[CG] Screening for Down syndrome in pregnancies that started as twin pregnancies but are only continuing as singletons

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Empty second sac

If an empty second sac is identified on ultrasound it appears the concentration of biochemical markers is unaffected and no different to a singleton pregnancy.
Therefore, offer these women biochemical screening as per hospital protocol for singleton pregnancy (either second trimester biochemical screening or nuchal translucency (NT) plus first trimester biochemistry)

Second sac contains fetal pole

If a second sac containing a fetal pole is identified on ultrasound it is possible there is a contribution to the biochemical markers from the non-continuing pregnancy. In this case, risk calculation should be based on maternal age and nuchal translucency alone – second trimester screening would be inappropriate.
Offer these women nuchal translucency (NT) screening (without biochemistry)