How Prenatal Testing Works


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 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.

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.