Thousands of parents are missing out on the chance to discover why their baby died during pregnancy or shortly after birth, say campaigners.
They say that although post-mortem examinations are usually offered, a combination of red tape and long waits mean that most parents say no.
The Royal College of Pathologists and charity Sands is calling for more government money to improve services.
More than 6,500 UK babies are stillborn or die shortly after birth each year.
While some of these cases are explained, many are not and hospitals can offer post-mortem examinations.
Unlike a coroner's post-mortem examination, parents have the right to refuse an examination.
Sands, which offers support to families who have suffered a stillbirth, says that far too few examinations are being carried out.
In 9% of cases, according to official figures, a post-mortem examination is never offered and in total 61% of perinatal deaths do not lead to one.
The charity says the complexity of the process can also be off-putting for newly bereaved parents and the staff who are helping them.
Some consent forms used in maternity units were up to 25 pages long.
The strain on pathology services can also mean long delays before the post-mortem examination can take place, meaning it can be weeks before the body can be released for a funeral, the charity says.
One woman who spoke to Sands said she was told that because of a shortage of pathologists, there could be a delay of up to six months, and the body would have to be preserved in formalin in the meantime.
Neal Long, chief executive of Sands, said: "While the decision to consent to post-mortem is very much a personal one, high quality pathology and bereavement services are essential to ensure bereaved parents have informed choice, and are not needlessly discouraged from consenting."
He said that in some cases, the information gained at a post-mortem examination could make a big difference to the care available during future pregnancies.
Dr Phil Cox, a consultant perinatal pathologist at the Birmingham Women's Hospital, said post-mortem examination should be available in a "timely fashion" - but this was impossible in some areas.
"Even in the West Midlands, where we are relatively well-funded, the service is constantly very stretched, despite the currently low consent rate."
Currently, the cost of hospital post-mortem examinations are covered by NHS pathology services.
A spokesman for the Department of Health said: "It is tragic for a family to lose a baby and it is important that they have the information they need to understand the cause of death and any health implications for the family.
"The Government is aware of shortages of specialist perinatal pathologists - we welcome the move by the NHS to concentrate perinatal pathology resources and expertise in specialist centres, in order to ensure high quality services can be provided with minimal delay."