Pregnant women have a lot to worry about—proper nutrition, sufficient exercise, and avoiding potentially dangerous foods are just a few of the things on an expecting mom’s mind. Then, too, many women (particularly first time moms) worry about whether their baby is developing correctly, whether they are at risk for a miscarriage, and whether their actions might adversely affect the pregnancy. For these reasons, seeing blood—a classic sign of impending miscarriage—after sex understandably induces panic in many women. Generally, however, blood after sex is both normal and completely harmless.
During pregnancy, the cervix becomes very vascular, and its blood supply increases tremendously. Many of the new blood vessels are close to the exterior surface of the cervix, and during sexual intercourse, the penis can bump and rub against this surface, generating friction. As a result of being bumped, fragile vessels in the cervix can rupture and bleed a small amount, leading to spotting that begins after sex and may last for several hours, note Heidi Murkoff and Sharon Mazel in their book, "What To Expect When You're Expecting."
Post-intercourse bleeding isn't dangerous, but it can be difficult to tell cervical bleeding from vaginal, which can be the bleeding caused by any number of other factors—it shouldn’t be a particularly large volume of blood. While there’s no concern that post-intercourse bleeding will cause a miscarriage, notes Dr. Raymond Poliakin in his book, “What You Didn’t Think To Ask Your Obstetrician,” large amounts of blood may indicate that it’s not cervical bleeding at all, but something else.
American Pregnancy advises women to check with their doctor if they bleed after intercourse; this helps to reassure the woman that her bleeding really is cervical, and ensures that there’s nothing wrong with the pregnancy. Even if intercourse always causes spotting, as long as a doctor has verified that the spotting is due to the intercourse itself, it’s considered safe to have sex throughout pregnancy.
The Mayo Clinic notes that it’s best not to have intercourse if a woman is experiencing any vaginal bleeding before sex begins. There are some conditions that can be exacerbated by intercourse during pregnancy—including placenta previa and an incompetent cervix. While bleeding after intercourse is generally considered innocuous, it’s only considered harmless post-sex bleeding if it actually began after sex. Bleeding that begins before sex, and then worsens afterward, may be the sign of something much more serious, and warrants a visit to the doctor.
Some women wonder whether intercourse can cause the cervix to open, leading to premature labor and delivery. This misconception probably arises from the mistaken belief that post-intercourse bleeding comes from the inside of the uterus, rather than the exterior surface of the cervix. The Mayo Clinic notes that while female orgasm can cause uterine contractions, they’re not the sort of contractions that expel a baby, meaning that they don’t lead to premature labor. Further, absent any other complications, sex can’t cause the cervix to open.