Prenatal testing can reassure expectant parents, while ruling out possible concerns.
Throughout your pregnancy, your healthcare provider will recommend one or more screening tests to assess fetal development and help improve the fetal outcome. A woman’s chance of delivering a healthy baby is about 97 percent. Screening tests help evaluate some of the inherited and environmental risks that occur the other 3 percent of the time. Some conditions are minor and treatable; others are of more concern.
To guarantee a good start for your baby, you might be offered a blood test and an ultrasound to aid in screening, and chorionic villus sampling (CVS) or amniocentesis to make or confirm a diagnosis.
A routine blood test (AFP/Quad Screen) administered between 15 and 22 weeks of pregnancy measures the amount of alpha-fetoprotein (AFP) and up to three other markers in the mother’s bloodstream. If results show an increased risk for a fetal abnormality, further specific diagnostic testing is ordered.
Ultrasound has been used in obstetrics for at least 30 years and is safe for the developing fetus. Typically, at 18 to 20 weeks of gestation, your healthcare provider may offer a screening ultrasound. The technique, also known as sonography, bounces low-power sound waves off the fetus; then these echoes are converted into electrical signals and analyzed by sophisticated computer software that produces moving and still pictures on a video monitor. The painless procedure usually takes about 15 to 20 minutes.
Two types of ultrasound are used: transabdominal and transvaginal. During a transabdominal ultrasound, the expectant mother lies on her back on an examination table. Her abdomen is covered with a warm thin layer of gel that improves the transmission of sound waves and enhances the image’s quality. The ultrasonographer places a transducer (a handheld device) on her abdomen and moves it around to direct sound waves and obtain images of the fetus.
For a transvaginal ultrasound, the expectant mother lies with her feet in stirrups. The technician or physician inserts a probe into the vagina. This technique offers better images in early pregnancy, as the uterus, ovaries and Fallopian tubes are closer to the vagina. An alternative technique, a translabial ultrasound, involves a probe at the vaginal opening.
A first trimester ultrasound determines the number of fetuses by locating and counting the gestational sacs that contain the embryo(s). Embryonic heart activity is noted, and the length of the embryo(s) is used to calculate gestational age and obtain the most accurate due date.
An ultrasound may be ordered to detect a suspected ectopic pregnancy (when the embryo implants outside the body of the uterus, most commonly in a Fallopian tube). When there is bleeding in early pregnancy or if the physician cannot detect a fetal heartbeat or movement by 12 weeks of pregnancy, an ultrasound will help rule out a miscarriage.
Ultrasound may detect birth defects as well, which enables expectant parents and the healthcare team to prepare for possible treatment, and gives them the opportunity to meet pediatric specialists who can answer questions ahead of time.
A second or third trimester ultrasound checks size of the fetus, fetal heartbeat, location of the placenta, amount of amniotic fluid and fetal anatomy: the heart, brain, spine, kidneys, bladder and stomach. Bleeding during these two last trimesters may signify a placental problem that also can be detected by ultrasound. Special care can cure some problems, or an early cesarean section may be recommended.
An ultrasound close to the delivery date or while in labor determines fetal presentation (head first or bottom first), alerting the provider that the fetus may be in a position that will not allow for a safe vaginal delivery.
Another kind of ultrasound used late in pregnancy, Doppler velocimetry measures blood flow in the umbilical cord that attaches the fetus to the placenta and in other fetal blood vessels. It indirectly measures adjustments that the fetus has made if the level of oxygen is low.
There is a new technology called 3-D (still) and 4-D (moving) ultrasound imaging that shows realistic dimensional images of the fetus. Kiosks in shopping malls have offered expect ant mothers such photos. Healthcare professionals discourage the use of nonmedical ultrasound examinations. If performed by someone not adequately trained in ultrasonography — using excessive power settings — these scans could be harmful to the fetus.
AMNIOCENTESIS AND CVS
When an ultrasound raises a suspicion of a concern, your physician might offer another level of testing. Ultrasound is used to guide two such tests — amniocentesis and chorionic villus sampling (CVS).
Amniocentesis may diagnose certain birth defects and chromosomal abnormalities. A thin needle is inserted through the mother’s abdominal and uterine walls into the amniotic sac. About one to two tablespoons of fluid are removed for laboratory analysis. The procedure is performed between 15 to 20 weeks of gestation and takes only a few minutes.
CVS, which obtains similar information to amniocentesis, is performed between 10 and 13 weeks of pregnancy. A small amount of chorionic villi (the forming placenta) is removed with a needle, employing a similar technique.
Both procedures pose a slight risk of miscarriage; therefore, each is performed for specific reasons only, such as when the woman is older than 35 and has an increased risk of Down syndrome, when a previous pregnancy resulted in a genetic birth defect, when the family has a history of birth defects (such as fragile X syndrome, which causes mental retardation) or cystic fibrosis (a lung disease), or when a first or second trimester blood screening or an ultrasound raises the risk for an abnormal fetus.
A third trimester amniocentesis might diagnose a suspected intrauterine infection, evaluate the severity of anemia in fetuses with Rh disease (a hemolytic disease caused by the incompatibility between the blood of mother and baby) and determine the maturity of the fetal lungs when an earlier delivery is being considered.
Prenatal diagnostic screenings are designed to reassure and rule out concerns. Your odds of having a healthy baby are in your favor, so avoid worrying unnecessarily. In most cases, screening tests will give peace of mind.
AIDING YOUR BABY’S HEALTHY DEVELOPMENT
- Certain behaviors can minimize the risk of some fetal defects.
- Eliminate drugs (especially those for acne), over-the-counter medications and herbal preparations, unless prescribed by a physician.
- Avoid alcohol and tobacco, including secondhand smoke.
- Take at least 400 micrograms of the B-vitamin folic acid daily, preferably starting before conception. It can prevent neural tube defects (see Nutrition: “You’re Pregnant! What Should You Eat?”). This amount is contained in most daily multivitamins. Prenatal vitamins contain one milligram (2 1/2 times this amount). Research studies show that an expectant mother cannot overdose on folic acid.
- Eat a balanced nutritious diet to maintain a healthy weight, seek prenatal care early and follow a physician’s recommendations for immunizations and medications.
Adair R. Heyl, Ph.D., is an assistant professor, and Peter S. Heyl, M.D., is an associate professor, at the Department of Obstetrics and Gynecology, Eastern Virginia Medical School, Norfolk.