Objective: To establish how different methods of estimating gestational age (GA) affect reliability of first-trimester screening for Down syndrome. Methods: Retrospective single-center study of 100 women with a viable singleton pregnancy, who had first-trimester screening. We calculated multiples of the median (MoM) for maternal-serum free beta human chorionic gonadotropin (free b-hCG) and pregnancy associated plasma protein-A (PAPP-A), derived from either last menstrual period (LMP) or ultrasound-dating scans. Results: In women with a regular cycle, LMP-derived estimates of GA were two days longer (range -11 to 18), than crown-rump length (CRL)-derived estimates of GA whereas this discrepancy was more pronounced in women who reported to have an irregular cycle, i.e., six days (range -7 to 32). Except for PAPP-A in the regular-cycle group, all differences were significant. Consequently, risk estimates are affected by the mode of estimating GA. In fact, LMP-based estimates revealed ten "screen-positive" cases compared to five "screen-positive" cases where GA was derived from dating-scans. Conclusion: Provided fixed values for nuchal translucency are applied, dating-scans reduce the number of screen-positive findings on the basis of biochemical screening. We recommend implementation of guidelines for Down syndrome creening based on CRL-dependent rather than LMPdependent parameters of GA.

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Keywords CRL, Dating, Down syndrome, First-trimester screening, Free b-hCG, LMP, PAPP-A, Trisomy 21
Persistent URL dx.doi.org/10.1515/JPM.2010.033, hdl.handle.net/1765/19757
Heesch, P.N.V, Struijk, P.C, Laudij, J.A.M, Steegers, E.A.P, & Wildschut, H.I.J. (2010). Estimating the effect of gestational age on test performance of combined first-trimester screening for down syndrome: A preliminary study. Journal of Perinatal Medicine: official journal of the World Association of Perinatal Medicine, 38(3), 305–309. doi:10.1515/JPM.2010.033