The feto-placental unit synthesises a variety of proteins and hormones which are secreted into the maternal circulation and amniotic fluid from early pregnancy. In pregnancies where the fetus has an autosomal trisomy, the normal concentration profiles of these markers in maternal serum and amniotic fluid are disturbed. These marker changes can be used to estimate the risk that a pregnancy is affected by Down's syndrome (or Trisomy 18) and thus allow the parents to make an informed decision regarding prenatal diagnosis by invasive testing. However, the factors which give rise to the varying patterns of marker concentrations in chromosomally abnormal pregnancies are poorly understood. The aim of this project was to investigate the underlying causes of abnormal marker concentrations in Down's syndrome, Trisomy 13, and Trisomy 18 pregnancies. The results of this investigation indicate that in Down's syndrome pregnancies, maternal serum levels of placental products reflect those found in the placenta; intact hCG, FβhCG and SP1 levels were elevated while PAPP-A and placental ALP levels were little changed. This suggests that transport of these proteins from the placenta into the maternal circulation is not affected but there is altered synthesis of hCG subunits and SP1. Hepatic synthesis of AFP does not appear to be altered in Down's syndrome pregnancies, but increased placental and reduced maternal serum levels of AFP point to a possible placental transport defect specific to AFP. Similarly reduced GGT levels in fetal intestine and in corresponding amniotic fluid from Down's syndrome pregnancies suggest that amniotic fluid GGT activity is of fetal intestinal origin since GGT activity was elevated in fetal liver and placental from the same series of Down's syndrome pregnancies.