New study establishes when pregnancy starts

41 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 nar

Contact: David Williamson
University of North Carolina at Chapel Hill

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