Prenatal Testing

Congratulations! You’re having a baby and your doctor has determined that yours is a “low-risk” pregnancy. You have good reason to believe that you will have a healthy, normal baby. Today, expectant mothers have the option of undergoing many excellent prenatal tests designed to provide them with more information about the health of their unborn babies.

Three to five of every 100 pregnant women carry a baby with a significant birth defect. This background risk applies to every pregnancy. The mother’s age, medical and family history, or the outcome of previous pregnancies may add to the background risk. As even low-risk pregnancies carry some degree of risk, women who undergo prenatal testing can increase their confidence that their babies are likely to be normal.

Genetic multiscreen
One of the most thorough and least invasive prenatal tests in use today is the genetic multiscreen, which consists of a blood test and an ultrasound examination. The blood test provides information about certain common birth defects, including spina bifida (an abnormal opening in the spine), and Down Syndrome (a birth defect associated with mental retardation).

Certain substances or “markers” are present in the blood of pregnant women. These include alpha feto-protein (AFP), human chorionic gonadotropin (hCG), and uterine estriol (UE). Too much or too little of any of these substances can be an indication of birth defects. Testing the mother’s blood in the 12th week of pregnancy confirms the levels of these substances.

An ultrasound examination, given a week after the blood test, looks for additional abnormalities. During an ultrasound, sound waves are used to create a “picture” of the fetus. The ultrasound helps to determine the position and size of the fetus, whether twins are present, and the location of the placenta.

If either the blood test or the ultrasound indicates a possible problem, your doctor might suggest further testing. For 20 years, amniocentesis has been the primary method available for assessment of the developing fetus. While blood tests can detect elevated levels of certain substances, amniocentesis actually tests for abnormalities in the fetus’s chromosomes.

During amniocentesis, the physician uses a thin needle to remove about three teaspoons of the amniotic fluid surrounding the fetus. That fluid is then tested to make sure its cells contain normal chromosomes.

Some couples have amniocentesis to test for inherited diseases such as cystic fibrosis, Tay-Sachs disease and sickle cell disease. Amniocentesis can also detect severe defects of the head and spinal column.

Amniocentesis poses a small risk to the pregnancy, but serious complications are extremely rare. The physician uses an ultrasound as a visual guide during the whole procedure to prevent the needle from touching the fetus. The most serious risk of amniocentesis is miscarriage, which happens in about one of every 200 cases.

Chorionic Villus Sampling
One alternative to amniocentesis is chorionic villus sampling (CVS). The chorionic villi are part of the tissues which surround the fetus and become the placenta. A sample of chorionic villi is taken either vaginally or through the abdomen in the same manner as amniocentesis.

This method is not for everyone, but it should be considered by anyone who wants prenatal testing results as early as possible in pregnancy. CVS can be carried out anytime from the 10th through the 12th week of pregnancy. CVS provides accurate diagnostic information, and the risk to pregnancy is about equal to the risks posed by amniocentesis. However, a mixture of the mother’s and fetus’s cells may inadvertently be withdrawn during CVS resulting in an inaccurate diagnosis.

It is important to remember that all of these tests work together to provide the most accurate diagnosis possible. A blood test that shows abnormal levels of hCG, AFP, or UE does not necessarily mean that your baby has a birth defect. The follow-up ultrasound may indicate your pregnancy is either not as far or farther along than you thought it was, which will affect the levels of these substances in your blood. If the cause of an abnormal blood test is still unclear, an amniocentesis can confirm or exclude certain birth defects. However, many birth defects cannot be seen until after the baby is born.

Unpredictable and often undetected, most birth defects come without warning to new parents. Abnormalities often occur in the very early weeks of the baby’s development, and we have, as yet, no way to prevent them. Prenatal testing gives us a way to diagnose many abnormalities early in pregnancy, giving a woman and her obstetrician different options for managing the pregnancy. It also allows parents and physicians to prepare for the birth of a baby with special needs.

Editorial provided by John Hobbins, MD, Chief of Obstetrics at University Hospital in Denver, Colorado, and Director of the Colorado High-Risk Maternity and Newborn Program.