Toward the middle of the first trimester, embryos are about a quarter-inch long in fetal position and look a lot like tadpoles. Optic cups on either side of the embryos' heads will later develop into eyes; a hole at the front of each embryo's head will become a mouth. Limb buds are starting to protrude, and they will develop into legs.
At 3 weeks, the see-through embryos have four distinct limbs. Between days 23 and 25, the toes start to separate. The embryos' mouths start to form, and their eyes become black. Their skulls are pointed since they're not fully formed, and their brains poke through.
By the end of the first trimester, the inch-long embryos have cartilage skeletons, ear holes and tongues. Meanwhile, the mother's nipples start to pink, meaning they become darker and swollen, and she may develop morning sickness.
In the second trimester, from approximately day 21 to day 40, the former embryos are now called fetuses and have started to develop kitten features.
Between days 25 and 28, the kittens' ears become pointed, their eyes develop lids, and their skulls look like heads instead of cones. Four weeks into gestation, their noses and muzzles look like cats'.
Twenty-eight to 32 days into gestation, fetuses are almost 2 inches long. Their toes are completely separated and their pads and claws have started to show.
The breeder or veterinarian can palpate the adult cat's belly around four to five weeks into gestation to confirm the pregnancy and to determine how many kittens she'll have. Typically, a litter has two to five kittens.
The fetuses' skin starts to thicken up around 5 weeks into gestation; their features continue to form. The whisker follicles will begin to show, although the unborn kittens are still hairless.
In the third and final trimester, cat fetuses look more and more like kittens.
At 38 to 44 days, their skin starts to wrinkle, their ears grow larger and their tails becomes longer. Fetuses at 6 weeks of development are 2 to 3 inches long, and hair follicles are forming all over.
Meanwhile, the mother's teats are getting bigger. She spends a lot of time grooming. She will start looking for a place to give birth, so this is when she needs to know where her nesting box is. Provide a box with space for her and the kittens, with some bedding, in a secluded space.
From 44 to 48 days the fetuses become fully furred with colorless hair, and their whiskers and eyebrows are long and thick. Noses gain color; tongues tends to stick out of mouths since they have grown so quickly.
At 7 weeks, fetal kittens are close to 5 inches long and start to practice grooming themselves.
From day 58 on, the fetal kittens are getting ready to be born. They're as long as 7 inches now, and their organs have fully developed. The mother will queen within a week, giving birth in her nesting box. The labor could take hours, depending on how many kittens she's been carrying.