First and foremost, bleeding during pregnancy can be dangerous.   It is not always, but you should at the very least call your physician if you have any signs of vaginal bleeding during pregnancy.

It is common to have some vaginal bleeding in the first trimester.  This can be anything from light spotting to sever bleeding with clots.  Between 20- and 30- percent of all pregnant women experience first-trimester bleeding.  Up to half of those who experience this may go on to have a miscarriage.  First trimester bleeding may be implantation bleeding (a small amount of spotting caused by the embryo implanting into the uterine wall).  Early bleeding may be a sign of an ectopic pregnancy.  An ectopic pregnancy is one in which the fetus is not inside the uterus.  An ectopic pregnancy may threaten the life of the mother.  First trimester bleeding can be a threatened, completed or incompleted miscarriage.  Postcoital bleeding may be common during pregnancy as well.  It can be caused by a blighted ovum.  A molar pregnancy (in which the fetus is not actually a baby but an abnormal tissue) can cause bleeding. 

Second- and third-trimester bleeding is considered abnormal.  This late-pregnancy bleeding is often caused by a problem with the placenta.  A “placental abruption” may occur when a normal placenta separates from the uterine wall prematurely and blood pools between the placenta and the uterus.  A “placentia previa” may occur when the placenta completely covers the opening of your womb (the cervix) and some of the blood vessels of the placenta stretch and rupture.  A uterine rupture is a splitting open of the uterus and may also cause late-pregnancy bleeding.  In rare cases late-pregnancy bleeding may be caused by a fetal vessel rupture, in which the vessels from the umbilical cord attach to the membranes instead of the placenta.  Other even more rare causes can be injuries or lesions of the cervix and vagina, including varicose veins, polyps and  cancer.

If you experience bleeding after the 28th week of pregnancy, you probably have a true emergency and should probably proceed to contact your doctor or go to the emergency room immediately.  This bleeding can be very mild to extremely brisk, and may or may not have accompanying abdominal pain.  This sort of bleeding complicates roughly 4 percent of pregnancies and is the most common cause of death of the mother in the U.S.

Bleeding During Pregnancy – Causes and Seeking a Doctor’s Opinion