First-Trimester Screening for Down Syndrome and Trisomy 18

Points to remember

  • Women with singleton pregnancies who will be age 35 or older at delivery should be offered prenatal diagnosis for fetal chromosomal defects.

  • With the exception of amniocentesis and chorionic villus sampling, all first- and second-trimester genetic tests, whether taken alone or integrated, determine only risk -- not the actual presence of a genetic defect.

  • Genetic counseling is an excellent resource for women and couples with questions or actual risk of genetic issues.

  • Risk from other factors such as environmental exposure, smoking and substance abuse, placental insufficiency and a faulty intrauterine environment cannot be determined through genetic testing, although ultrasound may be able to identify some selected congenital anomalies.

  • All genetic screening is optional and no one should feel pressured to take advantage of such testing.

  • False positives and false negatives occur. No screening test is infallible. Further testing such as amniocentesis may be recommended to follow up on results that may indicate higher than average risk.

  • Integrated screening is not helpful in the diagnosis of neural tube defects (NTDs). Alpha-fetoprotein, one of the components of the quad screen, detects risk for NTDs.

  • Earlier diagnosis creates more options. Pregnancies complicated by congenital anomalies or conditions can be monitored more thoroughly during prenatal care. Termination may be elected at an earlier gestation if desired. Pediatric and intensive care staff can be gathered for the birth. Parents and family can contact support resources to prepare for a baby with special needs.
  • Maternal serum alpha-fetoprotein evaluation is an effective screening test for neural tube defects and should be offered to all pregnant women. However, multiple-marker screening remains controversial and has not yet attained the status of standard of care.
  • There's no consensus as to the single best screening protocol for all patients.

What's Ahead?
In a perfect world, we could screen for all potential risks in all babies. But we have come a long way in a decade and there is much on the horizon — perhaps 5-, 8- and 10-marker screens. New ultrasound technology will give care providers the option of requesting less invasive testing while offering better analyses with fewer false positives and higher detection rates.

In the midst of all discussions about testing, it's important to keep things in perspective. Most babies are born strong and vigorous, and although all mothers should be offered multiple-marker screening between 15 and 21 weeks, currently there is not enough evidence to recommend more sophisticated nuchal translucency/integrated screening for all mothers-to-be.

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