The first child is a daughter. What does the doctor mean by matching the second child?

Except for medical needs, general hospitals prohibit the identification of fetal sex.

The examining doctor has to face many inquiries from the examinee every day, so what I tell you is of course ambiguous:

Pairing with the first daughter can be understood as a man and a woman, and it can also be understood as the second daughter (two daughters are a pair).

Just like a fortune teller. No matter what the result is, he can explain clearly.

I hope my answer will make you feel something.