Quantification of free fetal DNA in multiple pregnancies and relationship with chorionicity

Objective Free fetal DNA (ffDNA) in the maternal plasma appears to originate mainly from the trophoblast. We tested the hypothesis that ffDNA concentration is increased in multiple pregnancies where trophoblastic mass has been shown to be increased. Methods Quantitative real‐time PCR was used to measure the plasma concentration of DYS14 in singleton and twin pregnancies with one or two male fetuses. Royston and Wright's regression method was used to relate ffDNA to gestational age in singleton controls; z‐scores were calculated for the multiple pregnancy subgroups. Results Fifty‐five singleton and 65 twin pregnancies (36 with one and 29 with two male fetuses) were analysed. There was significantly higher ffDNA concentration in twin pregnancies with two male fetuses compared with pregnancies with one male fetus. In cases with two male fetuses, there was no statistically significant difference between monochorionic and dichorionic pregnancies. Conclusions There is higher ffDNA concentration in multiple pregnancies, and this must be taken into account for future quantitative ffDNA applications. Copyright © 2011 John Wiley & Sons, Ltd.