PLACENTAL MOSAICISM IS ASSOCIATED WITH UNEXPLAINED 2ND-TRIMESTER ELEVATION OF MSHCG LEVELS, BUT NOT WITH ELEVATION OF MSAFP LEVELS

In this patient-control study, we examined the impact of placental mos aicism on the concentrations of maternal serum human chorionic gonadot ropin (MShCG) and maternal serum alpha-fetoprotein (MSAFP) in the seco nd trimester of pregnancy. Patient and control groups were selected fr om 2347 women with a singleton pregnancy, who underwent chorionic vill ous sampling in the first trimester and from whom second-trimester ser um samples had been collected. The concentrations of both serum marker s, expressed in multiples of the median (MOM), in 35 women with confin ed placental mosaicism (CPM) were compared with those in 70 controls w ith uncomplicated pregnancies. Elevated MSAFP or MShCG was defined as a concentration of greater than or equal to 2 . 0 MOM. Of the 35 pregn ancies with CPM, none had an elevated MSAFP level, as opposed to two o ut of the 70 women (2 . 9 per cent) in the control group (P=NS). Nine women in the placental mosaicism group (26 per cent) had an MShCG leve l of greater than or equal to 2 . 0 MOM, compared with five in the con trol group (7 . 1 per cent; P=0 . 0135). Nineteen women in the placent al mosaicism group (54 per cent) were screen-positive for Down's syndr ome (cut-off 1:250), compared with 17 women (24 per cent) in the contr ol group (P=0 . 0042; relative risk=2 .). The three highest MShCG leve ls were found in pregnancies with CPM that involved trisomy 16; all th ese women delivered a small-for-gestational age (SGA) infant. CPM, esp ecially with trisomy 16, is associated with elevated levels of MShCG, but not with elevated levels of MSAFP. It is an important cause of fal se-positive results in serum screening programmes for fetal Down's syn drome. It is possible that abnormal MShCG levels in pregnancies with C PM result from a dysfunctional placenta, caused by chromosomally abnor mal areas. We therefore recommend increased surveillance of pregnancie s with unexplained elevated MShCG levels.