Just to add to the other comments, my son did the same thing only it lasted maybe 2 years, right from when he was born his Dad was the only one to calm him etc. He'd scream when he left for work or went out the room. It was heartbreaking for me. When I was at home with him he'd ask me over and over where's Daddy? when's Daddy home? My husband didn't really help he lapped up the attention and I ended up feeling like a spare part!
It only seemed to change when I fell pregnant, all of a sudden he became Mummies boy. I don't know whether it was because I softened or was too tired to discipline but it gave my husband a shock! And we started dishing out the same amount of discipline and having equal play times. I don't know if was a phase or something I was doing or even something my husband was doing but its passed now.
Perhaps your daughter is sensing the arrival of a new baby and is clinging to you as a safety net. Hopefully it should even out when the baby is here, she's not going to have a choice who she goes to!
