The second trimester screen is also known as the AFP4 or Quadruple screening and is typically done between 15-20 weeks. Approximately 80% of women who are carrying a pregnancy with Down’s syndrome will have a positive AFP4 screening. The false positive rate is 5%. Although the detection rate for trisomy 21 is lower than the first trimester screening, AFP4 screening does allow for the detection of spina bifida and anencephaly (abnormal fetal head). It will detect 75 to 80% of cases of open neural tube defect (spina bifida) and 95% of babies with anencephaly.

The State of California has recently instituted a program to combine first and second trimester screening which has increased the detection of Down’s syndrome to 90-95%.

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