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,” said Mads Melbye from the University of Copenhagen’s Department of Clinical Medicine, lead study author. “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.