Actually, I think Dona may be thinking of this:
Dr. Zev Williams from Brigham and Women's Hospital, Boston and colleagues were able to detect DSY14 Y chromosome-specific sequences in the circulation of 5 of 11 (45%) women who had donor egg pregnancies resulting in male offspring, the authors report. An earlier study reported a detection rate of 35% of women who had spontaneous male pregnancies.
The longest interval between delivery of a male offspring and detection of the circulating DSY14 sequence in Dr. Williams' patients was 9 years.
Their findings, the researchers say, "suggest the existence of an immunomodulatory mechanism that results in persistence of microchimerism with evasion of host immune surveillance."
You and Your Donor Egg Fetal Cells - The Well-Timed Period