A pregnant policewoman whose twins were delivered after her death suffered massive blood loss, a coroner has heard.

Pc Sarah Underhill, 37, collapsed and then died at Oxford John Radcliffe Hospital last October. Her twins James and Hannah were delivered by an emergency Caesarean section minutes after her death.

This morning, experts speaking at Oxford Coroner’s court said Mrs Underhill, of Didcot, died from amniotic fluid embolism.

This is when amniotic fluid enters the mother’s blood stream and, on rare occasions, this can lead to maternal collapse, massive blood loss, and is fatal within minutes.

Prof Sebastian Lucas, consultant pathologist at St Thomas’s Hospital London, said the condition was still regarded as an act of God.

He said the condition was undiagnosable and unpredictable.

He said: “Most of the AFE deaths are unpreventable and this is an example. There is no test you can do. It’s clinical guesswork. How the fluid gets into the maternal circulation, we don’t know. None of us have ever found the site of entry.”

Mrs Underhill, who conceived her twins through IVF, was admitted to hospital with pre-eclampsia, on October 2, 2008. She was due to give birth on October 6, but died in the early hours of October 5. Experts said the pre-eclampsia was not linked to her death.

The inquest continues.