Specific indications for ultrasonography include Investigation of abnormalities during the 1st trimester (eg, indicated by abnormal results of noninvasive maternal screening tests) Risk assessment for chromosomal abnormalities (eg, Down syndrome) including nuchal translucency measurement Need for detailed assessment of fetal anatomy (usually at about 16 to 20 weeks), possibly including fetal echocardiography at 20 weeks if risk of congenital heart defects is high (eg, in women who have type 1 diabetes or have had a child with a congenital heart defect) Detection of multifetal pregnancy, hydatidiform mole, polyhydramnios, placenta previa, or ectopic pregnancy Determination of placental location, fetal position and size, and size of the uterus in relation to given gestational dates (too small or too large) Ultrasonography is also used for needle guidance during chorionic villus sampling, amniocentesis, and fetal transfusion.