California's experience with low MS-AFP results.

The California AFP Screening Program was developed to offer pregnant women the opportunity to have their pregnancies screened for open neural tube defects (NTD). Because it is unwise to withhold potentially important clinical information about low MS-AFP results from the screened women and their physicians, follow-up diagnostic evaluations and counseling were offered to women with low MS-AFP results in addition to those with high results or positive family histories of NTD. Between April 7, 1986 and September 30, 1987, over 275,000 women voluntarily participated in the screening program. During the first year of the program 3,939 women were seen for follow-up evaluations due to low MS-AFP results. These evaluations occurred at state-approved AFP follow-up centers where ultrasonographic dating led to reinterpretation of 35% of the results. Of the remaining 2,552 (65%), 1,940 women (76%) had an amniocentesis. Sixteen Down syndrome fetuses were detected for a yield of 1/121 amniocenteses. Additional chromosome abnormalities included trisomies 18 and 13, Ullrich-Turner syndrome, (45,X), Klinefelter syndrome (47,XXY) and triploidy. The total yield of significant chromosome abnormalities was 1/78 amniocenteses.