Best time for prenatal screening of Down's syndrome?

Ans. d (14-18 weeks of gestation). (Ref. Harrison, Medicine, 15th ed., 375, 400, 407)Prenatal diagnosis of Down's Syndrome# Risk of having a fetus affected by Down's syndrome at an advanced maternal age (>35 years) is well known.# Fetal blood sampling and tissue sampling being invasive can be performed only in very selected cases.# Currently second trimester USG scanning has been shown to detect up to 60% of Down's syndrome fetuses.# The following findings on USG are useful:Soft marker of Down's syndromeCommentNuchal skin thicknessmeasured on an axial view (where the cavum septi pellucidi), cerebral pedicles, cerebellar hemispheres & cisterna magna are seen. A cut off of 6 mm is significant. Most sensitive and good markerChoroid plexus cysts-Cardiac defectsare noted in 50% children with DS.Femur, humerus lengthis a possible marker of underlying trisomy 21. Ratio of biparietal diameter to femur length and humerus length with expected length are good predictors with sensitivity of 37% and specificity of 47%.Clinodactyly, hypoplastic midphaiynx in the fifth finger, increased gap between 1st and 2nd toe Absent nasal bone-Abdominal findingsDuodenal atresia, omphalocoel, pylectasis ascites, echogenic bowel, should be seen.Duodenal atresia can be diagnosed as early as 14 weeks on a USG.