Usually screening for birth defects is done during first three months or first trimester. Usually these are performed between weeks 11 and 13 of pregnancy. Down syndrome, Trisomy 18, and heart defects are ruled out. If the results are suspicious, then tests are repeated or more sophisticated tests are done in second trimester, wherein other defects are also ruled out as well (chromosomal birth defects and abdominal wall defects).
Hope this helps. Take care!
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