Blood tests to determine the gender of a baby are 98 per cent accurate, provided they are carried out after the seventh week of pregnancy, scientists have revealed.

The study is one of the first into the effectiveness of the tests, which are sold privately in the UK.

US experts from the National Institute of Health, Bethesda, looked at 6,000 test results, and confirmed they were accurate in 98 per cent of cases. However, urine-based tests were completely unreliable, they reported in the Journal of the American Medical Association.

Dr Stephanie Devaney, who led the study, said such blood tests could be useful in clinical settings to assist in early detection for genetic conditions.

Critics have warned that such tests could be used for "family balancing", where couples might decide to continue with the pregnancy only if the sex of the baby fits with what they want.

 Baby blood test small