Though most doctors will give you a definition of when pregnancy begins, it’s not always the same one, according to a new survey.

Most of the polled obstetrician-gynecologists believe pregnancy begins when the sperm fertilizes the egg. But a minority say it doesn’t begin until a week later when the fertilized egg implants in the uterus - the definition given by the American College of Obstetrics and Gynecology (ACOG).

“People say that the medical profession has settled on this,” said Dr. Farr Curlin, the senior author of the study and a professor at the University of Chicago. “And what our data show rather clearly is that it is not at all settled among the medical profession.”

The definition of pregnancy can have a major impact on law and policy.

For example, embryonic stem cells are often derived from surplus embryos that aren’t implanted into a woman after in vitro fertilization.

And some contraceptives, such as intrauterine devices, prevent implantation. So if pregnancy is considered to begin at fertilization, “then you see why any technology that prevents implantation would be problematic,” Curlin said.

Federal policies have used implantation as the beginning of pregnancy.

For the survey, published in the American Journal of Obstetrics and Gynecology, Curlin and his colleagues sent questionnaires to more than 1,000 ob-gyns. The questions asked whether pregnancy begins at conception, at implantation, or if the doctor was unsure.

Most of the doctors, 57 out of every 100, said that pregnancy begins at conception, while 28 out of every 100 said it begins at implantation. The rest were unsure.

Implantation happens about a week after fertilization. That’s when the blastocyst, a tiny group of cells that will later become the foetus, embeds itself into the wall of the uterus.

Physicians who responded that they were religious or opposed to abortion or contraceptives that prevent implantation were more likely to believe that conception is the start of pregnancy.

Curlin said he was surprised that most of the doctors in his study disagree with ACOG, which is the leading organisation for this field of medicine.

ACOG did not respond to requests for comment.

“In this case, the science shows exactly what happens, but what you define as pregnancy is not what science can settle,” Curlin told Reuters Health.

One of the weaknesses in the survey is the use of the word “conception” rather than fertilization in the questionnaire. While conception is usually defined as fertilization, others interchange it with implantation, Curlin said.

The length of pregnancy - typically 40 weeks - remains the same regardless of how doctors define the beginning of pregnancy, because the 40 weeks are not counted from the time of fertilization or implantation, but from the time of the mother’s last menstrual period.

SOURCE:  American Journal of Obstetrics and Gynecology, online November 9, 2011