Implantation bleeding is vaginal bleeding that sometimes happens when a fertilized egg implants in the lining of the uterus. If implantation bleeding is heavy, a person should contact a doctor as early as possible.

Implantation bleeding is usually light bleeding or spotting that occurs in the first 1–2 weeks after conception.

Below, we look into the causes of heavy implantation bleeding and specific signs to watch out for.

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Implantation bleeding is generally easy to distinguish from a period because it is usually very light and only lasts a day or so. The blood is often pinkish or brown.

Heavier bleeding is not typical with implantation and may indicate a problem.

Anyone who experiences heavy bleeding in the first 12 weeks, or first trimester, of pregnancy should speak with their midwife, a doctor, or another healthcare provider as soon as possible.

The American College of Obstetricians and Gynecologists (ACOG) report that 15⁠–25% of people experience some bleeding in the first trimester of pregnancy. There can be many reasons.

However, heavy bleeding during the first trimester is usually a concern, and it may stem from one of the following causes.

Subchorionic hematoma

This involves the placenta separating from part of the uterus wall, allowing blood to pool in the space between.

Subchorionic hematomas are the most common cause of vaginal bleeding in people who are 10⁠–20 weeks pregnant.

While a person may have light or heavy vaginal bleeding and cramping, some people with subchorionic hematomas have no symptoms.

Anyone who may be experiencing this should speak with a doctor.

Ectopic pregnancy

An ectopic pregnancy occurs when a fertilized egg implants outside of the uterus.

About 90% of ectopic pregnancies implant in a fallopian tube, which leads from one ovary to the uterus. When this happens, the tube can burst, leading to potentially life threatening internal bleeding.

Ectopic pregnancies are not common, representing 1.3–2.4% of all pregnancies. Symptoms include:

  • vaginal bleeding that stops and starts and may be watery and dark brown
  • stomach pain that is typically low and on one side and may be sudden or gradual
  • pain where the shoulder ends and the arm begins
  • pain when using the bathroom
  • diarrhea
  • constipation

If a fallopian tube ruptures, a person might become pale and experience:

  • sudden, sharp, and intense stomach pain
  • dizziness
  • nausea
  • fainting

Someone who experiences any of these symptoms should receive immediate medical care.

Molar pregnancy

A molar pregnancy occurs when the placenta does not form correctly and instead develops into a mass of cysts. This is due to an irregularity in the embryo’s chromosomes.

This type of pregnancy may be complete, in which case there is irregular cell growth instead of an embryo. Or it may be partial, in which case there are signs of a fetus, but it cannot develop.

Molar pregnancies are rare, representing fewer than 1% of pregnancies in the United States. If a person does not receive medical attention, a molar pregnancy can cause a rare form of cancer called choriocarcinoma.

Symptoms and signs of a molar pregnancy include:

  • vaginal bleeding
  • grape-like cysts that come from the vagina
  • intense nausea
  • vomiting
  • pelvic pain or pressure
  • anemia
  • ovarian cysts
  • high blood pressure
  • an overactive thyroid
  • a uterus that is an unusual size

A person experiencing any of the above symptoms should contact their doctor immediately.

Pregnancy loss

This term describes a pregnancy ending before 24 weeks. ACOG report that pregnancy loss occurs in around 10% of known pregnancies.

There can be many causes. Symptoms include:

  • vaginal bleeding that may be light or heavy
  • passing blood clots
  • cramping
  • stomach pain
  • lower back pressure or pain
  • a change in vaginal discharge

If a person experiences any of these symptoms, they should contact their midwife or doctor immediately.

Pregnancy loss can be physically and emotionally challenging. It can help to speak with a counselor, a mental health professional, or members of a support group. Many resources are available, such as these from the March of Dimes.

Although bleeding can be common in the first trimester of pregnancy, contact a doctor or midwife about any bleeding.

A pregnant person with any of the following symptoms should receive immediate medical care:

  • heavy bleeding
  • bleeding with pain and cramping
  • bleeding with dizziness
  • pain in the stomach or pelvis

Implantation bleeding occurs in the first two weeks of pregnancy. It is generally light and lasts a day or two. Many other factors can also lead to light bleeding in early pregnancy.

Anyone who experiences heavy vaginal bleeding during pregnancy should consult a healthcare provider.

If heavy bleeding accompanies severe or other concerning symptoms, the person should receive urgent medical care.