Girls, I wanted to ask, we and our boyfriend want to get married, I'm …

Girls, I wanted to ask, we and our boyfriend want to get married, I'm 7 months pregnant, but if I take his surname now, then I have to redo all the documents and change my passport, if we get married and I leave my surname at registration, what surname will go to the child? My or father's? And how can I change the surname to my husband after childbirth? Tell me how to do it better.