Maternal serum screening in cases of mosaic and translocation Down syndrome

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

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.

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.

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%).