Vaginal births can quickly go wrong if, for example, the cord wraps around the baby’s neck.

The study was led by doctors from China’s national statistics office, Peking University, Harvard Medical School and New York University’s Wagner School of Public Service.

The 2010 W.H.O. survey erred by relying on figures from just 21 hospitals, mostly in cities, said Dr. Jan Blustein, a health policy specialist at N.Y.U. and co-author. The new study tallied 90 percent of the country’s births over seven years.

While a few cities like Shanghai had astronomical rates, half of all Chinese still live in rural areas where home births are more common and distances to hospitals are greater, so actual rates are closer to the ideal 10 to 20 percent range.

In Tibet, however, C-sections are so rare that mothers and children who could be saved from death are undoubtedly not getting the operations, Dr. Hellerstein said. C-sections save lives in breech or multiple births, for example, or when a mother has dangerously high blood pressure or a fetal heartbeat fails.

But babies born by C-section are more likely to hospitalized for breathing problems and more likely to suffer asthma and obesity later in life, possibly as a result of not getting microbes present in the birth canal. Mothers who have had C-sections also are more likely to hemorrhage or to have a uterine rupture in the next pregnancy.

Since 2009, China has been trying to control medically unnecessary cesareans by educating patients, doctors and midwives, and by warning individual hospitals when their rates are too high, said Dr. Jianmeng Liu, director of the Office for National Maternal and Child Health Statistics of China and a study co-author.

China’s C-section rate, the authors noted, is close to that of the United States, where it is slightly over 32 percent.