Screening for aneuploidy in first and second trimesters: is there an optimal paradigm?

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Abstract
This review serves to explore the recent literature regarding aneuploidy screening in both first and second trimesters. We aim to construct a comparative analysis of a range of proposed strategies for screening for trisomy 21. First trimester combined screening (sonographic nuchal translucency combined with serum markers pregnancy-associated plasma protein A and the free beta subunit of human chorionic gonadotrophin) has superseded second trimester serum screening (alpha-fetoprotein, total human chorionic gonadotrophin, unconjugated estriol with or without inhibin-A) as a screening paradigm for the detection of trisomy 21. This move is attributed to the recognition of superior detection rates, lower false-positive rates and earlier results associated with the former strategy. Septated cystic hygroma has been recognized as a distinct entity which confers a high risk of aneuploidy and structural malformations. Further advances in screening performance are achievable by combining the results of first and second-trimester screens in a sequential manner, with much interest generated by programs that only include second-trimester testing contingent upon first-trimester results. Screening strategies for aneuploidy continue to evolve, with the most recent evidence favouring a contingent sequential approach.