UNITED NATIONS -- If the developed world doesn't invest in efforts to close the widening gap between rich and poor, in less than 15 years 167 million children on the planet could be living in extreme poverty and 750 million women will have been forced into marriage as children, warns the United Nations children's agency.

UNICEF's "State of the World's Children" highlights the state of -- and risks of -- allowing the divisions to deepen not only in the poorest countries, but in the developed Western world.

"The United States has a record of the starkest inequality in education and in neonatal survival, and that's a reflection of underlying inequality and racial disparities," Dr. Kevin Watkins, principal author of the report and Director of the Overseas Development Institute, told CBS News. "It is not a financing issue in the U.S, it's a fairness issue."

Watkins, a Brit, says the same division between rich and poor was behind his own nation's shock Brexit vote to leave the European Union.

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"If you look at the major determinants of how people voted in the United Kingdom, there are some things that really stand out: People with high levels of education were much more likely to vote remain, people with lower levels of education were much more likely to vote for Brexit."

"Unfortunately there is that interconnection between rising inequality and disparity in opportunity and what we have seen happening in the U.K.," he told CBS News.

The 172 page UNICEF report has some heart-breaking tales highlighting the dismal circumstances facing the world's next generation.

One of the most shocking figures it contains is the prediction that three-quarters of a billion women will be child brides by 2030.

Marixie Mercado, media chief at UNICEF, says that while some progress has been made in recent years to address the problem, the increase in population and the unequal distribution of resources to the poorest countries means the overall number of child brides will not decrease.

Asia Khatun, from Bangladesh, is one of the human faces behind the worrying statistic.

Khatun was married two years ago at the age of 16 in an arranged marriage to Joylal, who was twice her age at 32. She had baby Siam, now 10 months old, with him. She was in school though the fifth grade, but had to leave to get a job in the garment sector as a messenger, where she worked from the age of 14 for two years, earning $50 a month.

Khatun told CBS News her husband's uncle arranged the marriage and asked for a dowry of 30,000 taka (around $380), which her family couldn't afford to pay. That led to their separation.

"I had the baby six months later at home, because we could not afford a hospital, and Joylal visited only once," she said. "I was afraid to get married. I wanted to establish myself first, but my auntie convinced my family, and they agreed, so I did it for the sake of my family and my parents' happiness."

"Now, I stay home and cook for my family; I am happy with Siam," Khatun said.

She would like to go back to school, but she's 18 now and there are no opportunities for adult education in her country.

Bangladesh, the UNICEF report says, has made solid progress in many areas such as reducing mortality rates, but it stands as a clear example of the disparities in wealth distribution. Access to antenatal care is just 36 percent for the poorest women, while 90 percent of the wealthiest Bangladeshis get the care they need.

Complications during pregnancy and childbirth are the second leading cause of death for girls between ages 15 and 19.

Jo Bourne, UNICEF's Global Chief of Education told CBS News there are 650 million primary and secondary-age school children globally; 124 million are out of school, and 40 percent of those in school are not learning to read, write or do simple arithmetic.

"What we have seen in the total number of children out of school is stagnation," Bourne said. "Countries are running to keep up, but with population growth, they're not running fast enough to improve on those numbers."

UNICEF hopes the report will be a call to action. The report calls for more information on those left behind by global development, and for new financing to address it.

"Inequity is neither inevitable, nor insurmountable," the report says.