Second trimester prenatal screening for Down syndrome: the associations between the levels of serum markers in successive pregnancies

OBJECTIVE: To investigate the association of Down syndrome screening results in successive pregnancies, and assess the impacts of including previous screening results in the current risk estimation on screening performance. METHODS: The study was based on 56,951 women who had triple marker screening in two or more singleton pregnancies in the Ontario Maternal Serum Screening (MSS) program between October 1993 and September 2000. The problem of recurrent false positive results was examined by comparing screening results from different pregnancies in the same individuals. Between-pregnancy associations in the levels of serum markers were estimated using correlation analysis. A published method was used to adjust current risk estimation for previous screening results. The effect of this adjustment was assessed by comparing screening performances prior and subsequent to the adjustment. RESULTS: The observed false positive rate (FPR) in subsequent pregnancies was 2.5 times higher than that expected (26.4% vs 10.7%) among women who screened positive in one previous pregnancy, and 3.9 times higher (47.4% vs 12.1%) among women who screened positive in two previous pregnancies. Adjusting for a previous screening result will significantly reduce the recurrent FPR without compromising detection. CONCLUSION: Risk estimation for Down syndrome may be adjusted using the screening result from a previous pregnancy.