P01.06: Can a second‐trimester scan modify the likelihood of fetal aneuploidy in women older than 35 years of age?

Abstract

To evaluate how a second-trimester scan can modify the a-priori likelihood of having a fetus with aneuploidy in women older than 35 years of age From January 2002 through November of 2005, 967 women older than 35 years of age at delivery were scanned consecutively between 18 and 22 weeks of gestation. 100 women were lost to follow-up and were excluded from the study, for a total of 867 patients. Six ultrasonographic parameters were evaluated and the presence of at least one of these counted as a positive sonogram: 1) nuchal fold > 6 mm, 2) intracardiac hyperechogenic focus, 3) hyperechogenic bowel, 4) mild pyelectasis, 5) short humerus, 6) short femur. Neonatal records as well as Karyotypes for those patients who had a diagnostic procedure were reviewed postnatally. Trisomy 21, 18 and monosomy X were included as abnormal for the purpose of this study. The prevalence of aneuploidy in our group based on comparably aged women did not differ significantly from the expected (P = 0.88). An abnormal sonogram was significantly associated with Down syndrome (P = 0.002). However, a positive test had only about a 4% chance of being indicative of a truly affected child. This is still a four fold increase over the expected occurrence. In women 35–39 y.o. (86% of our group) a negative sonogram almost always signified the absence of aneuploidy (PV− = 99.83%, likelihood ratio = 0.2). In women over 40 y.o. (14% of our group) a negative sonogram was not significantly associated with the absence of the child being diagnosed with aneuploidy (PV− = 98.5%). An abnormal sonogram was associated with an increased risk aneuploidy that was four-fold in women both 35–39 and 40+. A negative sonogram appeared to correlate with the absence of disease only in women 35–39.