Confined placental mosaicism

Confined placental mosaicism (CPM) represents a discrepancy between the chromosomal makeup of the cells in the placenta and the cells in the fetus. CPM was first described by Kalousek and Dill in 1983. CPM is diagnosed when some trisomic cells are detected on chorionic villus sampling and only normal cells are found on a subsequent prenatal test, such as amniocentesis or fetal blood sampling. In theory, CPM is when the trisomic cells are found only in the placenta. CPM is detected in approximately 1-2% of ongoing pregnancies that are studied by chorionic villus sampling (CVS) at 10 to 12 weeks of pregnancy. Chorionic villus sampling is a prenatal procedure which involves a placental biopsy. Most commonly when CPM is found it represents a trisomic cell line in the placenta and a normal diploid chromosome complement in the baby. However, the fetus is involved in about 10% of cases.
Pathogenesis.
CPM occurs in one of two ways:
Several factors influence the pattern of normal and abnormal cells in the developing embryo. Reduced or improved replication rates of the trisomic cells could affect the number of abnormal cells compared to the number of normal cells. The abnormal cells may fail to differentiate or function properly and could be lost. It is also possible that there is no selection against the abnormal cells, but their presence could compromise the pregnancy on a whole.
Types.
There are three types of confined placental mosaicism depending on the cells involved at the time of the error:
Prognosis.
Most pregnancies that are diagnosed with confined placental mosaicism continue to term with no complications and the children develop normally.
However, some pregnancies with CPM experience prenatal or perinatal complications. The pregnancy loss rate in pregnancies with confined placental mosaicism, diagnosed by chorionic villus sampling, is higher than among pregnancies without placental mosaicism. It may be that sometimes the presence of significant numbers of abnormal cells in the placenta interferes with proper placental function. An impaired placenta cannot support the pregnancy and this may lead to the loss of a chromosomally normal baby. On the other hand, an apparently normal diploid fetus may experience problems with growth or development due to the effects of uniparental disomy (UPD). Intrauterine growth restriction (IUGR) has been reported in a number of CPM cases. In follow-up studies adequate postnatal catch-up growth has been demonstrated, which may suggest a placental cause of the IUGR.
When predicting the likely effects (if any) of CPM detected in the first trimester, several potentially interactive factors may be playing a role, including: