Maternal serum screening in cases of mosaic and translocation Down syndrome.

OBJECTIVES: To determine if the second-trimester maternal serum markers (MSM) screening for Down syndrome (DS) is efficient in DS mosaicism or structural rearrangement cases.

METHOD: DS mosaic or translocation cases were reviewed from databases of routine MSM DS screening. The control group consisted of 977 trisomy 21 cases included in a series of 854 902 patients (routine screening). DS risk was calculated by combination of maternal age and MSM [alpha-fetoprotein (AFP) and human choriogonadotrophin (hCG) or free beta-hCG and/or uE3] expressed in multiples of median (MoM). Mosaic DS cases were divided into three groups, < 10%, 10-49%, and >or= 50% trisomy 21 cells. Translocation DS cases were divided into three groups, isochromosome, Robertsonian, or reciprocal translocation. Detection rate (DR) and MoMs were evaluated in each group.

RESULTS: As many as 76 cases of nonstandard trisomy 21 were collected. For mosaic DS cases (n = 43) DR was 69.8% (not significantly different from the 70.8% of control group). When mosaicism was less than 10%, the DR dropped to 25%. For translocation DS cases (n = 33) DR was 75.7% (not significantly different from control group) whatever the types of translocation.

CONCLUSION: In the nonstandard DS cases, second-trimester MSMs gave the same detection rate as for standard trisomy 21, except the cases with low-level mosaicism (<10%).