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.
Of those 189 pregnancies, 141 lasted at least six weeks past the last menstrual cycle, and the other 48 ended in early loss, the scientist said. Among pregnancies lasting six weeks or more, the first detectable rise in the level of a hormone known as chorionic gonadotropin (hCG) -- an indicator of successful attachment of a fertilized egg to the uterus wall -- occurred six to 12 days after ovulation.
"The risk of early loss was strongly related to the time of implantation," the authors wrote. "Early loss was least likely when implantation occurred by the ninth day (13 early losses among 102 pregnancies, or 13 percent) rising to 26 percent (14 of 53 pregnancies) when implantation occurred on the 10th day, 52 percent (12 of 23) on the 11th day and 82 percent (9 of 11) with implantation after day 11."
Three pregnancies in which the first rise in hormone occurred after day 12 ended by themselves.
"This study was first intended to identify the proportion of pregnancies that were lost very early before women knew they were pregnant," Wilcox said. "But we also were able to look at the date on which pregnancy could first be detected in relation to ovulation.
"The first detection of pregnancy is really the first detection of hCG, which rises very rapidly once the fertilized egg attaches to the uterus. That's in fact the most hidden event of pregnancy and has never been seen before."
Fertilization begins about a week earlier when egg and sperm unite, he said. Many fertilized eggs, however, never succeed in clinging to the uterine wall where they can grow into an embryo, fetus and, eventually, a baby.
"This is the first really concrete information we have about when pregnancy starts in humans - how long after fertilization does pregnancy begin," Wilcox said. "It probably has implications for assisted reproduction -- the way medicine is able to manipulate reproductive processes to encourage pregnancy - possibly by increasing the narrow implantation 'window.' We may want to be careful doing this though. We don't want to shut down a mother's natural ability to screen out some potential pregnancies that might not be doing well due to chromosomal abnormalities or other developmental problems."
Note: Wilcox can be reached at 919-541-4660 until Tuesday at noon when he will leave for research meetings in Baltimore for several days. During his absence, he will carry a cell phone, 919-219-8197. His secretary also will relay messages to him.
Journal
New England Journal of Medicine