Chapter 1: Fetal Assessment

PRENATAL DIAGNOSIS

Prenatal diagnosis refers to those testing modalities used during pregnancy to screen and diagnose fetal aneuploidies and anomalies. Counseling is recommended, ideally in the first trimester, prior to pursuing any type of prenatal screening or diagnostic testing. Screening tests provide risk assessment and determine whether the individual is at increased risk for having a fetus with a genetic condition. Diagnostic testing determines if the fetus is affected by a chromosomal or genetic abnormality by analyzing the fetal DNA through invasive testing. Evolving technology has made the use, interpretation, and choice of appropriate test for each individual challenging for both patients and providers.

FETAL SCREENING TESTS

I. NUCHAL TRANSLUCENCY (NT)

An ultrasound measurement of the amount of fluid behind the neck of the fetus, done between 10 0/7 and 13 6/7 weeks¹ when the crown-rump length (CRL) measures 38 to 45 and 84 mm, respectively. An increased NT is defined as a numerical value of ≥3 mm or greater than the 95th to 99th percentile for gestational age. An increase in the NT correlates with an elevated risk for chromosomal abnormalities such as trisomy 21, trisomy 18, and Turner syndrome and other genetic diseases such as Noonan syndrome. In addition, gestations with increased NT have an elevated risk of adverse pregnancy outcomes, including fetal cardiac defects and intrauterine fetal demise, even when karyotype is normal. A measurement of NT alone has a low detection rate for trisomy 21 (64%–70%); therefore, it is not recommended as a sole screening test for aneuploidy.

¹This nomenclature refers to the number of weeks and the number of days in an incomplete week of gestation.

II. COMBINED FIRST-TRIMESTER SCREENING

NT combined with a measurement of the maternal serum markers, free β-human chorionic gonadotropin (β-hCG) and pregnancy-associated plasma protein A (PAPP-A), is used to calculate the risk for trisomies 18 and 21. It is performed between 10 3/7 and 13 6/7 weeks’ gestation. It is an effective screening tool, with a detection rate of 82% to 87% for trisomy 21 at a 5% false-positive screen rate. Free β-hCG is elevated and PAPP-A is decreased in a pregnancy affected by Down syndrome. A Cochrane review comparing the performance of first-trimester ultrasound markers with or without serum markers for Down syndrome screening found that the combination of maternal age, NT, and serum markers (PAPP-A and free β-hCG) performs better than the use of ultrasound markers alone (except nasal bone) with or without maternal age.

III. SECOND-TRIMESTER SCREENING

The quadruple screen (or quad screen) involves analyzing levels of 4 maternal circulating factors: maternal serum α-fetoprotein (MSAFP), total hCG, unconjugated estriol, and inhibin A between 15 and 22 6/7 weeks’ gestation. The quad screen is used to calculate the risk for trisomies 18 and 21 and open ...