Image copyright Getty Images Image caption Thirty-three maternal deaths per hour is too many, say experts

Maternal deaths have fallen worldwide, dropping by 45% since 1990, according to new figures.

The statistics, released by the World Health Organization, also provide new evidence on the causes of women dying in pregnancy or childbirth.

The WHO says most are preventable, highlighting the need for more investment in pregnancy care.

In 1990, more than half a million women died in pregnancy or childbirth - by 2013 the figure was 289,000.

But the WHO say the figure it still too high - with 33 maternal deaths around the world every hour.

The gap between rich and poor countries is wide. In sub-Saharan Africa, a 15-year-old girl has a one in 40 risk of dying during pregnancy or childbirth. In Europe, that risk is one in 3,300.

The WHO says the study highlights the need for poorer countries to invest more in health care.

But there is also evidence that in some rich countries such as the United States, maternal mortality is rising, say health experts.

More than one in four maternal deaths is caused by pre-existing conditions which complicate pregnancy, such as diabetes, HIV, malaria and obesity.