THE TIME OF APPEARANCE AND DISAPPEARANCE OF FETAL DNA FROM THE MATERNAL CIRCULATION

A single copy Y-chromosome DNA sequence was amplified using the polyme rase chain reaction (PCR) from the peripheral blood of 30 women who ha d achieved a pregnancy through an in vitro fertilization (IVF) program me. The time of conception was known precisely and was con firmed by s erial ultrasound scans. Conceptions were dated as the number of weeks after fertilization plus 2, to give a time equivalent to the obstetric menstrual dating of the pregnancy (LMP). Y-chromosome-specific DNA wa s detected in all pregnancies with a male fetus (18/30). The earliest detection was at 4 weeks and 5 days, and the latest at 7 weeks and 1 d ay. Y-chromosome-specific sequences were no longer detected in any of the male pregnancies 8 weeks after delivery. No Y-chromosome sequences were detected in any of the pregnancies where only female babies were delivered. This demonstrates that fetal DNA appears in the maternal c irculation early in the first trimester, that it can be identified in all pregnancies tested by 7 weeks, that it continues to be present thr oughout pregnancy, and that it has been cleared from the maternal circ ulation 2 months after parturition. Early non-invasive prenatal diagno sis for aneuploidies and inherited disorders will be possible in all p regnancies if fetal cells can be isolated free from maternal contamina tion (or identified accurately in the presence of maternal cells) with out problems of contamination from previous pregnancies.