Abortion is not considered one of the breast cancer risk factors, which include age, obesity, and family history. Research has found no link between abortion and an increased risk of breast cancer. While a small batch of studies may suggest a possible connection, an overwhelming amount of research indicates otherwise.

Concerns over a possible link between abortion and breast cancer have to do with changes in hormone levels during an abortion. Hormones like estrogen and progesterone can fuel the abnormal growth of breast cells.

There are two types of abortion:

  • Spontaneous abortion, or miscarriage, is the unintentional loss of a baby in the first five months of pregnancy.
  • Induced abortion is a procedure that’s done to end a pregnancy.

Researchers have studied the effects of both types of abortion on breast cancer, and they haven’t found a connection.

Many of the studies that find no link between abortion and breast cancer are prospective cohort studies. In these studies, researchers start with a group of women who don’t have breast cancer. Then they follow those women over time to see if they develop breast cancer.

One of the biggest studies on the subject was published in The New England Journal of Medicine in 1997. The study looked at 1.5 million women. Researchers adjusted for known breast cancer risk factors. They found no link between induced abortions and breast cancer.

Other studies have come to similar conclusions:

  • A 2004 analysis in The Lancet reviewed data from 53 studies that included 83,000 women with breast cancer. It found neither spontaneous nor induced abortions increased breast cancer risk.
  • A 2008 Archives of Internal Medicine study of more than 100,000 women also found no link between induced or spontaneous abortions and breast cancer incidence.
  • A 2015 review didn’t find enough evidence to confirm any link.

A few retrospective case-control studies have found a connection between abortion and breast cancer. These studies compare women who have breast cancer with women who don’t by asking about their past health history. It can be hard to get accurate results in these types of studies because some people may not remember exactly what they did in the past. Also, because abortion can be a controversial topic, some women may be hesitant to talk about it.

Some studies have found a link between abortion and breast cancer:

  • A 2014 Chinese meta-analysis published in Cancer Causes & Control looked at 36 studies and found that induced abortion was associated with breast cancer risk.
  • A 2012 Chinese study of 1,300 women also found a link between abortion and breast cancer.

Although the studies don’t all agree, many medical groups say most of the evidence does not show a link between abortion and breast cancer. These groups include the National Cancer Institute and the American College of Obstetricians and Gynecologists (ACOG).

An abortion is a medical procedure, and it can have risks. Some bleeding and cramping is normal afterward.

Signs of more serious side effects include:

  • excessive bleeding
  • severe pain
  • high fever
  • smelly discharge from the vagina

Complications from an abortion can include:

  • infection in the uterus
  • excess bleeding
  • damage to the cervix or uterus
  • incomplete abortion that requires another procedure
  • premature birth in future pregnancies

Women who are exposed to higher levels of estrogen — for example, because they have had their menstrual periods for a longer length of time or take birth control — have a slightly higher breast cancer risk.

Other risks include:

  • Age. Most breast cancers are diagnosed in women over 50.
  • Genes. Mutations to BRCA1, BRCA2, and other genes that run in families increase the risk.
  • Early periods or late menopause. The earlier a woman’s period starts and the later it stops, the longer her body is exposed to estrogen.
  • Late pregnancy or no pregnancy. Getting pregnant for the first time after age 30 or not having children can increase your risk.
  • Taking birth control pills or hormone therapy. These pills contain estrogen, which can encourage breast cancer growth.
  • Obesity. Women who are overweight or inactive are more likely to get breast cancer.
  • Alcohol use. The more alcohol you drink, the more your risk increases.

Regardless of any controversy over abortion policy, most medical groups agree that the procedure itself doesn’t appear to increase breast cancer risk.