The placenta -- sometimes called the afterbirth -- is the organ that is vital to a baby's growth and nutrition. The placenta implants in the wall of the uterus. The developing child is connected to the placenta through the umbilical cord. Fetal blood flows through the placenta. It can exchange nutrients -- oxygen, sugar, amino acids -- with maternal blood. It removes waste from the fetus -- carbon dioxide, for example -— in a similar fashion. Lots of blood flows through the placenta for both of these actions.

In rare cases, the placenta implants in a location that leads to some or all of the cervix being covered by placental tissue. This is known as a placenta previa. The cervix is the opening at the bottom of the uterus. It dilates during labor to allow the baby to exit the womb and pass through the vagina to be born. As this potential space opens it exposes underlying tissue. In most cases, this tissue is simply the amniotic sac (bag of water). However, in cases of placenta previa, the underlying tissue is the placenta and the fetal blood flowing through it. This creates the potential for bleeding.

Bleeding is the principle symptom of a placenta previa. Doctors are taught that painless bleeding in pregnancy should raise concern for a previa. Pressure, pain, tenderness and tightness are not usually symptoms I associate with a placenta previa, unless a woman is having contractions. Contractions may cause bleeding from a previa, or bleeding from a previa may cause painful contractions.

Painful bleeding is more consistent with placental abruption, a separation of the placenta from the wall of the uterus.

Doctors vary in what they recommend for women with a placenta previa who have not had any bleeding. Many recommend that nothing be placed in the vagina (so called "pelvic rest"): no sex, no tampons, no douching. Some may have other recommendations about work or exercise.

What is most important is that women with a placenta previa call their doctor at once if bleeding occurs and go promptly for evaluation. Bleeding can be heavy and, on occasion, place a mother's or baby's health at risk. Such risks may lead doctors to recommend an early delivery. Because a placenta previa covers the birth canal, Caesarean delivery is almost always recommended.

If bleeding occurs before term but does not warrant an early delivery, patients may be placed on limited activity or bedrest. Often there will be an initial period of observation in the hospital to assure the patient and doctor that a first bleed will not be followed by more trouble.