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How Prenatal Testing Works

Fetal Monitoring

Fetal blood sampling can be done from 18 weeks to full term. In this procedure, fetal blood is obtained from the umbilical cord for analysis. Like amniocentesis, the doctor (aided by ultrasound) inserts a needle through the mother's abdomen into the umbilical cord and withdraws a blood sample. The blood sample is sent to a lab for analysis. The genetic results from fetal blood sampling can be obtained much faster than with amniocentesis because no tissue culturing is involved. In addition, this technique can be used to transfuse the fetus with compatible blood in the event that the baby's and mother's Rh factors do not match. The overall risk of the fetal blood sampling procedure is 0.5 - 1 percent.

Fetal monitoring is usually done in the third trimester (for high-risk pregnancies, pre-term labor or overdue babies) and during delivery. This test involves strapping an electronic monitor to the mother's abdomen that measures the electrical activities of the following:

  • fetal heart beat
  • mother's heart beat
  • mother's uterine muscle contractions

The baby's movements inside the uterus can be assessed and correlated with its heart rate. There are two types of test done:

  • Non-stress test - the baby's heart rate should increase when the baby moves (about 15 beats for 15 seconds at least twice in a 20 minute period).
  • Stress test - the baby's heart rate should increase when the uterus contracts. Uterine contractions are caused by infusing a medication (Pitocin) or by stimulating the mother's nipples.

These tests are used by obstetricians to determine how well the baby will handle the stress of delivery.

As you can see, there are a wide variety of ways to measure your baby's growth and development before it is born. Prenatal testing is completely normal during pregnancy and can often detect problems early, before they progress.