Pregnancy & Baby

Study Guide: Prenatal Tests

Prenatal doctor visits come with lots of screenings and tests, some of which are designed to determine the health of your baby. The mere thought can make parents-to-be anxious.

“Some women just don’t want to know” if their babies have any genetic disorders or defects, says Arelis Figueroa, M.D., an O.B./Gyn with Touro Infirmary. “They say, ‘I don’t want to know because I don’t want to worry.’”  But she says that knowing that their baby may need special attention can help parents be prepared; she also notes that parents need to think about what they’ll do with the results before they consent to any screenings or tests. With her help, we look at what’s currently available.

Maternal serum screening

This test, also called the quad screening, is offered to all pregnant women, regardless of age or risk factor. It’s conducted during the second trimester to look for increased risks for neural tube defects, Down syndrome (Trisomy 21), Trisomy 13 and Trisomy 18. The latter two genetic disorders are so catastrophic that 90-95 percent of babies do not survive their first year of life.

“All women are offered this maternal serum screening regardless of their age because we compare risk factors—and everyone has a risk factor; the risk factor just increases after the age of 35,” explains Dr. Figueroa. False positives can happen, she says, “if the gestational age is not correct, if the patient’s age is not correct,” and so on. If a test is positive, the doctor will recommend additional testing. And “false” negatives? Dr. Figueroa explains that it’s a little misleading, as it’s a screening tool, and not a diagnostic test.

“It’s just telling you if your risk is increased compared with someone else your same age.”

Sequential screening.

This involves a first trimester nuchal translucency screening—an ultrasound measurement of the nuchal folds of the back of the developing baby’s neck—and blood tests drawn from the mother the same day, followed by the maternal serum screening in the second trimester.

“It increases the detection rate, decreases that false negative, false positive” you get with only using the maternal serum screening, explains Dr. Figueroa. If both the first and second part of the sequential screenings are negative, the detection rate is 95 percent accurate for Down syndrome, Trisomy 13 and Trisomy 18, she says.

Genetic (level 2) ultrasound.

These are also used in the second trimester for screening purposes. “It’s an anatomic survey,” explains Dr. Figueroa. “But by itself, the sensitivity is not very high—about 73 to 74 percent—for finding risks for a trisomy. That’s important because you’ll have patients who’ll say, ‘Oh, I had two ultrasounds, and everything was fine.’ Unfortunately, ultrasounds cannot see everything.”

Amniocentesis. This invasive diagnostic test is 99 percent accurate for diagnosing chromosome disorders, and can detect approximately 95 percent of open neural tube defects. It’s performed in the second trimester, and can test for nearly all chromosomal disorders, several hundred genetic disorders, and neural tube defects.

Chorionic villus sampling (CVS) is another invasive diagnostic test; this one has 98 percent accuracy. It’s performed at the end of the first trimester (10 to 13 weeks) and tests for genetic and chromosomal disorders, but not neural tube defects. Both amniocentesis and CVS carry very small risk of miscarriage (1 in 200 to 1 in 400).

Cell-free fetal DNA
. This new, non-invasive diagnostic screening is 98 percent accurate. Anytime from week 10 to 32 in her pregnancy, the mother has her blood drawn; her baby’s cells are then isolated. It detects Down syndrome and Trisomies 13 and 18, and has a false positive rate of .5 percent; those who get a positive result may be advised to have a diagnostic test—either CVS (if it’s early enough) or amniocentesis. Currently, the cell-free fetal DNA test is only available to patients with risk factors.

“It is a great option for someone who doesn’t want to have an amniocentesis,” Dr. Figueroa says. “I’m getting goose bumps right now. I had a patient who had an abnormal maternal screening. She then had a cell-free fetal. And when I gave her those results…  She was very happy.”


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