The studies supporting this did not control for factors not related to abortion or miscarriage, and hence the causes of this correlation have not been determined, although multiple possibilities have been suggested.Some purported risks of abortion are promoted primarily by anti-abortion groups,
but lack scientific support. For example, the question of a link between induced abortion and breast cancer has been investigated extensively. Major medical and scientific bodies (including the WHO, National Cancer Institute, American Cancer Society, Royal College of OBGYN and American Congress of OBGYN) have concluded that abortion does not cause breast cancer.In the past even illegality has not automatically meant that the abortions were unsafe. Referring to the U.S., historian Linda Gordon states: "In fact, illegal abortions in this country have an impressive safety record. ": 25  According to Rickie Solinger,

A related myth, promulgated by a broad spectrum of people concerned about abortion and public policy, is that before legalization abortionists were dirty and dangerous back-alley butchers.... [T]he historical evidence does not support such claims. : 4 

Authors Jerome Bates and Edward Zawadzki describe the case of an illegal abortionist in the eastern U.S. in the early 20th century who was proud of having successfully completed 13,844 abortions without any fatality. : 59 
In 1870s New York City the famous abortionist/midwife Madame Restell (Anna Trow Lohman) appears to have lost very few women among her more than 100,000 patients—a lower mortality rate than the childbirth mortality rate at the time.
Rates of such abortions may be difficult to measure because they can be reported variously as miscarriage, "induced miscarriage", "menstrual regulation", "mini-abortion", and "regulation of a delayed/suspended menstruation".Forty percent of the worlds women are able to access therapeutic and elective abortions within gestational limits, while an additional 35 percent have access to legal abortion if they meet certain physical, mental, or socioeconomic criteria. While maternal mortality seldom results from safe abortions, unsafe abortions result in 70,000 deaths and 5 million disabilities per year. Complications of unsafe abortion account for approximately an eighth of maternal mortalities worldwide, though this varies by region. Secondary infertility caused by an unsafe abortion affects an estimated 24 million women. The rate of unsafe abortions has increased from 44% to 49% between 1995 and 2008. Health education, access to family planning, and improvements in health care during and after abortion have been proposed to address this phenomenon. Incidence
There are two commonly used methods of measuring the incidence of abortion:

Abortion rate – number of abortions annually per 1,000 women between 15 and 44 years of age; some sources use a range of 15–49. Abortion percentage – number of abortions out of 100 known pregnancies; pregnancies include live births, abortions, and miscarriages.In many places, where abortion is illegal or carries a heavy social stigma, medical reporting of abortion is not reliable. For this reason, estimates of the incidence of abortion must be made without determining certainty related to standard error.