An amniocentesis screening test is more than 99% accurate in diagnosing genetic disorders such as Down syndrome and is usually performed at 15-20 weeks of pregnancy (but sometimes the screening will be performed as early as the 11th week or as late as the 39th week). Results are usually available within 10 days.
Here are some reasons why a physician will request an amniocentesis screening:
- the mother is older (usually over age 35)
- abnormal results in another screening test; an amniocentesis is used to double check the results of the earlier screening.
- to determine the baby’s gender
- to determine the baby’s paternity (DNA obtained is compared with DNA collected from the possible father)
- a prior pregnancy resulted in a child with a chromosomal abnormality (down syndrome), enzyme deficiency (cystic fibrosis), or a metabolic disorder.
- the parents have genes for an inherited disorder (for instance, Tay-Sachs, muscular dystrophy, cystic fibrosis, or sickle-cell anemia, or if the mother is the carrier for an X-linked genetic disorder such as hemophilia)
- the baby has a suspected infection (for example, toxoplasmosis)
- there may be a reason to deliver the baby before the due date
Here is what happens during the procedure:
- The patient lies down on the exam table.
- The doctor uses an ultrasound to determine where the baby and placenta are in order to avoid them.
- The doctor guides a thin needle through the abdomen and uterus and then withdraws a small sample (2 to 4 teaspoons) of amniotic fluid.
- Total time: 30 minutes
- Some patients say it doesn’t hurt at all; others say they feel pressure or cramping.
The amniotic fluid in the sac surrounding your baby contains cells that are shed from the baby. At the lab, a technician will use a microscope to look for defects in the baby’s cells’ chromosomes. For instance, an extra chromosome might indicate Down syndrome or trisomy 18. High levels of alpha-feto protein (AFP) in the amniotic fluid may indicate a neural tube defect such as anencephaly (absence of a large part of the brain or skull) or “open” spina bifida (when the spinal cord sticks out through an opening in the spine).
Complications from amniocentesis are rare (1-2 percent of cases), but may include the following symptoms (Be sure to report any of these symptoms IMMEDIATELY to your physician):
- leaking of amniotic fluid
- vaginal bleeding (2-3 percent of cases)
- cramping
- a very slight chance of injury to the baby or miscarriage (the latter is more common the earlier on in the pregnancy the amniocentesis is performed)