According to China's health ministry, 336 million abortions have been performed since the country instituted its one-child policy in 1971. How do researchers count abortions?
In countries where it's legal, they ask health care providers to document each abortion. Often, government health agencies ask (or, in China's case, require) abortion providers to submit information to a central agency about each abortion performed. China's abortion data has vacillated from year to year, which indicates that the country's official data may not be reliable. Health care providers and regional governments may have incentives to overreport or underreport abortions. Since prefectures must demonstrate their adherence to China's one-child policy, they may overreport abortion numbers. Unmarried women may be underrepresented in China's official numbers. Private-sector abortions, which are illegal but on the rise, are absent from governmental tallies.
More-reliable abortion data is available for other countries where abortion is legal. In some European countries, abortion providers are required by law to report abortions, or they must report abortions in order to be paid for the procedure by a governmental health care provider. (Private-sector abortions may still be left out of such tallies.) In some Northern European countries, where abortion is less stigmatized than it is in other areas of the world, abortion reports include information about the woman's age, gestational stage, type of procedure and type of facility where the abortion took place.
In the United States, the Centers for Disease Control and Prevention collects data from every state government's health agency (plus separate agencies in New York City and Washington, D.C.). In most states, facilities where abortions are performed are required to report abortions to state agencies, although state agencies aren't required to report their numbers to the CDC. The CDC encourages agencies to ask abortion providers for the woman's age, gestational stage, race, ethnicity, abortion type, the woman's marital status and the woman's previous abortions and births, but not every state collects all this information, and some states (California, Delaware, Maryland, and New Hampshire) don't report abortions to the CDC at all. A nonprofit called the Guttmacher Institute, however, approaches every known abortion facility nationwide to collect more-complete data. Guttmacher researchers first send questionnaires to abortion facilities, then they follow up with phone calls; the Guttmacher Institute also takes governmental tallies into consideration. According to the most recent Guttmacher report on American abortion incidence, 1.21 million abortions were performed in 2008.
In countries where abortion is illegal or heavily stigmatized, tallying abortions is much trickier. One common method of counting illegal abortions is to examine hospital reports of women admitted due to complications following an illegal abortion and supplement that data by surveying local reproductive health experts for their estimates of the number of illegal abortions performed. (Of course, not all abortion complications are reported as such in countries where abortions are illegal. Some abortion complications are reported as miscarriages, and the fact that it's difficult to distinguish between symptoms of mifepristone-induced abortion and miscarriage make this method even more challenging.) Other methods include surveying illegal abortion providers about the number of abortions they perform and surveying women about their personal reproductive history. Statistical methods can also be helpful; one researcher has used information on contraception availability and overall fertility to create mathematical models that predict abortion rates in areas where abortions are illegal.