i also have a bold patch at the back of my head, a size of a 2 pence, and I'm 7 months pregnant, and this is my first pregnancy. its very common because lack of vitamins. any way try to keep your hair up so people wont notice it, at the moment the doctors wont be able to help because your pregnant but after the baby's born you can then get it checked out from a skin specialist or you can try the Chinese shops which have herbal stuff that will help your hair grow from the root.
good luck