proestrus lasts only a day or two and is basically a courtship phase. Exposure to tomcats will trigger the rest of her heat cycle. If she's not fixed and you don't want kittens, keep her indoors until she's spayed because she can't get pregnant yet.
Time to Get Pregnant
Estrus, the second stage, lasts four to six days. Tabby is more vocal and acts up quite a bit, sometimes rolling about the floor, weaving in and out of your legs and even biting or grabbing at you. Her vocal cries will attract suitors. Interestrus follows, and what happens next depends on what happened during estrus. If Tabby hasn't mated she'll stay in interestrus for a week or two and then start a new cycle again. If she mated and ovulated but failed to conceive, she'll have a false pregnancy that lasts about a month. If she did get pregnant, she'll deliver kittens in about 63 days.
A Time of Rest
In the northern hemisphere, cats have a three-month period of reproductive rest, usually ranging from November to January. Their ovaries are inactive and they can't conceive when they're in anestrus. Older female cats who no longer have heat cycles are permanently in anestrus.
Other Heat Cycle Facts
Cats are most fertile between 1 1/2 years and 8 years of age. Some cats experience their first heat as young as 3 1/2 months, though the average age is between five and nine months. If Tabby gets pregnant, a slim chance exists that she could experience another heat cycle between the third and sixth week of pregnancy. It's rare for a cat to become pregnant with additional kittens during that time. If that happens, Tabby could deliver a second litter of kittens while still nursing her first litter.