Tests Done Under Special Circumstances

If problems with the baby's development are detected by other means or the parents have risk factors for various genetic diseases, special tests may be required. For example, if the mother is age 34 or older, she has an increased risk of having a baby with Down syndrome. So, the parents, in consultation with the obstetrician, may elect to sample fluid or tissue from the baby to determine its genetic make-up. These samples can be obtained through three procedures: In these tests, the doctor uses a needle or suction tube to sample fluids or the baby's tissue with the aid of ultrasound to see what he/she is doing. The tissue/fluid samples are analyzed in a laboratory to determine the number of chromosomes (karyotype) and other biochemical tests (e.g., AFP). The results from the tests can be used to make decisions about possibly terminating the pregnancy or preparing the parents to deal with any genetic defects (genetic counseling).

Amniocentesis
Amniocentesis is a test in which the doctor samples the fluid (amniotic fluid) that surrounds the growing baby in the uterus. Floating in the amniotic fluid, there are cells from the baby as well as fluids from the baby (urine). The cells can be grown and the fluid analyzed for various biochemical markers.

Amniocentesis is usually done between 15 to 18 weeks and for mothers with two or more risk factors (e.g., older than age 34, and/or family history of Down syndrome or other genetic diseases). If you were to have this test done, you would go into an examination room with your doctor, an ultrasound technician, and your partner or other family member. The procedure goes like this:

  1. The technician swabs your abdomen with an antiseptic (Betadine).
  2. The technician will use ultrasound to determine where the baby is in your uterus and where there are pockets of fluid away from the baby.
  3. Once these areas are located, your doctor will insert a needle through your abdomen and into the uterus. The doctor constantly monitors the ultrasound to avoid poking the baby.
  4. The doctor withdraws about 30 to 60 ml of amniotic fluid, which the baby will replace within a day.
  5. The doctor places this fluid in one or more sterile cups, marks the cups, and sends them off to a laboratory.
  6. The doctor removes the needle and places a bandage over the site.
  7. You may have another ultrasound exam to assess the baby's health after the procedure.
It takes about two to three weeks before the results of the amniocentesis can be reported.


Diagram showing the sampling and analysis procedures of amniocentesis

Amniocentesis has a quoted risk of 0.5 percent, meaning that 1 in 200 procedures have some type of complication (e.g., infection, miscarriage or poking the baby with the needle). In most cases, these percentages are much lower, especially since ultrasound has been used to guide the doctor.

Chroionic Villus Sampling
Like amniocentesis, chorionic villus sampling (CVS) is done to obtain a sample of the baby's tissue for determining genetic disorders. In contrast, CVS obtains tissue from the placenta (chorion) instead of fluid. Because the chorion is derived from the baby, not the mother, it bears the genetic characteristics of the baby. The procedure is similar to amniocentesis except that the tissue can be taken by inserting a needle through the abdomen or a sample tube through the cervix.


Diagram showing the sampling and analysis procedures of chorionic villus sampling

Like amniocentesis, CVS is done when the mother has one or more risk factors for genetic diseases. CVS can be done earlier than amniocentesis, usually in the latter part of the first trimester ( between weeks nine and 11). The results of CVS can be obtained sooner than amniocentesis because no culturing of the tissue is required. However, because CVS only gets tissue, some of the biochemical tests done with amniocentesis cannot be done with CVS. Also, CVS carries a higher risk (about 1 percent), mostly because it is a newer procedure relative to amniocentesis.

Fetal Blood Sampling
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
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.