Former Irish President Mary Robinson was just making polite conversation when she asked an Ethiopian teenager about her wedding day.

The 16-year-old had already been married a year.

"She looked at me with the saddest eyes and said, 'I had to drop out of school,'" Robinson said in a telephone interview.

"That conveyed to me the reality," said Robinson, the first woman to serve as Ireland's president and former U.N. high commissioner for human rights. "Her life, as far as she is concerned, had more or less ended."

Robinson said keeping girls in school was one of the most important things policymakers could do to address the coming challenges of an ever-increasing population, predicted by the United Nations to reach 7 billion at the end of the month.

"European countries are concerned about aging populations as is Japan, but this is much less of an issue than the huge bulge of people which we are going to see over the next 40 years when the population goes from 7 billion to 9 billion people," she said.

"Almost all of that increase will be in poor developing countries, so that we have a very big demographic challenge."

Family planning experts worry in particular about the looming population boom in sub-Saharan Africa.

In May, the United Nations projected the world population would reach 9.3 billion in 2050 and 10.1 billion by 2100. Much of that growth will come from Africa, where the population is growing at 2.3 percent a year -- more than double Asia's 1 percent growth rate. If that rate stays consistent, which is not certain, Africa's population will more than triple to 3.6 billion by 2100 from the current 1 billion.

Joel Cohen, a professor of population studies at Rockefeller University and Columbia University in New York, said universal secondary education offered a way to reduce population in high-fertility regions.

In addition to providing information about contraception, a secondary education motivates women to reduce their own fertility, improve the health of their children and allows them to move from a mind-set of having many children in the hopes that some will survive to improving the quality of each child's life, Cohen wrote in the journal Nature.

(Read by Emily Cheng. Emily Cheng is a journalist at the China Daily Website.)