Placenta Previa


Placenta previa is one of the most common causes of bleeding in pregnancy. This is when the placenta is positioned in a way that blocks the cervix partially or completely. The bleeding happens since the placenta is such a vascular organ. If it thins out, it slides off the base of attachment. It can separate.

Previa usually happens in the second trimester since the cervix is normally thick in the first trimester.

First trimester previa:

It is very common for the placenta to attach to the area on or near the cervix. There isn’t much room to fit the growing baby and the placenta because the uterus isn’t big quite yet in the first 12 weeks. As the uterus grows, the placenta usually migrates away. This is why it isn’t so common in the first trimester.

Second trimester previa:

Placenta previa is serious. The placenta is the vascular part of pregnancy. It is adhered to the inside of the uterus. It allows oxygen and nutrients to flow from the maternal side to the fetal side, into the umbilical arteries, to the baby. It is extremely important for this union of the placenta and uterine wall not to separate before delivery of the baby, as it would weaken the supply of blood to the baby and create problems for mother with bleeding during pregnancy. This separation is called placenta Abruption.

If the placenta is positioned low in the uterus, this poses some problems.

– The cervix is blocked. Labor and delivery will be catastrophic if the baby has no way out. When cervix is completely blocked this is called, “total previa”. “Partial previa” is when the area is partially blocked.
– A low attachment would mean it is attached on much thinner tissue of the uterus. Muscular layers are located higher up. This low lying placenta can result in the lower uterine lining not having enough muscle to contract and pinch off the bleeding openings from the maternal side, after delivery. An emergency hysterectomy may be needed if hemorrhaging is excessive. It also can be life threatening.

Why is Placenta Previa common?
Since there is limited surface area inside the uterus, partial previa can often take place. A low lying placenta is even more common. The upper part of the uterus will enlarge faster than the lower segment of the uterus when it grows. This will then have the placenta that lies over both areas grow away from the cervical os. This is what we call “placental migration”. The placenta actually is not moving. The tissue that it is embedded on is what is expanding.

Ideally, by the time the third trimester rolls around, there’s a good chance that the placenta will be out of the way, resolving the low lying placenta issue. Vaginal delivery can still take place if the low lying placenta is at the edge of the cervix. A C-section is required if total previa is present.

What is a major risk of placenta previa?
Abruption which is the separation of the placenta from the uterine wall before delivery is a major risk.

What is the treatment for placenta previa?
Depending on how far along you are in your pregnancy, as well as how much bleeding you are experiencing determines the treatment. Bed rest and reduced activities in your day may be recommended if you are diagnosed after the 20th week of pregnancy and no bleeding. Any bleeding during pregnancy should be reason for bed rest and advised to refrain from sexual activity. You should be closely monitored by your doctor.

Most complete previas will resolve by term. If you are bleeding, your doctor may admit you into the hospital until you and your baby are both stable. Even then, bed rest may still be in your requirements until your baby is delivered.

What are possible complications?
Baby Complications:
– secondary to acute blood loss
– intrauterine growth retardation from poor placental perfusion
– increased incidence of congenital anomalies

Mother Complications:
– Hemorrhage can be life threatening
– Cesarean delivery (c-section)
– Higher risk of postpartum hemorrhaging
– Higher risk of placenta accrete (When placenta attaches directly to uterine muscle)


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