Can sonograms detect Down syndrome and other defects the baby can have?

Yes. Sonogram of heart- called echocardiogram will, detect most cardiac anomalies associated with down's syndrome.
Many are missed. The best sonographers miss fetuses affected with Down syndrome. Only half will have a cardiac defect. The OK sonographers miss about 50%. Now, the best and most accurate screening test is not a fetal ultrasound, it is a blood analysis of fetal DNA in your bloodstream, done as early as 9 weeks0 days by one lab., 10w0d by 3 others, all 99.99% accurate. Confirmation by CVS or amnio recommended.