A study of two women in West Yorkshire shows they died because they had sex too soon after giving birth.

Both women were in their 20s and had had several children.

The first, aged 22, died eight days after having her third child.

The second, aged 29, died five days after giving birth to her fourth child.

Both women had had a normal delivery and collapsed while having sex.

The researchers said death was due to air bubbles or embolisms being introduced into the body and travelling up to the heart and brain.

This was because the blood vessels in a woman's pelvic area widened during pregnancy and took around three weeks to readjust after birth.

The researchers said it was more common for women to have air embolisms if they had deep penetrative sex with their knees against their chest.

This was because the uterus would be above the right wall of the heart, increasing air pressure on the heart.

Very uncommon

The researchers stressed that these kinds of deaths were very uncommon.

Middle-aged men with heart problems were much more likely to die during sexual intercourse.

For women, other causes of death by air embolism during or after birth include induced abortion, oral sex and forceps delivery.

Dr Philip Batman, one of the pathologists involved in the study, said this was because all of these introduced air into the woman's bloodstream.

He said doctors used to advise women to refrain from sex after birth for around six weeks - until they had attended an after-birth clinic.

But new, more general guidelines suggested that women should abstain until they felt comfortable.

Dr Batman favoured the former advice and said women should be warned of the small risk factor.

"There are risks to all things and it would be unusual if there weren't, " he said.