The Confidential Enquiry into Maternal and Child Health (CEMACH), found there were 3,600 stillbirths in 2005.

Stillbirths are deaths of unborn babies after the 24th week of pregnancy.

They are commonest among the under-20s and over-40s and ethnic minorities.

Numbers could rise as more women give birth later, CEMACH warned.

CEMACH looked at pregnancies in England, Wales and Northern Ireland.

Stillbirths declined steadily from the 1950s to the 1990s. Since then, there has been some success in reducing the number of stillbirths in multiple pregnancies, and in babies born prematurely, but the incidence in singleton pregnancies has remained level.

There were 668,681 live births in England, Wales and Northern Ireland in 2005.

Overall, the stillbirth rate was 5.5 per 1,000 births. A third of stillbirths occurred when a baby had reached full-term.

For women under 20, the rate is 6.6 per 1,000 and for those over 40 it is 7.2 per 1,000 births.

Black and Asian women are up to twice as likely to have a stillborn baby when compared with white women, the report said.

In over 50% of cases, doctors do not know why a baby is stillborn, although there is a theory that many of the cases could be linked to a baby failing to grow properly in the womb.

Known causes of stillbirth include congenital malformations, where a baby's brain, heart or other organ have failed to develop properly; maternal haemorrhage; or asphyxiation during childbirth.

'Deep-seated reasons'

In terms of non-medical factors, deprivation appears to be a major contributory factor, with mothers in very poor areas twice as likely to have a stillborn baby or to have babies who die in the first month of life.

Unplanned home births also carry a risk, the report said.

The researchers say that the quality of research into the causes of stillbirth has been affected by a fall in the number of post mortem examinations.

In 1993, 58% of stillbirths and deaths in the first four weeks of life after birth resulted in a post mortem, but the figure fell to 39% in 2005.

The report also called for better targeting of women at risk of their baby dying, including those who are socially deprived and from ethnic backgrounds.

The report said: "Given the steadily increasing average maternal age in the UK, maintaining perinatal mortality at current rates may be a challenge in the years ahead."

"Public health initiatives aimed at reducing the incidence of stillbirth require a better understanding of its causes."

Tragic problem

Richard Congdon, chief executive of CEMACH, said: "The rapid improvement in recent years in the survival prospects of babies who have been born very prematurely is highly welcome.

"But unfortunately there seem to be deep-seated reasons why we are not seeing the same improvement in the number of stillbirths.

"We believe this needs attention."

Jim Dornan, vice president of the Royal College of Obstetricians and Gynaecologists (RCOG), added: "The results of the latest CEMACH study of perinatal mortality shows that the occurrence of stillbirth is still a major and tragic problem in this country.

"Advances in technology mean that neonatal mortality through premature births has fallen but stillbirths present us with a challenge.

"We need to know more about why stillbirths happen.

"The best way forward is to collate the key data we need to identify the patterns and associations behind stillbirth so that we can discover the most promising areas for further research into its prevention."

A Department of Health spokesman said: "The rate of stillbirths, despite much research, has remained at a similar and stubborn level to that of the early 90's.

"Research has not yet pinpointed a specific cause for this phenomenon which is why it is essential that vital work into this area continues."