Abortion and Breast Cancer Risk

Scientists have been studying for decades whether abortion increases the risk of breast cancer. Some studies have suggested that it does, but those studies had major limitations. Higher-quality, more reliable studies show that abortion doesn’t affect breast cancer risk. 

Medical organizations like the American Cancer Society, the National Cancer Institute, and the American College of Obstetricians and Gynecologists agree that induced abortion doesn’t affect breast cancer risk. (Induced abortion is what you probably think about when you think of abortion: It’s when a pregnant person chooses to end their pregnancy, either with medicine or surgery.) 

Studies have also found no link between breast cancer and miscarriage, which is sometimes called spontaneous abortion.

Some factors related to the reproductive system — like your age at your first period and whether you’ve used hormonal birth control methods like the pill or an IUD — can affect your breast cancer risk. Researchers have wondered whether abortion might have a similar effect. 

Pregnancy and ending a pregnancy both change estrogen and progesterone levels in the body. Since these sex hormones can fuel hormone receptor-positive breast cancer, the thought is that abortion could throw these hormones out of whack and increase the risk of breast cancer.

Specifically, sex hormones surge during pregnancy, leading to permanent changes in breast cells during the first trimester that get them ready to make milk. If a pregnancy is ended during this time, these changes stop, which some believe could increase the risk of cancerous changes in the breast cells. But research shows this isn’t the case.

Politics have also played a role: Opponents of abortion have claimed for decades that abortion increases breast cancer risk and that this is a reason to ban the procedure. But these campaigns aren’t based on credible science.

Scientists are confident there’s no link between breast cancer and abortion. Nevertheless, some U.S. states mandate that doctors warn people of a breast cancer risk before they have an abortion.

Large prospective studies show that having an abortion doesn’t increase the risk of breast cancer.

Prospective studies are considered more trustworthy because they look forward instead of backward: Instead of drawing conclusions about things that have already happened, they track events as they happen, which makes it easier for researchers to gather accurate data. The prospective studies about abortion include participants who have and haven’t had an abortion, then track over the years whether they develop breast cancer.

One of the most significant prospective studies was published in 1997 in the The New England Journal of Medicine. Researchers analyzed national health records — including abortion and breast cancer information — of 1.5 million Danish women born between 1935 and 1978. They found that there was no relationship between abortion and breast cancer risk.

More recently, a 2018 meta-analysis study — which is a study that analyzes data from a group of studies — also found no link. Published in the journal Medicine, the study analyzed all high-quality data that had been collected on abortion and breast cancer risk up to that point. It found that, among women who had never given birth, there was no relationship between having one abortion — or two or more abortions — and breast cancer. 

Certain studies have found that induced abortion increases breast cancer risk. However, many of these are too small to be useful. Some lump miscarriage and induced abortion together, even though they have different effects on the body. And some don’t consider potentially important factors like the participant’s age at the time of the abortion or whether they’ve given birth before — which are important factors because they can affect risk factors like sex hormones.  

These are also mostly retrospective studies, meaning they look back at participants' pasts to compare those with breast cancer and those without. In a retrospective study, researchers ask both groups whether they’ve ever had an abortion. However, since participants may misremember or choose not to disclose whether they’ve had an abortion due to stigma, these studies are less reliable. 

In the past, and sometimes still today, miscarriage has been referred to as a spontaneous abortion. This is why some researchers have lumped together miscarriage and induced abortion when studying their effect on breast cancer risk. Although the two have similar effects on the body, there are differences, including on sex hormones. Still, research shows that miscarriage also does not increase breast cancer risk.