Placenta previa

Abnormal placenta locations during pregnancy can be a serious complication during pregnancy.  Placenta previa is associated with bleeding during pregnancy and can cause significant and life threatening bleeding.  There are three types of placenta previa-total, partial, or marginal.  In total placenta previa, the placenta completely covers the cervical os (opening),  whereas in partial placenta previa, only a portion of the cervix is covered.  In a marginal placenta, the placenta does not cover the cervix but is nearby the opening and may cause bleeding during contractions.
Placenta previa occurs in about 1 in 200 pregnancies and occurrs more often in women greater than 35 years, African American women, women who have been previously pregnant, women who smoke, women who live at higher altitude, or in women with a previous cesarean section. 
Approximately 5-15% of women will be diagnosed with a placenta previa at their 20 week anatomy scan.  90% of these women will have a normal placental location at term.  This is due to the lower aspect of the uterus growing during pregnancy which will then cause the edge of the placenta to move farther away from the cervix.
The clinical symptoms of placenta previa are painless bleeding.  This bleeding can range from spotting to hemorrhage requiring emergent surgery and delivery, although this is rare.  One third of women with a previa experience bleeding before 30 weeks gestation, 1/3 experience bleeding between 30-35 weeks gestation, and 1/3 experience bleeding after 35 weeks gestation.  If a women does experience bleeding, it is imperative for them to contact their physician for examination.  If a women is remote from term (37 weeks), their bleeding is stabilized and they may receive a blood transfusion.  If a women is at term, delivery is recommended.
Conditions associated with placenta previa that adversely affect the pregnancy include poor fetal growth, preterm contraction or rupture of membranes, abnormal presentation of the fetus (ie breech), and placenta accreta.  Placenta accreta is a serious complication of pregnancy in which the placenta invades the uterus.  Approximately 2/3 of women with a placenta accreta will need a hysterectomy at the time of delivery secondary to the significant bleeding associated with an accreta. 
Depending on a woman's amount of bleeding due to the previa, she may be placed on activity restrictions including no heavy lifting, no intercourse, or even modified bed rest. 
Placenta previa is a serious complication in pregnancy but most women can be safely managed by their obstetrician to result in a term or near term delivery.