The placenta is a bed of cells formed inside the uterus (womb) during pregnancy. The purpose of the placenta is to carry nourishment and oxygen from the mother to the fetus and to carry waste products and carbon dioxide from the fetus to the mother through the umbilical cord.

The placenta is usually formed along the upper part of the uterus, allowing enough space for the fetus to grow. In placenta previa, the placenta starts forming very low in the uterus or even over the cervix (the opening of the uterus that leads to the vagina). This obstruction impairs normal vaginal delivery of the baby at birth.

There are 3 types of placenta previa:

• complete placenta previa: The internal cervical opening is completely covered by the placenta.
• partial placenta previa: The internal cervical opening is partially covered by the placenta.
• marginal placenta previa: The placenta is at the edge of the internal cervical opening.

A “low-lying” placenta does not cover or touch the cervix, but is within 2 centimetres of the opening. Placenta previa is estimated to occur in 1 in 250 pregnancies.

The causes and risk factors for placenta previa are:

• The fertilized egg implanted very low in the uterus, causing the placenta to form close to or over the cervical opening.
• The lining of the uterus (endometrium) has abnormalities such as fibroids or scarring (from previous previa, incisions, caesarean sections or abortions).
• The placenta formed abnormally.
• The pregnancy is multiple (i.e., twins or triplets). The chances of developing placenta previa are doubled for these pregnancies.
• The mother may have had several previous pregnancies. The chances of developing placenta previa are increased to 1 in 20 for women who have had 6 or more pregnancies.
• The mother smokes or uses cocaine. Smoking and cocaine use can increase the risk of this condition.
• The mother is older. The risk of developing placenta previa is 3 times as high in women over 30 years of age as in women under 20 years of age.
• The pregnancy has been conceived with the help of assisted reproductive technology, such as in-vitro fertilization.

Slight and occasional bleeding may occur during the first and second trimester of pregnancy. The colour of the blood may be bright red and it may start and stop, then restart after several days or weeks.

Sudden and excessive bleeding may occur in the third trimester of pregnancy.

Bleeding is usually not accompanied by pain, although uterine cramping may occur at the time of bleeding in some women. In 7% to 30% of women there may be no bleeding at all.

The bleeding occurs because as the pregnancy progresses, the placenta gets detached from the uterine walls. In the third trimester the uterine walls become thinner and spread to accommodate the growing fetus. If placenta previa is present, the placenta is attached very low on the uterine wall. This thinning makes the placenta stretch and tear away from the uterine wall, leading to bleeding.

The following are the possible complications of placenta previa:

• slow fetal growth due to insufficient blood supply
• fetal anemia
• fetal distress due to low oxygen supply
• shock and death of the mother if the bleeding is excessive
• infection and formation of blood clots
• blood loss requiring transfusion

If you are diagnosed with complete placenta previa, your doctor may advise you to have a caesarean section for the delivery of your baby, usually between weeks 36 and 37 of pregnancy.

The condition may be diagnosed by an ultrasound showing the position of the placenta. If it is detected late in the first or early in the second trimester, often the placenta's location will shift as the uterus enlarges. This can be followed by serial ultrasound. Some women may even remain undiagnosed until delivery, especially in cases of marginal placenta previa. Painless bleeding from the vagina during the third trimester of pregnancy may also indicate the condition.

In many cases, a diagnosis of placenta previa in the early weeks (usually before Week 20) may correct itself as the pregnancy progresses. In other cases, placenta previa may be missed if the placenta is located behind the baby's head.

If you have been diagnosed with placenta previa, your doctor will avoid digital (which means using the fingers) examination of the vagina and the cervix, as this could cause the placenta to tear and lead to heavy bleeding.

In addition to painless vaginal bleeding, other signs that your doctor may look for to confirm a diagnosis of placenta previa include:

• abdominal ultrasound during the second trimester showing the low placement of the placenta
• low-lying placenta revealed by transvaginal or transperineal ultrasound