What could that mean?

So we went to our 13 weeks prenatal appointment today. Doctor checked baby’s heartbeat with Doppler and we heard. It was wonderful. 150 hpm. Then the doctor started referring to baby as her. I was like what? I didn’t ask anything, but then she said she would refer the baby as a her until we knew the gender in about 5-6 weeks. Why would she refer to the baby is a her? Could she know something? I know it’s to early.