Causes of Pregnancy Bleeding
Summary: Vaginal bleeding during early pregnancy or at any point needs to be reported to your health care provider.  However it s fairly common in the first trimester, occurring in 20-30% of all pregnancies.  If you are passing clots, in pain or bleeding heavily call your physician.

Bleeding from the vagina in the last six months of your pregnancy is always abnormal and a cause for concern.  If this happens after the 7th month this is a real emergency, even if there is no pain.  Hemorrhage is the main cause of maternal deaths in the U.S.  It occurs in approximately 4% of all pregnancies.

Causes of 1st trimester bleeding - anytime you experience this during the first trimester you should seek medical help and contact your doctor or health care provider.

Implantation bleeding - some women will have spotting or bleeding early in the pregnancy when the embryonic baby is implanting into the uterine wall.  This is no cause for worry.

Threatened miscarriage - can cause you to  bleed and also cause cramping.  An ultra sound confirms the fetus is still in the uterus, but your baby could still be still be at risk.  You may be ordered to bed.  However this may or may not work.  It can be caused by an infection, being dehydrated, trauma, a medication, or for no known reason.  There is absolutely no evidence that having sex, emotional upset or lifting something heavy.

A complete miscarriage - also called spontaneous abortion, is the most common cause during the first trimester.

Incomplete miscarriage - (miscarriage happening)  If your cervix is open and you are passing blood and tissue the miscarriage is not completed.

Blighted ovum - ultrasound will show pregnancy but the embryo hasn’t developed in the right place.

Intra uterine fetal death - determined by ultrasound, it can happen at any stage of the pregnancy but rarely occurs in the 2nd or 3rd trimester unless the placenta separates from the uterine wall.

Ectopic pregnancy - (called a tubal pregnancy) In this situation the fertilized egg implants in a fallopian tube.  The tube can rupture and the bleeding can be life threatening.

The risk factors for this are: a prior ectopic pregnancy, fallopian tube surgery, pelvic inflammatory disease, smoking, IUD, daily douching and history of infertility.
    
Molar pregnancy (gestational trophoblastic disease) - ultrasound may show the fetus is not a baby but abnormal tissue.  This rarely can be cancerous and can spread through the body.

Late pregnancy bleeding - is any sign of bleeding from the fourth month to ninth month.  The most common cause is a problem with the placenta.  Sometimes this can be due to  vaginal and cervical abnormalities.

Here are some conditions which cause late term bleeding.

Placenta previa - some of the blood vessels stretch and rupture and causes bleeding.  It occurs in approximately 20% of third trimester bleeding.  Here are the risk factors for the condition: multiple pregnancies, prior placenta previa, previous c-section.

Placental abruption - A normal placenta separates from the uterine wall too early.  Risk factors: more than four pregnancies, abnormal position of the baby, forceps delivery, baby’s shoulder caught on pubic bone, use of too much pitocin(strengthens labor).

Fetal vessel rupture  - (1 in every 1000 pregnancies)  The baby’s umbilical blood vessels attach to the membrane instead of the placenta and the vessels lie across the birth canal.

More specific information on the symptoms and treatment of these conditions can be found at here