• fetal DNA, cell free fetal DNA

Fetal cells are consistently found in the maternal circulation, and polymerase chain reaction based studies have led to the identification of cell free fetal DNA (fetal DNA) in maternal blood. Approximately 1.2 nucleated fetal cells/ml of whole blood from women carrying a male fetus were detectable,¹ and relative enrichment of fetal DNA was detected in the maternal plasma and serum.² The amount of fetal DNA in the maternal blood increases with progression of pregnancy, and 3.4–6.2% of the total maternal plasma DNA during pregnancy was of fetal origin.³ Therefore, cell free fetal DNA in pregnant women’s plasma is useful for non-invasive prenatal diagnosis, especially for detection of fetal sex,^3,4 RhD blood type,^5–7 and gene mutations of paternal origin.^8–10 Previous studies indicated that pregnant women with pre-eclampsia,¹¹ placenta previa¹² and fetal chromosome abnormalities¹³ tend to have elevated levels of fetal DNA in their plasma. Since functional or structural abnormalities of the placenta and destruction of the trophoblast may be associated with these diseases,¹⁴ it is suggested that cell free fetal DNA is of placental origin. This implies that quantitative analysis of fetal DNA may be valuable to screen for placental dysfunction. Ng et al¹⁵ recently reported that placental mRNA is present in the maternal circulation, and suggested that the same might occur for placental DNA,¹⁶ However, no direct evidence has been given for placenta derived cell free fetal DNA in the maternal blood, although its clinical use is growing.¹⁷

Confined placental mosaicism, which is defined by the presence of abnormal karyotypes only in the placenta while the fetus itself is usually diploid,¹⁸ may occur through a loss of the extra chromosome in a trisomic zygote during an early mitotic cell division in only the …