Over all, Unicef estimates that more than 125 million girls and women have undergone the practice and that 30 million girls are at risk of it over the coming decade. The report, released Monday, is the first in which Unicef assessed the practice among all age groups based on household survey data from all of the 29 countries. Its last report, issued eight years ago, was based on 30 surveys in 20 of the countries; the new study includes 74 surveys done in 29 countries over two decades.

The report depicts progress against female genital cutting as halting and uneven. It also offers a portrait of nations where its prevalence is still stunningly high. In addition to Egypt, where 91 percent of women 15 to 49 have undergone the practice, countries with the highest percentages of women who have been cut include Somalia, at 98 percent; Guinea, at 96 percent; Djibouti, at 93 percent; Eritrea and Mali, at 89 percent; and Sierra Leone and Sudan, at 88 percent.

Unicef found that the steepest declines in the prevalence of the practice, also known as female genital mutilation, have occurred in Kenya, one of Africa’s most dynamic and developed nations, and — most surprisingly — in the Central African Republic, one of its poorest and least developed.

Researchers now say the prevalence of the practice in these two countries began to fall four or five decades ago. They said the progress made sense in Kenya, where efforts to stop female genital cutting stretch to the early 1900s, but they were at a loss to explain why it had plunged in the Central African Republic, to 24 percent in 2010 from 43 percent in the mid-1990s.

“We have no idea, not even a guess,” said Bettina Shell-Duncan, an anthropology professor at the University of Washington who was a consultant on the report. Professor Shell-Duncan said researchers needed to get to the Central African Republic soon to figure out what was happening there.