LAGOS, Nigeria (AP) - One South African mother, just 19, named her newborn "Enough" and shrugged off a nurse who questioned whether she was old enough to know how many children she wanted.
In Nigeria, newborn twins have to share a bassinet in a crowded hospital that doesn't have enough electricity.
"Where there is life, there is hope," their mother said. But as the world's population surpasses 7 billion, fears were stirred anew about how the planet will cope with the needs of so many humans.
The United Nations marked the milestone Monday, even though it is impossible to pinpoint the arrival of the globe's 7 billionth occupant because millions of people are born and die each day.
At Lagos Island Maternity Hospital, the strain of caring for a burgeoning population was evident. The droning roar of a generator could be heard throughout one hot ward, where it powered ceiling fans and incubators. While Nigeria is oil-rich, it does not produce nearly enough power for its more than 160 million people.
Seun Dupe, a 32-year-old hairdresser who gave birth to the twins on Oct. 23, remained an optimist despite the staggering burden facing Africa's most populous nation and other developing countries. Her babies spent Monday squirming beneath a bundled-up mosquito net. She has yet to decide on their names.
Dupe was confident that new lives will ensure Nigeria's future as "a great nation."
Nigeria's megacity of Lagos is expected someday to surpass Cairo as the continent's most populous.
U.N. Secretary-General Ban Ki-moon said the day was "not about one newborn or even one generation" but "about our entire human family."
At a news conference in New York, he noted "a world of contradictions" - famine in the Horn of Africa, fighting in Syria and elsewhere and widespread protests against economic inequality.
"Seven billion population is a challenge," he said, and "at the same time, an opportunity, depending upon how the international community prepares for that challenge."
Demographers say it took until 1804 for the world to reach its first billion people and a century more until it hit 2 billion in 1927. Soon the numbers began to cascade: 3 billion in 1959, 4 billion in 1974, 5 billion in 1987, 6 billion in 1998.
The U.N. estimates the world population will reach 8 billion by 2025 and 10 billion by 2083. But the numbers could vary widely, depending on life expectancy, access to birth control, infant mortality rates and other factors.