Pregnancy can be an exciting time for expectant parents; it can also be a time of concern over the health of the baby. Medical technology has led to the development of many prenatal screening tools, which are used to identify mothers and babies who are at an increased risk for developing a disease or condition.

Prenatal screening results do not give a “yes” or “no” answer about the presence of a disease or whether it will develop, only if it is more or less likely. Most women who test positive for a screening don’t actually have the condition; it’s just a way to rule out people who are at low risk and identify those at high risk.

Birth Defects

Birth defects are physical abnormalities that may be caused by inherited genes, genetic mutations, or environmental factors. They cause deformities or disabilities that can’t always be treated or cured. Approximately 250 birth defects can be diagnosed in a fetus (unborn baby); those most often identified by screenings are chromosome abnormalities like Down's syndrome and neural tube defects (NTDs) like anencephaly and spina bifida .

Routine Screenings to Monitor Health of the Mother

A woman’s health directly impacts the health of her fetus. At the first prenatal care appointment, the doctor will review the mother’s family and personal health history and offer routine screenings so issues that could threaten the fetus can be controlled or anticipated. A woman’s participation in any screening is completely voluntary.

Routine screenings include blood pressure reading; urine tests (to detect urinary tract infection, protein in the urine, and signs of diabetes); microbiological examinations for infections such as gonorrhea , chlamydia , and Group B Streptococci; and blood tests. Blood tests include tests for blood type and Rh factor; sexually transmitted diseases, hepatitis, and HIV/AIDS ; antibodies for Rubella (German measles); anemia ; type 1 diabetes ; and whether the mother has, or is a carrier of, hereditary diseases like cystic fibrosis , sickle cell disease , Tay-Sachs disease , and beta thalassemia.

Screenings to Monitor Health of the Fetus

Screenings also enable doctors to monitor the health of a fetus during its development. In many cases, when screenings produce abnormal results, subsequent testing will be recommended; usually a combination of two or more of these screenings will produce the most reliable results. Some screenings are routine, while others are prompted by certain criteria, such as a mother’s age or abnormal results of a routine screening.

Maternal Serum Alpha-Fetoprotein (MSAFP)

MSAFP, known as Enhanced AFP, AFP3, or triple screen, is a blood test designed to identify NTDs. It detects Alpha-fetoprotein (AFP) and also measures the levels of two pregnancy hormones―estriol and human chorionic gonadotropin (HCG).

Elevated levels of AFP can indicate NTDs, as well as the presence of twins or a miscalculated date of conception; low levels indicate possible Down's syndrome. This routine screening has most commonly been done between the 16th and 18th week of pregnancy, and detects about 70% of fetuses with Down's syndrome, and 85% with NTDs. Unfortunately, the test is positive in more than 5% of pregnancies, most of which are fully normal. These “false positive” tests must be followed up with further studies, adding expense and worry.

Some doctors have started to use the “quadruple screen,” adding a fourth test (“inhibin-A”) in an effort to increase the accuracy of screening. While the quadruple screen may be better than the triple during the second trimester (after 13 weeks from the last menstrual period), recent recommendations endorse triple screening during the first trimester as long as necessary follow-up testing is readily available and can be accurately interpreted by the doctors doing the tests.

Amniocentesis

Available since the 1970s, amniocentesis involves obtaining a sample of amniotic fluid by inserting a thin needle into the uterus. Chromosomes in the fluid are evaluated for Down's syndrome, genetic birth defects, and other abnormalities—and if desired, the baby’s sex.

Amniocentesis is usually performed between the 15th and 18th week; if performed earlier, it is associated with an increased risk of miscarriage and foot deformities. It is offered to women who will be age 35 or older at delivery, have family history of genetic abnormalities, or had an abnormal MSAFP.

Chorionic Villi Screening (CVS)

CVS involves removing a sample of the placenta to detect chromosomal abnormalities such as Down's syndrome. The test provides more information than amniocentesis, and can be done earlier in the pregnancy, at 10-12 weeks, which provides additional time for counseling and decision-making. It has a slightly higher risk of damage to the fetus, and because it does not test amniotic fluid, it should be combined with the MSAFP to screen for NTDs.

Ultrasound

This popular diagnostic test utilizes sound waves to produce a picture of the baby on a computer screen. It can determine the age of a fetus with a high degree of accuracy, and is commonly recommended to identify twins, evaluate fetal structure, study breathing, and measure amount of amniotic fluid. It is usually performed between the 16th and 18th weeks, and is often used to confirm results of other screens.

Non-stress Test (NST)/Biophysical Profile (BPP)

These tests are sometimes performed with high-risk pregnancies late in the third trimester, to monitor the baby’s heartbeat and observe fetal activity, muscle tone, and breathing, as well as amniotic fluid volume.

Nuchal Translucency (NT)

This relatively new screening assesses a baby’s risk for Down's syndrome in the first trimester. It uses ultrasound to measure the space in the tissue at the back of the fetus’ neck, where fluid tends to accumulate if Down's syndrome is present. This painless test confers no risk to the pregnancy, and while it cannot produce a definite diagnosis like the more invasive tests, such as CVS and amniocentesis would give, it can indicate the need for further testing.

Percutaneous Umbilical Blood Sampling

This is the most accurate way to diagnose Down's syndrome, but it carries the highest risk of miscarriage. It is used to confirm positive results of a CVS or amniocentesis and cannot be performed until the 18th to 22nd week of pregnancy.

Psychological Aspects of Screening

Screenings can be stressful for the mother and father, especially when results are abnormal and they may now be faced with making difficult decisions given the screening information. While the screenings may help prepare parents for unexpected outcomes of the pregnancy, there is often no treatment that can be given while the fetus is still in utero.

Nonetheless most people who test positive for a screening will test negative in follow-up diagnostic tests.

Genetic counseling, an important component of prenatal screenings, can assist parents in understanding what the results mean, evaluating treatment options, and considering the possible need for further testing.