Bleeding during pregnancy is common, especially in the first trimester, but the causes vary widely depending on when it occurs. Some are harmless, like the light spotting that happens when a fertilized egg attaches to the uterine wall. Others, like placental abruption or ectopic pregnancy, require immediate medical attention. Understanding what’s behind the bleeding, and when it signals something serious, can help you respond appropriately.
First Trimester Causes
Bleeding in the first 12 weeks of pregnancy is surprisingly common and often not a sign of trouble. The most frequent cause is implantation bleeding, which occurs when the fertilized egg burrows into the lining of the uterus. This typically shows up as light spotting a week or two after conception and can be easy to mistake for a light period.
Another common cause is a subchorionic hematoma, which is a blood clot that forms between the amniotic sac and the uterine wall. These hematomas can produce anything from light spotting to heavier bleeding, and many resolve on their own without affecting the pregnancy. Your cervix also becomes more sensitive during pregnancy because of increased blood flow, so spotting after sex or a pelvic exam is normal and usually nothing to worry about.
More serious first trimester causes include miscarriage and ectopic pregnancy. A miscarriage often involves heavy bleeding with cramping, though some begin with light spotting that worsens over days. An ectopic pregnancy, where the embryo implants outside the uterus (usually in a fallopian tube), can cause bleeding along with sharp pain on one side of the pelvis or lower abdomen. Ectopic pregnancies cannot continue and need treatment quickly to prevent dangerous internal bleeding.
Molar Pregnancy
A molar pregnancy is a rare but important cause of first trimester bleeding. It happens when abnormal tissue grows in the uterus instead of a normal embryo. Symptoms include dark brown to bright red vaginal bleeding during the first three months, severe nausea and vomiting, pelvic pressure, and sometimes the passage of small, grapelike cysts from the vagina.
If a molar pregnancy isn’t detected early, the uterus may grow unusually fast and appear too large for the stage of pregnancy. It can also trigger high blood pressure, ovarian cysts, and an overactive thyroid. A molar pregnancy requires treatment to remove the abnormal tissue, and follow-up monitoring of pregnancy hormone levels to make sure no tissue remains.
Second and Third Trimester Causes
Bleeding later in pregnancy has a different set of causes, and most of them deserve prompt evaluation.
Placenta Previa
Placenta previa occurs when the placenta partially or completely covers the cervix. It affects about 1 in every 200 pregnancies. The hallmark symptom is bright red vaginal bleeding in the third trimester that typically comes without pain or cramping. Some women have a single episode; others bleed repeatedly. Placenta previa is usually diagnosed on ultrasound, often during a routine scan, and the delivery plan depends on whether the placenta moves away from the cervix as the uterus grows.
Placental Abruption
Placental abruption is more common, occurring in about 1 in every 100 births. It happens when the placenta separates from the uterine wall before delivery. Unlike placenta previa, abruption typically produces dark red bleeding accompanied by abdominal pain, and the uterus may feel tender or rigid. This is an emergency situation because the baby can lose its blood and oxygen supply.
Bloody Show
Not all late pregnancy bleeding is dangerous. “Bloody show” is a normal sign that your body is preparing for labor. As the cervix softens and begins to open, it can release a small amount of blood mixed with mucus. This discharge is usually pink, red, or brown, has a jelly-like or stringy texture, and amounts to no more than a tablespoon or two. After 37 weeks, this is a routine part of the process. Before 37 weeks, any bleeding or mucus discharge should be evaluated, as it could indicate preterm labor.
Non-Obstetric Causes
Some pregnancy bleeding has nothing to do with the pregnancy itself. Cervical polyps, small growths on the cervix caused by chronic inflammation or infections, can bleed when touched. They’re uncommon during pregnancy and usually small, but if one develops, your provider will decide whether to remove it based on its size, location, and your overall condition.
Vaginal or cervical infections, including sexually transmitted infections, yeast infections, and bacterial infections, can also cause spotting. The cervix has increased blood flow during pregnancy, making it more fragile and prone to bleeding from irritation or inflammation. These infections are treatable during pregnancy, and addressing them promptly helps prevent complications.
How Bleeding Gets Evaluated
When you report bleeding during pregnancy, your provider typically starts with a pelvic exam to look at the cervix and check for discharge, lesions, polyps, or any tissue in the cervical opening. A transvaginal ultrasound is the key imaging tool. It confirms that the pregnancy is inside the uterus, checks for a heartbeat, and can identify issues like subchorionic hematomas or placenta previa.
Blood tests play an important role too. Pregnancy hormone levels (hCG) help your provider interpret the ultrasound results. In a healthy early pregnancy, hCG levels roughly double every 1.4 to 2.1 days. Levels that rise more slowly than expected, or that are lower than they should be for how far along you are, can point toward an ectopic pregnancy or a miscarriage in progress. If the situation isn’t urgent, your provider may track hCG levels over several days on an outpatient basis to see how the numbers trend.
Blood typing is also checked, because if you have Rh-negative blood and your baby is Rh-positive, bleeding can cause your immune system to produce antibodies against the baby’s blood cells. A simple injection prevents this.
When Bleeding Is an Emergency
Light spotting, especially in the first trimester, is often manageable with a phone call to your provider and a follow-up appointment. But certain patterns of bleeding warrant going straight to an emergency room. Heavy bleeding that soaks through a pad in under two hours, or that requires you to change pads every hour for several hours in a row, is a red flag regardless of what trimester you’re in.
Other warning signs that accompany bleeding and signal an emergency include severe abdominal or pelvic pain, dizziness or feeling faint, fever, and passing clots or tissue. Bleeding in the second or third trimester is generally treated with more urgency than early spotting, because the placental causes that are more common later in pregnancy can threaten both the baby’s and the mother’s health quickly. Any bleeding after 20 weeks that isn’t clearly a small amount of bloody show should be evaluated promptly.