Birman are very social cats and they normally love the company of others of the same breed. Also he has been use to another cat there so is maybe grieving for your other cat. It is very common also that a cat can pick up on your pregnancy my persian was like he was stuck to my whilst i was pregnant and didnt leave the side of our daughter when she was born. In photos he was in the end of almost each picture. If at all worried it might be worth talking to the vet for suggestions. You will be so busy when the baby comes that it probably woundnt be the best time to get another cat as they do take up your time.. Hope you get your problem sorted. And good luck with the forthcoming birth.