About 20% of women have some bleeding during the first 12 weeks of pregnancy. Possible causes of first trimester bleeding include:

Implantation bleeding. You may experience some normal spotting within the first six to 12 days after you conceive as the fertilized egg implants itself in the lining of the uterus. Some women don't realize they are pregnant because they mistake this bleeding for a light period. Usually the bleeding is very light and lasts from a few hours to a few days.

Miscarriage. Because miscarriage is most common during the first 12 weeks of pregnancy, it tends to be one of the biggest concerns with first trimester bleeding. About half of women who bleed in pregnancy eventually miscarry, but that doesn't necessarily mean that if you're bleeding you've lost the baby, especially if you don't have any other symptoms.

Other symptoms of miscarriage are strong cramps in the lower abdomen and tissue passing through the vagina.

Ectopic pregnancy. In an ectopic pregnancy, the fertilized embryo implants outside of the uterus, usually in the fallopian tube. If the embryo keeps growing, it can cause the fallopian tube to burst, which can be life-threatening to the mother. Although ectopic pregnancy is potentially dangerous, it only occurs in about 2% of pregnancies.

Other symptoms of ectopic pregnancy are strong cramps or pain in the lower abdomen, and lightheadedness.

Molar pregnancy (also called gestational trophoblastic disease). This is a very rare condition in which abnormal tissue grows inside the uterus instead of a baby. In rare cases, the tissue is cancerous and can spread to other parts of the body.

Other symptoms of molar pregnancy are severe nausea and vomiting, and rapid enlargement of the uterus.

Additional causes of bleeding in early pregnancy include:

• Cervical changes. During pregnancy, extra blood flows to the cervix. Intercourse or a Pap test, which cause contact with the cervix, can trigger bleeding. This type of bleeding isn't cause for concern.
• Infection. Any infection of the cervix, vagina, or a sexually transmitted infection (such as chlamydia, gonorrhea, or herpes) can cause bleeding in the first trimester.

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