Trisomy 21. Second-trimester ultrasound.

Area of Science:

• Prenatal diagnosis
• Medical imaging
• Genetics

Background:

• Ultrasound screening for fetal chromosomal abnormalities, particularly Down syndrome (trisomy 21), is a valuable tool.
• Sonographic findings for trisomy 21 can be subtle, with normal amniotic fluid, placentation, and growth often observed.
• Many chromosomal abnormalities share overlapping sonographic findings with Down syndrome and phenotypically normal fetuses.

Purpose of the Study:

• To highlight the continued importance and effectiveness of second-trimester ultrasound screening for Down syndrome.
• To emphasize the role of trained sonographers in risk assessment for trisomy 21.
• To underscore the necessity for parental understanding of screening test limitations and confirmatory testing.

Main Methods:

• Second-trimester ultrasound examination.
• Assessment of fetal markers, including nuchal thickness.
• Consideration of karyotype determination when major structural abnormalities are identified.

Main Results:

• Nuchal thickness in the first or second trimester is a clinically useful marker for trisomy 21.
• Ultrasound findings for trisomy 21 can be non-specific, with significant overlap with normal fetuses.
• The predictive value of ultrasound markers is influenced by population, biochemical markers, and historical factors.

Conclusions:

• Ultrasound remains a powerful tool in prenatal genetic evaluation, used alone or with biochemical markers.
• Despite limitations, ultrasound plays a crucial role in prenatal diagnosis until non-invasive fetal cell isolation is widely available.
• Karyotype determination is recommended if major structural abnormalities are detected via ultrasound.