Placenta previa

Definition

Placenta previa is a condition that may occur during pregnancy when the placenta implants in the lower part of the uterus and obstructs the cervical opening to the vagina (birth canal).

Causes, incidence, and risk factors

Possible causes of placenta previa include a scarred endometrium (lining of the uterus), a large placenta, an abnormal uterus, or abnormal formation of the placenta. The incidence of placenta previa is approximately 1 out of 200 births.

The incidence increases with each pregnancy, and it is estimated that women who have had 6 or more previous deliveries may be at risk in as many as 1 in 20 births. The incidence of placenta previa is doubled in multiple pregnancy (more than one baby). Risk factors include multiparity (previous deliveries), multiple pregnancy, previous myomectomy (removal of uterine fibroids through an incision in the uterus), and a previous C-section (if the scar is low and close to the vaginal cervix region).

Symptoms

• Spotting during the first and second trimesters
  
• Sudden, painless, and profuse vaginal bleeding in pregnancy during the third trimester (usually after 28 weeks)
  
• Uterine cramping occuring with onset of bleeding
  

Note: Bleeding may not occur until after labor starts in some cases. Labor sometimes starts within several days after initial heavy, vaginal bleeding.

Signs and tests

The uterus is usually soft and relaxed. The infant position is oblique ( // ) or transverse ( == ) in about 15% of cases. Fetal distress is not usually present unless a cord accident occurs, or vaginal blood loss has been heavy enough to induce maternal shock or placenta abruptio.

An Abdominal ultrasound performed during the second trimester indicates low positioning of the placenta. Transvaginal or transperineal ultrasound can help physicians determine the position of a low-lying placenta.

Treatment

The course of treatment depends on the amount of abnormal uterine bleeding, whether the fetus is developed enough to survive outside the uterus, the amount of placenta over the cervix, the position of the fetus, the parity (number of previous births) for the mother, and the presence or absence of labor.

Early in pregnancy, transfusions may be given to replace maternal blood loss. Medications may be given to prevent premature labor, prolonging pregnancy to at least 36 weeks. Beyond 36 weeks, the benefits of additional infant maturity have to be weighed against the potential for major hemorrhage.