TESTS OF FETAL WELL-BEING
Since the development of electronic fetal monitoring and the growth of the field of genetics
in the late 1960s, prenatal tests of fetal well-being have evolved. In addition, the value of some basic techniques, such as the mother counting fetal movements, are being used more often.
Amniocentesis During an amniocentesis, a sample of the amniotic fluid surrounding the fetus is obtained. This fluid contains cells from the fetus that can be cultured, or grown, to determine chromosomal makeup, fetal lung maturity, and other information about the fetus. Amniocentesis carries a risk of fetal death of less than 1 percent, particularly when performed by skilled practitioners.
Chorionic Villus Sampling (CVS)
In this procedure, a small amount of tissue is removed from the fetus at 9-10 weeks gestation. The genetic material of the tissue sample is tested for chromosomal abnormalities. The advantage of this technique is that it can be performed very early in pregnancy, and the results are generally available before the end of the first trimester. The risks of fetal death as a result of the procedure are about the same as those of amniocentesis, or about 1 percent.
Percutaneous Umbilical Blood Sampling
During this procedure, also known as cordocentesis, blood is obtained directly from the umbilical cord using
ultrasound guidance. Chromosomal and biochemistry tests can be performed on this pure fetal blood sample. These results can be available within minutes to hours. This relatively new procedure carries a higher risk of fetal loss than those above, somewhere in the range of two to five percent.
Fetal Monitoring Techniques
Electronic fetal monitoring has become commonplace in the evaluation of fetal well-being. Tracings of the fetal heart rate and uterine activity are used to evaluate the response to contractions, fetal movement, or external stimuli. These tests are begun after 28 weeks gestation, the time in pregnancy when a fetus has a reasonable chance of living outside the uterine environment, if that environment is determined to be potentially dangerous. The nonstress test, which evaluates the fetal heart-rate response to fetal movement, is the most common method of antepartum (prebirth) screening for well-being. If the fetus is in some jeopardy, there will be no characteristic accelerations of the fetal heart rate accompanying fetal movement. Another common antepartum screening test is the contraction stress test. Contractions of the uterus are stimulated and the fetal heart-rate response is monitored. Certain types of decelerations in the heart rate during the contraction indicate that the placenta does not have an adequate reserve of oxygen to supply the fetus.
Ultrasound
Ultrasound has become an integral part of the evaluation and monitoring of pregnancy. Sound waves are passed over the maternal abdomen, and images of the fetus and surrounding tissues are observed on a viewing screen. Ultrasound monitors the growth of the fetal head, limbs, kidneys, liver, lungs, and brain. The technique can also be used to evaluate fetal movement, breathing, the amount of amniotic fluid, and the condition of the placenta. Information gained from the use of ultrasound is useful throughout pregnancy to determine gestational age, the presence of more than one fetus, any fetal malformations and defects, and the position of the fetus and placenta.