A: The placenta is the vascular structure that nourishes your baby throughout pregnancy. Oxygen, nutrients and infection-fighting substances travel to your baby through the umbilical cord. In turn, the placenta removes waste products from the baby’s blood.

An ultrasound exam is used to visualize the location of the placenta. The placenta most commonly attaches to the upper region of the uterus, an area away from the cervix. This is called the upper segment or fundus of the uterus. If the placenta lies close to the cervix, it is called low lying.

It’s not uncommon for the placenta to be lying low at the time of an 18- and 20-week ultrasound. If part of the placenta covers the cervix, it is called placenta previa. This is different from a low-lying placenta. When placenta previa exists, the cervical opening is blocked, and a vaginal delivery is not possible. In this situation, a Caesarean section is the safest means of delivery.

A low-lying placenta is normal during the second trimester. The lower segment of the uterus does not fully develop until 36 weeks of pregnancy. As the uterus grows, the lower segment becomes thinner. A low-lying placenta often shifts more to the upper segment of the uterus, further away from the cervix.

If your placenta appeared to be very close to your cervix on an earlier ultrasound, a second ultrasound may be requested to check the location of the placenta towards the end of your pregnancy. If the placenta is still “low” but does not cover the cervix, it is more likely you will be allowed to go into labor naturally, and your chances of having a C-section will not be based on the location of the placenta.

In regards to your other concern, a low-lying placenta does not put you at risk for preterm labor.