Low-lying placenta (Placenta previa)

• A low-lying placenta happens during pregnancy, when the placenta develops low down in the uterus.
• Most cases of low-lying placenta are found during routine ultrasounds that are performed during the second trimester of pregnancy.
• Most women who are diagnosed with a low-lying placenta early in pregnancy have no problems later in the pregnancy, as the placenta moves up higher into the uterus and away from the cervix.
• For some women, the placenta remains low-lying during the weeks leading to labor. In these cases, the mother and baby will be more closely monitored; a caesarean delivery may be planned to prevent the danger of complications to the mother and baby.

What is a low-lying placenta?

The placenta is the organ that develops on the lining of the uterus (womb) when a woman becomes pregnant. The placenta is attached to the fetus by the umbilical cord.

The placenta serves many important functions during pregnancy, including:

• It produces hormones that help and direct fetal growth and development;
• It protects the fetus against bacterial infections, and;
• It is where the mother's blood supply and the fetal blood supply meet - oxygen and nutrients pass from the mother to the fetus, and waste products pass the other way.

The placenta attaches to the site where the fertilized egg implants into the lining of the uterus. In most cases, the placenta attaches towards the upper part of the uterus, away from the cervix, which is the opening through which the baby passes during delivery. In some pregnancies, the placenta instead attaches to a lower part of the uterus, but as the uterus continues to grow and expand, the placenta often moves higher into the uterus.

If the placenta is still lying low in the uterus by the 20^th week of pregnancy, it is called a low-lying placenta or placenta praevia. There are a few types of low-lying placenta; some types may partially or even completely block the cervical opening.

Signs and symptoms

Many women may not experience any symptoms with a low-lying placenta. For women who do experience symptoms, these may include:

Vaginal bleeding

Vaginal bleeding may be experienced during the second half of pregnancy. The bleeding is usually painless and may stop without treatment, only to return days or weeks later. The bleeding can range from light to heavy.

Blood loss

Some women may experience severe enough blood loss that it causes anemia. Symptoms of anemia include pale skin, a fast, weak pulse and breathlessness.

Symptoms associated with early labor

Some women may experience symptoms often associated with labor, such as regular contractions and aches or pains in the lower abdomen.

Causes and Risk factors

It is not clear why some women develop a low-lying placenta, but there are factors that are known to increase the risk. These include:

• Smoking;
• Being over the age of 35;
• A previous pregnancy;
• Being pregnant with more than one baby i.e., twins/triplets;
• A previous caesarean section;
• Have previously had low-lying placenta, and;
• Cocaine use.

Types

There are three main types of low-lying placentas. They are classified by their position in the uterus:

• Low-lying placenta praevia - the placenta is located close to the cervical opening without covering it;
• Partial or marginal placenta praevia - the placenta partially blocks the cervical opening, and;
• Complete placenta praevia - the placenta covers and blocks the cervical opening.

The various positions of low-lying placenta. 

Methods for diagnosis

Ultrasound

Most cases of low-lying placenta are found during routine ultrasounds that are performed during the second trimester of pregnancy.

Some cases of low-lying placenta may require more scans, to more accurately confirm the position of the placenta. A popular scan for diagnosing a low-lying placenta is a transvaginal ultrasound, in which your doctor inserts a wand-like ultrasound device into the vagina to better see the placenta.

Physical examination

Your doctor may also perform a physical examination in order to determine the position of the baby within the uterus, because in some cases of placenta praevia, the baby may be positioned sideways or bottom-first. The position of the baby may affect whether a vaginal birth is possible.

Types of treatment

The type of treatment required for a low-lying placenta may vary depending on the:

• Amount of bleeding;
• Stage of pregnancy;
• Health of the mother and baby, and;
• Position of the baby and placenta.

Further ultrasound scans are performed later in pregnancy to confirm the position of the placenta. In cases where the placenta is blocking the cervix, vaginal delivery will not be possible and a caesarean section will need to be scheduled.

• Women who experience mild vaginal bleeding may not require hospitalization and may be advised to get plenty of rest at home. You may be advised by your doctor to avoid sexual activity or heavy strenuous activity.
• If the bleeding is more serious, the baby and mother may be monitored, either during a hospital stay or at home. In these cases, a vaginal delivery may still be possible, although some women may need to undergo a caesarean section.
• Women with very heavy bleeding may need to remain in hospital, with more severe cases requiring blood transfusion and medication to prevent preterm (before 37 weeks) labor. Women who experience more severe symptoms and whose cervix is blocked usually undergo caesarean delivery.

Potential complications

There may be some complications associated with having a low-lying placenta. The complications can affect either the mother or her baby, and may vary depending on the:

• Position of the placenta;
• Stage of the pregnancy;
• Health of the mother and baby;
• Amount of vaginal bleeding, and;
• Position of the baby within the uterus.

Potential complications for the mother

Hysterectomy

Women with low-lying placenta who have previously undergone a caesarean section may have scar tissue on their uterine walls that puts them at an increased risk of developing:

• Placenta accrete - in which the placenta remains firmly attached to the uterus;
• Placenta increta - in which the placenta has grown into the uterus, or;
• Placenta percreta - in which the placenta has grown through the uterus.

Such conditions may be confirmed by using computerized tomography (CT) or magnetic resonance imaging (MRI) scans. The treatment usually involves a hysterectomy after the baby has been delivered, as the placenta cannot be separated from the uterus.

Heavy bleeding

Some women with low-lying placenta may experience particularly heavy bleeding during labor, or after the baby has been delivered. If blood loss is severe enough, a blood transfusion may be required.

Placental abruption

Women with low-lying placenta have an increased risk of developing a placental abruption, in which the placenta separates partially or even fully from the wall of the uterus. Depending on the degree of abruption, some women may experience severe blood loss and require a blood transfusion. Severe cases can cause shock, affect other organs of the body and increase the risk of death.

Postpartum endometritis

Some women may be at an increased risk of developing an infection of the inner lining of their uterus, called postpartum endometritis.

Potential complications for the baby

Depending on the health of the baby and the degree of distress, blood loss and lack of oxygen that they may have experienced before, during and after labor, some complications may include:

• Congenital malformation;
• Blood loss;
• Fetal anemia;
• Low birth weight;
• Jaundice;
• A longer hospital stay;
• Increased risk for sudden infant death syndrome (SIDS), and;
• Death.

Prognosis

Most women who are diagnosed with a low-lying placenta early in pregnancy have no problems later in the pregnancy, as the placenta moves up higher into the uterus and away from the cervix. Some women whose low-lying placenta persists into their third trimester will go on to a preterm delivery.

Prevention

Although the reasons why some women develop a low-lying placenta during their pregnancy are not clearly understood, getting good prenatal care may help identify complications early on. In addition, the chances of developing a low-lying placenta may be reduced by avoiding the known risk factors.