Childbirth is painful for women because Eve sinned.
Why is it so painful for women to give birth to children? Sure, science can explain what causes the pain, but what caused nature to work in this way?
GOD caused it.
When Adam and Eve sinned in the Garden of Eden, GOD explained to each of them specific consequences of their rebellion.
16 To the woman [GOD] said, “I will greatly multiply your pain and your groaning, and in pain you shall bring forth children. Your recourse will be to your husband, and he shall rule over you.”
(Genesis 2:16 OSB)
Ask any mother about the worst pain she has felt. For a large percentage of moms, the answer is child birth.
Why is this the case? Sin.
Actions have consequences, and labor in delivery is one such consequence of living in a world full of sin.