Week 34 of pregnancy reduces breast cancer risk: study

Women's bodies undergo a "striking" change during a specific week of pregnancy that can significantly reduce their risk of developing breast cancer later in life, scientists said Tuesday.

Previous research has highlighted how women under the age of 30 can reduce their risk of contracting breast cancer later in life by having a baby.

But a new study by experts in Denmark and Norway claims to have identified the precise week of pregnancy when the change occurs.

"If you deliver a child at week 33 you get the child, which is great, but you don't get the bonus of having a lower risk of breast cancer for the rest of your life," Mads Melbye, from the University of Copenhagen's Department of Clinical Medicine and lead study author, told AFP.

"It's a very distinct change in risk when you go from week 33 to week 34." A typical pregnancy lasts 40 weeks and a baby born before 37 weeks is considered premature.

Melbye and his team studied a huge database of nearly four million women in Denmark and Norway stretching back almost 40 years.

It listed the age at which each of them gave birth, how far into a pregnancy each birth occurred and whether or not they contracted breast cancer later in life.

They found that women who gave birth after 34 weeks had an average 13.6 percent lower risk of developing breast cancer compared to women who had no children.

For pregnancies that ended a week earlier, the reduction in risk -- while still there -- was only 2.4 percent.

Melbye said what changes in women during this vital week of gestation remains a mystery.

"We don't know what that is, but knowing that you have to get to this point of the pregnancy makes it much easier for researchers to focus because we have to know what happens around that week to understand this," he said.

He said it may be possible a woman's body sends a signal after 34 weeks of pregnancy to boost immunity against environmental causes of breast cancer. "To the best of our knowledge it must have something to do with a specific biological effect that the cells reach at 34 weeks."