Cats are polyestrus, which means females can have many heat periods in each breeding season. Each heat period lasts between 5-14 days. If she is mated, the cat will usually go out of heat within 24 hours.

Breeding season in the Northern hemisphere is during the months of late winter and early spring (March-September). Indoor-cats or cats living in warm climates can go into heat and become pregnant more than once during the breeding season.

When in heat, the cat may display several signs. These may vary between breeds and even between individual cats. Signs include the famous calling (loud meowing), increased appetite and restlessness. Many cats begin to roll on the floor and demand to be petted. Some cats may begin to spray urine around the house.

If the cat is not mated and does not become pregnant, she can go into heat repeatedly every 2-3 weeks. This means that if your cat goes into heat - you can't simply keep her locked up at home, waiting for it to go away. Unless you spay the cat, she will keep going into heat practically every month.

Pregnancy

Feline pregnancy, or gestation, usually lasts 63-65 days. The length of the pregnancy, from ovulation to birth can vary between different breeds - anything between 58-70 days is considered within normal range.

You should be able tell if your cat is pregnant by the second or third week of the pregnancy, as the nipples of the pregnant female become enlarged and change color to deep pink. Later on, the growing abdomen will become more visible and leave little room for doubt.

Take the cat to see the vet for a prenatal check-up. Your vet will be able to confirm the pregnancy using an ultrasound or other tests. He or she will also set a course of tests and future check-ups as necessary.

Occasionally, a cat may exhibit a condition called pseudo- pregnancy or false pregnancy, where elevated levels of hormones cause symptoms that look much like a pregnancy. This condition can last for several weeks and then gradually fade away.

Care During Pregnancy

A young and healthy pregnant cat usually needs little special care other than extra attention to her nutrition.

She may experience nausea and morning sickness for a few days during mid-pregnancy, due to hormonal changes. She may also show a decrease in appetite and may even go off her food entirely a few days before the birth. If she stops eating for more than a couple of days, or if you notice a decrease in appetite for more than 3-4 days, consult your vet.

You should never medicate a cat unless your vet instructs to do so. This is especially important during pregnancy, where relatively safe and common drugs can be harmful. Let your vet know even if you only suspect your cat may be pregnant before any drugs are prescribed.

Source: http://www.petcaretips.net/heat_pregnancy_cat.html