They smell the pregnancy hormones. :)
when i was pregnant with my son i didnt know that cats can hear the babies heart when your pregnant until my grandma told me that. no one knew that i was pregnant except my grandma because i went there to her house and i was 5 months pregnant and she noticed that her cat thats a female always came to me and wanted me to pet her and she always laid on my stomach. and when i did tell everyone that i was pregnant she already knew that i was but she wanted me to tell my mom and i told her when i was 5 months along=). so yes cats can hear and since when a women is pregnant=).
My girl cat knew before I did that I was pregnant. They can sense or smell our hormone change I think. She is a total daddies cat but when I'm pregnant she is all over my belly. LOL
She is also doing the "love paw" thing on my breasts as she lays on my belly, tmi, but I haven't started leaking so I shoo her away from that! I dont need my body thinking its time to start feeding the baby lol! I am sure when the baby is born it'll think I purr from all the time the cat spends purring on my belly lol.
I definitely think my cats can tell Im pregnant. My female cat has been super affectionate for the past few weeks, she likes to put her paws on my belly while she sleeps beside me. Now Ive noticed the past week my male cat has become very sookey and follows me around alot! Id say your cat knows something is going on!!