Placenta previa is a rare condition in which the placenta grows in the lower part of a pregnant woman's uterus and covers all or part of the cervix.

The placenta is the organ that nourishes the developing fetus.

Possible causes of placenta previa include:

• Abnormal formation of the placenta
• Abnormal uterus
• Large placenta
• Scarred lining of the uterus (endometrium)

Placenta previa is very rare. The risk increases with each pregnancy, especially in women who have had six or more deliveries. The rate of placenta previa is also higher in women who are carrying more than one baby (multiple pregnancy).

Risk factors include:

• Multiple pregnancy
• Previous deliveries (multiparity)
• Previous C-section (if the scar is low and close to the vaginal cervix region)
• Previous removal of uterine fibroids through a surgical cut in the uterus (myomectomy)
• Smoking

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

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

The uterus is usually soft and relaxed. In a small number of cases, the infant is in an unusual position. Fetal distress doesn't usually occur unless there is a cord accident, or vaginal blood loss has been heavy enough to cause placenta abruptio or shock in the mother.

An abdominal ultrasound during the second trimester can show whether the placenta is low. Transvaginal or transperineal ultrasound can help the doctor see the position of a low-lying placenta.