Nov 18 (Reuters) - Most U.S. doctors believe pregnancy starts when the sperm fertilizes the egg, a survey shows, contradicting the position of a key medical group with a view that could potentially affect U.S. policy and laws regarding contraception and research.

The American College of Obstetrics and Gynecology (ACOG), the leading organization for this field of medicine, defines pregnancy as beginning when the fertilized egg implants in the uterus, roughly a week after fertilization.

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