Modifying risk for aneuploidy with second-trimester ultrasound after a positive serum screen.

Prenatal diagnosis for aneuploidy (primarily Down syndrome) has evolved over the past 4 decades. It started as a screening process using maternal age of 35 years or older as a risk factor to offer patients the option for prenatal diagnosis. The actual diagnosis used an invasive procedure (amniocentesis) to obtain fetal cells for processing to determine fetal karyotype. This had a potential risk for miscarriage. The development of noninvasive prenatal screening to better identify pregnant patients at high risk for Down syndrome improved the ability to detect cases of aneuploidy and limit amniocentesis to only patients considered at high risk. This approach has a higher detection rate and a lower procedure-related rate of fetal loss than use of maternal age of 35 years or older alone. This article presents an overview of how prenatal diagnosis has evolved and then focuses on the current status of using ultrasound to evaluate patients considered to be screen-positive for Down syndrome based on first-trimester screening (10-14 weeks) or second-trimester (15-22 weeks) maternal serum analyte screening.