It’s normal for your uterus to contract when you orgasm. It’s always done that—now it’s just a good deal bigger so you notice it a whole lot more.
This won’t hurt baby, and it doesn’t mean you’re going into labor. Don’t worry, unless the cramping is especially severe, lasts more than a few minutes, intensifies, or seems to be turning into a series of contractions. If this happens, or if you have bright-red bleeding (not just spotting), call your OB right away. It could signal a miscarriage.
— Excerpted from The Baby Bump: 100s of Secrets to Surviving Those Nine Long Months
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