LAGOS, Nigeria -- 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 public 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.
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."
In South Africa, Nozipho Goqo, an unemployed 19-year-old from Johannesburg, gave birth Monday to a boy, her first child. She gave him a Zulu name -- Gwakwanele -- that means "enough."
A nurse at Charlotte Maxeke, a sprawling teaching hospital that serves a large region in and around the city, teased Goqo that she was too young to know whether this would be her last baby. Goqo smiled and said she was sure.
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.
India, which struggles with a deeply held preference for sons and a skewed sex ratio because of millions of aborted female fetuses, is using the day to highlight that issue.
Meanwhile, China, which at 1.34 billion people is the world's most populous nation, said it would stand by its one-child policy, a set of restrictions launched three decades ago limiting most urban families to one child and most rural families to two.
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