While bleeding or spotting during pregnancy is not abnormal, it's certainly unexpected—and unnerving. Learn the causes of bleeding during pregnancy, how to tell when it's serious, and what you should do it if happens to you.

Any type of bleeding while pregnant is terrifying, even for the most level-headed mom-to-be. The good news: Spotting, or light bleeding in early pregnancy that's not heavy like your menstrual period, occurs for nearly one-third of all moms-to-be and often poses no threat to mom or baby. It's common for a small percentage of pregnant women to have spotting after intercourse, for example, and others may have bleeding for reasons that have nothing to do with pregnancy, such as infections or tears to the vaginal wall. "The vast majority of spotting is harmless," says Alyssa Stephenson-Famy, M.D., Maternal-Fetal Medicine Specialist at the University of Washington, Seattle. But bleeding, no matter how scant, can be indicative of a variety of complications, including miscarriage, ectopic pregnancy, and placenta previa, and thus should never be ignored. Here are the various reasons you may experience bleeding while pregnant, when to know whether you should call your doctor, and tips for effectively communicating your symptoms to your healthcare provider.

Causes of bleeding or spotting in the first 20 weeks

Doctors estimate that 25 to 40 percent of women will experience some vaginal bleeding during early pregnancy and more often than not the pregnancy will progress totally normally, Dr. Stephenson-Famy says. According to the American Pregnancy Association, there are a number of possible causes of innocuous spotting or bleeding in the first half of pregnancy, including:

  • Implantation bleeding: The implanting of the egg in the lining of the uterus occurs about 4 weeks into your pregnancy as the fertilized egg attaches to your uterine wall. If you notice a small amount of bleeding about a week to 10 days after conception, implantation bleeding is likely the cause and is nothing to worry about.
  • Sexual intercourse: During the second and third trimester, your cervix becomes swollen because of the increased blood supply in the area. As a result, vigorous intercourse may cause spotting during pregnancy.
  • Infections: Some women have cervical bleeding because of an infection, usually a sexually transmitted disease such as chlamydia. In this case, the underlying condition needs to be treated.
  • Internal exam done by your obstetrician or midwifeIt's not uncommon to bleed after a Pap smear or pelvic exam, which in many practices is conducted between the sixth and 12th week of pregnancy. Spotting may occur within 24 hours after the visit and usually goes away within a day.

Sometimes bleeding during the first half of your pregnancy can be a sign of a more serious condition, however, such as:

  • Subchorionic hemorrhage, which is bleeding around the placenta. Although it is possible to continue with a normal pregnancy after this type of bleeding occurs, prompt diagnosis and treatment is vital. "Most subchorionic hemorrhages resolve, but it does put the woman at an increased risk for other complications such as preterm labor," Dr. Stephenson-Famy says.
  • Chemical pregnancy occurs when an egg is fertilized but never fully implants in the uterus.
  • Miscarriage (either threatened or imminent), which is the spontaneous loss of a pregnancy in the first 20 weeks. Often, the bleeding or spotting that occurs during a miscarriage will be accompanied by other symptoms, such as cramping or abdominal pain.
  • Ectopic pregnancy, which occurs when a fertilized egg implants somewhere other than the uterus, most often in a fallopian tube. Sometimes called a tubal pregnancy, an ectopic pregnancy cannot progress normally and may be life-threatening to the mother if left undiagnosed.
  • Molar pregnancy, a nonviable pregnancy characterized by an abnormal growth on the placenta, and, usually, an abnormal fetus.

Causes of bleeding or spotting past 20 weeks

Although the risk of miscarriage (known as a stillbirth after the first 20 weeks) diminishes greatly after the first trimester, and many of the early complications are no longer a factor (such as ectopic and molar pregnancies), bleeding during the second half of pregnancy should be taken very seriously, especially if it's ongoing, Dr. Stephenson-Famy says. Causes for bleeding in the second half of pregnancy include:

  • Sexual intercourse
  • Cervical checks, especially late in the third trimester when they become more frequent
  • Placenta previa, which is when the placenta covers the cervix either partially or completely
  • Placental abruption, in which the placenta tears away from the wall of the uterus, can cause severe vaginal bleeding and is life-threatening to both Mom and Baby. According to the American Academy of Family Physicians, a placental abruption is the most common cause of serious bleeding during late pregnancy. The condition is rare and occurs only in about one percent of all pregnancies.
  • Preterm labor, in which vaginal bleeding is accompanied by cramping or contractions, diarrhea, pelvic pressure, or back pain before 37 weeks, could have serious repercussions for the baby if not managed. After 37 weeks, these symptoms could be a normal start to labor.

How to know when spotting or bleeding during pregnancy is serious

Vaginal bleeding during pregnancy is not uncommon, but it's also usually not normal. It can indicate many things, depending on whether it's heavy or light, how long it lasts, what color it is, and at what point in the pregnancy it occurs. Signs that bleeding is due to serious conditions such as miscarriage, problems with the placenta, or preterm labor, include heavy bleeding (similar to menstrual bleeding), bleeding in conjunction with cramps and/or fever, or bleeding and passing some tissue. The color of the blood is also important: Blood that's bright red is usually more worrisome than blood that's brownish.

When to call your doctor when you're spotting or bleeding during pregnancy

No matter when it occurs, even if you have less than a dime-size spot of blood, call your doctors or midwife immediately to be safe, as any vaginal bleeding during pregnancy can be a symptom of a larger problem. Be prepared to answer detailed questions about the amount of blood you've lost and a description of how you're feeling overall, advises Laura Riley, M.D., author of Pregnancy: The Ultimate Week-by-Week Pregnancy Guide. Your practitioner will decide whether there's a cause for concern. If this is the case, she may want to monitor the situation by having you stay off your feet for a few hours and call back, or have you come in immediately. She may also order an ultrasound to see what's going on with the fetus. Dr. Riley says you should insist on being seen if you have any vaginal bleeding that makes you feel faint or soaks through a sanitary pad. You should also be seen if the bleeding is persistent or accompanied by pain or a fever.

Unfortunately, if the bleeding is due to a miscarriage, there's nothing that can be done.

If the bleeding is a sign of preterm labor, your doctor will take steps to prevent it, possibly recommending bed rest or medication.

If the bleeding is due to placenta previa—a condition in which the placenta covers the mouth of the uterus—then your doctor may put you on modified bed rest. She'll probably avoid conducting an internal vaginal exam, and will probably recommend that you have a c-section when your baby is ready to be born. A scheduled c-section is usually done at 38 or 39 weeks, but women with placenta previa often bleed in the third trimester. If this is the case, the c-section may be done earlier.