in my experience, we first noticed because they started getting barrel bellies! Then soon before they give birth there b00bs start filling, the back ones first.
Getting fat and you can feel the kittens moving if you feel their belly, also the nipples start losing a bit of hair around them and get more prominent. A cat is pregnant for 65 days and at about 60 days will be tired, eat a lot more and start looking around the house for somewhere to give birth, if you suspect your cat is pregnant get a box somewhere quiet and keep putting her in it so she knows where to go when the time comes.
Hi,
At about 21 days after mating, they do what is known as 'pink-up', which means that the nipples will become slightly swollen, and pinkish red at the bases. From around five weeks of pregnancy, you will see the gentle swelling, which then obviously gets bigger. A few days before they are going to have them, you will be able to feel the glands around the nipples filling out with milk. Kittens generally arrive between 63-65 days, but can actually be anything from 59-70 days.