If your female cat isn't spayed and has reached feline puberty -- generally about the age of 5 months -- she can get pregnant. It's hard to tell if a cat is pregnant just by observing physical symptoms until she's quite far along. Your veterinarian can detect pregnancy by palpating your cat's abdomen and feeling for fetuses fairly early on, but can't predict the exact number of kittens she's carrying until approximately day 40 of pregnancy.
A cat's gestational period averages about two months from conception to birth. If Kitty was in heat and outdoors, she might have copulated several times during her estrus, perhaps with multiple males. That makes pinpointing the potential birth date a bit complicated, unless you bred her deliberately or knew there was only a short window when she could have gotten pregnant. A cat in heat desperately wants to breed, so she'll try to get out of the house any way possible. Even if she escaped only for a brief period, there's a good chance she found herself a tomcat.
Early Signs of Pregnancy
Early signs of feline pregnancy are quite subtle. Between two to three weeks into her pregnancy, a cat's nipples become more red and enlarged. However, milk won't leak from the nipples until shortly before she gives birth. Some cats experience morning sickness, generally between the third and fourth week of gestation. A cat might not eat for a day or two, or throw up her meals. Your cat's personality might change -- for the better. Pregnant cats often become more affectionate with their people.
Cats gain weight when expecting, but most of that gain comes as the due date approaches. Her overall weight gain depends on how many kittens she's carrying. Because early weight gain is minimal compared with the gain as the kittens develop, you might notice that your cat suddenly appeared to put on pounds. That's usually only a week or two before she gives birth. If your cat is long-haired, you might be less aware of weight gain, as the hair somewhat hides a big belly. If your cat is carrying just one or two kittens, changes in size are less detectable.
Your vet can detect feline pregnancy after the second week via palpation, although most veterinarians prefer to use ultrasound at roughly the same time period to make a pregnancy diagnosis. If your vet detects a heartbeat in the fetuses during the ultrasound, the pregnancy is at least three weeks along. After approximately the 40th day, or sixth week of pregnancy, an X-ray can confirm the number of kittens. That's when their skeletons mineralize and each individual fetus can be seen clearly on a radiograph.