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.
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."
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.
Across the maternity ward, Dora Monnagaratoe cuddled her newborn son in a bed. The 40-year-old maid named her fourth baby Tebogo, or "we are thankful" in the Sotho language.
"It's three girls and one boy now," said Monnagaratoe, herself one of eight children. "It's fine."
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.
In Uttar Pradesh, India – the most populous state in the world's second-most populous country – officials said they would appoint seven girls born Monday to symbolize the 7 billion.
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.
"It would be a fitting moment if the 7 billionth baby is a girl born in rural India," said Dr. Madhu Gupta, a gynecologist. "It would help in bringing the global focus back on girls, who are subject to inequality and bias."
According to U.S. government estimates, India has 893 girls for every 1,000 boys at birth, compared with 955 girls per 1,000 boys in the United States.
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.
"Overpopulation remains one of the major challenges to social and economic development," Li Bin, director of the State Population and Family Planning Commission, told the official Xinhua News Agency. He said the population of China would hit 1.45 billion in 2020.
While the Beijing government says its strict family planning policy has helped propel the country's rapidly growing economy, it has also brought many problems.
Soon, demographers say, there won't be enough young Chinese to support its enormous elderly population. China, like India, also has a highly skewed sex ratio, with aid groups saying sex-selective abortions have resulted in an estimated 43 million fewer girls than there should be, given the overall population.
India, with 1.2 billion people, is expected to overtake China around 2030, when the Indian population reaches an estimated 1.6 billion.
Associated Press writers Edith M. Lederer at the United Nations, Donna Bryson in Johannesburg, Tim Sullivan in New Delhi and Biswajeet Banerjee in Lucknow, India, contributed to this report.Below, see stunning photos from National Geographic of populations around the world. National Geographic magazine's year-long series on population is now available as a free app for iPad. Check it out here or visit their website.