GENEVA, Oct 29 (Reuters) - Nearly 40 percent of the 128 million babies born worldwide every year are not officially registered, and two thirds of deaths also go undocumented, the World Health Organisation (WHO) said on Monday.

The U.N. agency said incomplete birth and death registries in many developing countries "means they cannot count how many people are born and how many die, and they cannot record how long they live or what kills them".

"When deaths go uncounted and causes of death are not documented, governments are unable to design effective health policies, measure their impact or know whether health budgets are being well-spent," it said in a statement.

Only 31 of the WHO's 193 member states are believed to have reliable cause-of-death statistics.

The Health Metrics Network -- which is supported by the Bill & Melinda Gates Foundation and the British, U.S. and Danish governments -- is helping Cambodia, Sierra Leone and Syria improve their civil registration systems.

Sally Stansfield, executive secretary of that WHO-hosted network, said surveys and surveillance projects had helped fill in the gaps in some countries though serious problems remained.

"It's a major challenge," she said.

Children whose births are not registered are less likely to benefit from basic human rights, social, political, civic and economic, the WHO said.

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