American women are more likely to die in childbirth than they were two decades ago, making the United States one of the few countries where the risks from childbirth have risen in the past generation, World Health Organization data showed on Tuesday.

No other country recorded such a large percentage increase, although a few other rich countries also failed to keep maternal mortality in check. In Canada, deaths rose from 6 to 11 per 100,000 births between 1990 and 2013. Many European countries and Japan have mortality rates in single figures.

The WHO tracks maternal mortality as one of the "Millennium Development Goals" that the United Nations set for 2015. Death rates have fallen by 45 per cent globally since 1990, to an estimated 289,000 women in 2013.

Giving birth in the United States remains far safer than in most countries, with only 28 maternal deaths per 100,000 live births in 2013. But that is 136 per cent higher than the 1990 mortality rate, when only 12 mothers died for every 100,000 births, the data showed.

China has cut its rate by two-thirds since 1990, with 32 women dying for every 100,000 live births in 2013.

WHO experts said the increase in the U.S. mortality rate may be a statistical blip. Or it might be due to increased risks from obesity, diabetes and older women giving birth.

Marleen Temmerman, the director of reproductive health and research at WHO, said more analysis was needed.

"It's difficult to say how many deaths are really related to increasing age, but what we know is older age in pregnant women is contributing more to the risk for diabetes and more hypertension related problems," she told a news conference.

WHO death-rates expert Colin Mathers said improved data collection could also affect the figures.

"There's also a situation in the U.S. where there are a considerable proportion of the population without health insurance. That may also be affecting things," he said.

Globally, most maternal deaths are caused by severe bleeding, high blood pressure, infections and obstructed labour. More than one in four is caused by malaria, HIV/AIDS, obesity or diabetes. Abortion complications account for 8 per cent of deaths.

Although the world will miss the U.N. target of a 75 per cent decline from 1990, many countries have made big improvements, according to the data, published jointly by the WHO, UNICEF, the U.N. Population Fund and the World Bank.

In 1990, 23 countries had at least 1,000 maternal deaths per 100,000 live births. By 2013, only Sierra Leone remained above that threshold, but it had still managed to more than halve its death rate over the period.

Two countries — India and Nigeria — account for more than a third of the total number of deaths globally. Sub- Saharan Africa remains the most dangerous place to have a baby.

Maternal mortality has worsened in a handful of poor countries — the Philippines, Suriname, Cuba, Venezuela and Tonga. Almost everywhere else has slashed death rates, although the WHO says the annual death toll remains far too high.

"It's still like two plane crashes every day," Temmerman said.