If your cat is pregnant, you may wonder how her kittens are developing. Compared to the nine-month gestation time for humans, the average 65-day cat gestation seems short. In this time the kittens go through many changes, particularly during the last four weeks.
The First Few Weeks
You may not even realize your cat is pregnant during the first few weeks of her pregnancy. It takes almost a week for the clusters of cells, known as blastulas, to implant themselves along the uterine wall. At this point they're known as embryos and are about .06 inches long—not even a tenth of an inch. About two weeks into pregnancy, as the cells continue to divide, the organs, skin, spinal cords and placentas will begin to develop. At three weeks the embryos are a bit over an inch long and have their internal skeletal systems. Their faces are also formed at around this point. This is the time when pregnant cats, known as queens, begin to exhibit signs of pregnancy, such as nausea.
Weeks Four Through Six
Around the beginning of the fourth week, when the embryos become known as fetuses, they're about 1.5 inches long. Their teeth are forming and their bones are calcifying. By the fifth week you might notice that your cat has gained weight. The kittens are about 2.5 inches long and they've developed quite a bit in the last week. Their ears are formed, their eyelids are developed to protect their developing eyes from surrounding fluids and they may kick and head-butt their litter mates as their space becomes tighter. They don't have fur yet, but they are starting to form claws.
By the end of the sixth week the kittens are more than 3.5 inches long and they've grown their first little bits of fur. Their tails have lengthened and their ears are bigger. Your cat may be a bit restless as she begins her search for an appropriate place to have her kittens.
Weeks Seven and Eight
During the last two weeks, the kittens make great progress. Their sense of smell is developing, as are their tongues, which will help them groom themselves. They begin to increase size every day as they add fat to their bodies.
The Last Week Before Birth
Most kittens need to make it to 60 days in their mother's womb to be mature enough to survive. At this point they'll be about seven inches long and their organs should be developed enough that they'll be able to make it outside the womb. The last several days before birth, they are basically putting on weight until your cat is ready to deliver. Each kitten will weigh around 3.5 ounces at birth.