Identification of a duplication on chromosome 21 in a pregnant patient through prenatal cell-free DNA screening over multiple pregnancies

Prenatal cell free DNA (cfDNA) screening is a widely used method to screen for fetal aneuploidies in pregnancy. While designed to identify fetal aneuploidies, deletions and duplications present in the pregnant patient have also been identified through cfDNA. To provide reliable results and limit false positives and false negatives, a cfDNA screen should be able to differentiate between findings of patient origin and fetal origin in order to correctly interpret fetal status. Here we present a case of chromosome 21 (chr21) duplications found in the same pregnant individual across three separate pregnancies, one singleton, one affected singleton with trisomy 21, and a twin pregnancy, without precluding the interpretation of fetal aneuploidy.