First and second trimester maternal serum markers in pregnancies with a vanishing twin.

OBJECTIVE: The aim of this study was to assess the concentration of the first and second trimester maternal serum markers in pregnancies with a vanishing twin. METHODS: This is a retrospective case-control study of pregnancies screened for Down syndrome in one Ontario center. Singleton pregnancies with ultrasound evidence of a vanishing twin were identified, and each was matched with five normal singleton controls for ethnicity, maternal age, gestational age, and blood sampling date. The median MoM of the first and second trimester serum markers was compared between cases and controls. The differences were assessed using the Mann-Whitney U-test. RESULTS: The study included 174 pregnancies that had a vanishing twin. Compared with control pregnancies, pregnancy associated plasma protein A increased by 21% (p = 0.0026), alpha-fetoprotein (AFP) increased by 10% (p < 0.0001), and dimeric inhibin A (DIA) increased by 13% (p = 0.0470) in pregnancies with a vanishing twin. Unconjugated oestriol and total human chorionic gonadotrophin were not significantly changed in these pregnancies. CONCLUSIONS: Pregnancy associated plasma protein A is not an adequate marker for pregnancies with a vanishing twin. The impact of elevated AFP on risk estimation is offset by that of DIA to certain extent. Further studies are needed to establish an adequate adjustment method for AFP and DIA to improve the accuracy of screening results for these pregnancies.

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.