Underscoring historic recent gains in global health, the number of children younger than 5 who die this year will fall to 7.7 million, down from 11.9 million two decades ago, according to new estimates by population health experts.

But as much of the world makes strides in reducing child mortality, the U.S. is increasingly lagging and ranks 42nd globally, behind much of Europe as well as the United Arab Emirates, Cuba and Chile.

Twenty years ago, the U.S. ranked 29th in the child mortality rate, according to data analyzed by the Institute for Health Metrics and Evaluation at the University of Washington.

The estimates, derived from modeling based on international birth records and other sources, are being published Monday in the British medical journal the Lancet.

Singapore, the country with the lowest child mortality rate in the world at 2.5 deaths per 1,000 children, cut its rate by two-thirds between 1990 and 2010. Serbia and Malaysia, which were ranked behind the U.S. in 1990, cut their rates by nearly 70% and now are ranked higher.

The U.S., which is projected to have 6.7 deaths per 1,000 children this year, saw a 42% decline in child mortality, a pace that is on par with Kazakhstan, Sierra Leone and Angola.

“There are an awful lot of people who think we have the best medical system in the world,” said Dr. Christopher Murray, who directs the institute and is an author of the study. “The data is so contrary to that.”

Even many countries that already had low child mortality rates, such as Sweden and France, were able to cut their rates more rapidly than the U.S. over the last two decades.

“It’s really just hard to fathom,” said Laura Beavers, national Kids Count coordinator for the Annie E. Casey Foundation, one of the nation’s leading advocates for children’s health.

The U.S. mortality rates defy traditional explanations, such as a nation’s diversity, high number of immigrants and persistent pockets of poverty, Murray said.

Australia, another diverse country with a large immigrant population, cut its child mortality rate over the last two decades more than the U.S. Australia now ranks 26th in the world.

Murray said high child mortality rates were not limited to black and Latino populations in the U.S. In fact, researchers have found high rates among higher-income whites, a group that traditionally has better access to medical care.

The data instead suggest broader problems with the nation’s fragmented, poorly planned healthcare system, Murray and other healthcare experts say.

Although the U.S. spends nearly twice as much per capita on healthcare as most other industrialized countries, researchers are finding substantially higher levels of preventable deaths from diseases such as diabetes and pneumonia.

Another recent study by the Institute for Health Metrics and Evaluation found that the rate of deaths among women giving birth has actually increased in the U.S. over the last two decades.

“We certainly have outstanding medical science and centers of excellence that rival the best in the world,” said Cathy Schoen, an expert on global health systems at the nonpartisan Commonwealth Fund. “But many other countries have been putting many more resources into thinking about how they can improve. … They have been far more strategic.”

That is one of the main reasons the recently enacted healthcare law is so important, many healthcare experts say. The bill not only expands insurance coverage but gives providers incentives to improve quality and better coordinate care and makes it easier for Americans to get preventive medical care.

There is more encouraging data about progress elsewhere in the world.

Although child mortality remains extremely high in several regions — including sub-Saharan Africa, where in some countries 1 in 7 children die before their fifth birthday — mortality rates are falling at an accelerating rate, according to the institute’s research.

That in part reflects efforts to expand vaccinations for diseases such as measles and to give antiretroviral drugs to pregnant women infected with HIV, said Dr. Mickey Chopra, chief of health and associate director of programs at UNICEF.

Chopra and others said initiatives to distribute mosquito netting to reduce malaria infections, provide Vitamin A supplements to children and encourage more breast-feeding are also having an effect.

Global public health advocates hope to be able to make more progress as efforts get underway to distribute antibiotics to combat pneumonia and dysentery in the developing world. “I am even more excited about the next five years,” Chopra said, adding that such progress was almost inconceivable a few years ago.

Researchers found the fastest rates of decline over the last two decades in many countries in Latin America and North Africa.

Other countries with slow rates of decline include Britain, New Zealand and South Korea, which have all fallen in the international rankings since 1990. All three are still ahead of the U.S.

noam.levey@latimes.com