When does pregnancy begin?

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

Most of the polled obstetrician-gynaecologists believe pregnancy begins when the sperm fertilises the egg. But a minority says it doesn't begin until a week later when the fertilised 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 fertilizations.

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," Dr Curlin said.

Implantation the beginning?

Federal policies have used implantation as the beginning of pregnancy. For the survey, published online in the American Journal of Obstetrics and Gynecology, Dr Curlin and his colleagues sent questionnaires to more than 1,000 obstetricians and gynaecologists.

Most (57%) said that pregnancy begins at conception, while 28% said it begins at implantation. The rest were unsure. Physicians who said they are religious or opposed to abortion or contraceptives that prevent implantation were more likely to believe that conception is the start of pregnancy.

Dr 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," Dr Curlin said.

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, Dr Curlin said.

(Reuters Health, Kerry Gren, November 2011)

Read more:

Pregnancy