<prompt> Best time for prenatal screening of Downs syndrome?8-10 weeks of gestation12-14 weeks of gestation10-12 weeks of gestation14-18 weeks of gestation  <response>: Ans. d 14-18 weeks of gestation. Ref. Harrison, Medicine, 15th ed., 375, 400, 407Prenatal diagnosis of Downs Syndrome Risk of having a fetus affected by Downs 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 Downs syndrome fetuses. The following findings on USG are useful:Soft marker of Downs 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. 