Objective: To evaluate the risk of fetal involvement when trisomy 8 mosaicism (T8M) is detected in chorionic villus samples (CVS). Methods: A retrospective descriptive study of registered pregnancies in Denmark with T8M in CVS identified through a database search and a review of published cases of T8M found through a systematic literature search and inclusion of cross references. Pregnancies with T8M in CVS and no additional numerical chromosomal aberrations were included. Results: A total of 37 Danish cases and 60 published cases were included. T8M detected in a CVS was associated with fetal involvement in 18 out of 97 pregnancies (18.6% [95%CI: 11.4-27.7]). Eight out of 70 (11.4% [95%CI: 5.1-21.3]) interpreted prenatally to be confined placental mosaicism (CPM) were subsequently found to be true fetal mosaicisms (TFM). Conclusion: T8M detected in CVS poses a significant risk of fetal involvement, and examination of amniotic fluid (AF) and/or fetal tissue should be offered. However, a normal result of AF still has a considerable residual risk of fetal involvement. Genetic counselling at an early gestational age is essential, and follow-up ultrasonography should be performed to predict fetal involvement if possible.

Objective: To evaluate the risk of fetal involvement when trisomy 8 mosaicism (T8M) is detected in chorionic villus samples (CVS). Design: A retrospective descriptive study of registered cases in Denmark and a systematic literature review. Setting: Cases of T8M in CVS registered in Denmark between January 1983 and March 2019 and published literature until March 2019. Sample: A total of 37 registered pregnancies in Denmark and 60 published cases. Methods: Registered pregnancies with T8M in CVS were identified through a database search. Published cases of T8M were found through a systematic literature search and backward snowballing. Pregnancies with T8M in CVS and no additional numerical chromosomal aberrations were included. Main outcome measures: Fetal involvement defined as T8M in amniotic fluid (AF) or fetal tissue. Results: T8M detected in a CVS was associated with fetal involvement in 18 out of 97 pregnancies ). Eight out of 70 (11.4% [95%CI: 5.1-21.3]) interpreted prenatally to be confined placental mosaicism (CPM) were found to be true fetal mosaicisms (TFM). Conclusion: T8M detected in CVS poses a significant risk of fetal involvement, and examination of AF and/or fetal tissue should be offered. However, a normal result of AF still has a considerable residual risk of fetal involvement.Genetic counselling at an early gestational age is essential, and follow-up ultrasonography should be performed to predict fetal involvement if possible.