Sforno on Genesis 35:17/ספורנו על בראשית ל״ה:י״ז

1אל תראי, do not be afraid that the infant about to be born will be a girl although your pains are so great that there is reason for suspecting this. The Talmud in Niddah 31 claims that the birth pangs associated with the birth of female children are greater than those associated with the birth of male children.

2כי גם זה לך בן, even though you are experiencing all these pains, the child is a male.