CHAPEL HILL - With help from healthy women trying to conceive babies, North Carolina scientists have uncovered the most precise information yet about when pregnancy starts in humans.
Fertilized eggs attach themselves to the lining of the womb six to 12 days after ovulation, the research shows. In most successful pregnancies, that implantation - the real start of pregnancy -- occurs on day eight, nine or 10 following ovulation. Day eight appears to be the most successful.
The later the attachment takes place, the more likely a pregnancy will end on its own, the scientists found.
Conceivably -- no pun intended - as a natural protective mechanism, the uterus tends to reject fertilized eggs that take too long to adhere to the lining because they may be less fit, the researchers say. On day 11, more than 50 percent of pregnancies fail and on day 12, that number jumps to more 80 percent.
A report on the findings appears in Thursday's issue (June 10) of the New England Journal of Medicine. Lead author is Dr. Allen J. Wilcox, chief of the National Institute of Environmental Health Sciences' epidemiology branch and adjunct professor of epidemiology at the University of North Carolina at Chapel Hill School of Public Health. Co-authors are Drs. Donna Day Baird and Clarice Weinberg, both also of NIEHS and UNC-CH.
"This is a very basic piece of reproductive biology that will probably find its way into the textbooks," Wilcox said. "It's a step forward in terms of what we understand about pregnancy. Eventually, it could have an impact on patient care, but it is not going to change the way physicians treat their patients immediately."
Researchers collected daily urine samples for up to six months from 221 healthy N.C. women attempting to conceive after stopping contraception, he said. Of 199 conceptions, enough information was available on 189 for analysis.