Is childbearing sinful?

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Yes

After delivering a child, a woman must kill a lamb and a dove as a sin offering.

And when the days of her purifying are fulfilled, for a son, or for a daughter, she shall bring a lamb of the first year for a burnt offering, and a young pigeon, or dove, for a sin offering, unto the door of the tabernacle of the congregation, unto the priest: Who shall offer it before the LORD, and make an atonement for her. Leviticus 12:6-7

No

God commands us to have children, so it can't be sinful.

Be fruitful and multiply. Genesis 1:28, 9:1, 9:7, 35:11

Women are saved by childbearing. (If it was sinful, they couldn't be saved by it.)

Notwithstanding she shall be saved in childbearing. 1 Timothy 2:15