Colette Murphy remembers walking through the hospital in her 34th week of pregnancy, pleading with a registrar to allow her to come back and have her baby checked every week. "But he told me the baby was head down, really growing well, and said I was probably going to have a nice big healthy 7lb baby."

Four weeks earlier, when she had her routine, 30-week check-up, the consultant noticed the baby's heart rate had slowed, and Colette was admitted to hospital for a week. Her baby's heart rate came back up, but Colette was tracked carefully until week 34. "But this registrar insisted they now didn't need to see me for a month - his exact words to me were, 'Well, what can go wrong? Nothing can go wrong!'"

At 37 weeks, Colette went into labour. When she arrived in hospital, the midwives couldn't find the baby's heartbeat. "After two hours of tests, they told us our baby was dead. Our little boy Ethan was delivered that evening. He had blond hair, he was perfect, but teeny-tiny, just 4lb 5oz."

Eleven babies a day are stillborn in the UK - and the overall number has risen in each of the past two recorded years. Babies in multiple pregnancies, babies of older mothers and obese mothers are known to be at greater risk. But Colette was in none of these categories. Ethan fell into the 70% of stillbirths which are officially recorded as "unexplained".

"We lose 10 times more babies to stillbirth than to sudden infant death," says Professor Jason Gardosi, director of the NHS's Perinatal Institute. "Sids cases are very important for us to try to understand, but stillbirths happen to many more families and need at least as much recognition of their importance."

Sids campaigners have achieved a remarkable 70% drop in the number of cot deaths in the past 20 years, largely due to the Back to Sleep campaign. Professor Gardosi is frustrated at the lack of an equivalent push to reduce the number of stillbirths.

He believes a large number of the stillbirths categorised as "unexplained" are signposted by failure to grow in the womb and thus potentially avoidable. His research quotes the finding that in the UK only about 25% of babies who are small for gestational age are picked up antenatally in routine practice - a figure that drops to 15% if the pregnancy is considered "low risk".

"There is no national strategy in place. Consistently, year on year, we found two-thirds of stillbirths categorised as unexplained. But when we looked, we found the majority were babies who didn't reach their growth potential. Many stillbirths are of quite mature babies, but whose weight is well below the normal limit."

Normally, after the 12- and 20-week scans, midwives will measure a woman's bump either with their hands or using a tape-measure to check fundal height, ie the distance between the pubic bone and the top of the uterus. "We calculate roughly the bump should grow one centimetre per week of pregnancy," says community midwife Fiona Carver. "But we might wait until it's about four centimetres less than we expect it to be before we organise another scan to check the baby's growth."

But the one-centimetre-a-week rule doesn't take a lot of the variables into account. Is it a first pregnancy? If yes, the bump is likely to be smaller. What's the mother's ethnic group? Generally, a Bangladeshi mother is going to have a smaller baby than the average Caucasian mother.

"We've tracked 152,000 pregnancies here in the West Midlands with our customised antenatal growth chart," says Pat McGeown, head midwife at the Perinatal Institute. "We draw up the potential growth curve for this particular baby, considering the mother's height and weight, her ethnic origin, which pregnancy it is and the size of any previous babies. We've trained midwives to do the fundal height measurement every two to three weeks from 26 weeks onwards. Ideally the measuring is done by the same person."

But although the customised growth chart is freely available to download, there is no standardised record-keeping, and most maternity units still use the one-size-fits-all way of measuring a baby's growth.

"Mostly the growth chart serves to reassure mothers when everything is going fine," says Jason Gardosi, "but we feel very strongly that we need to try to improve the antenatal detection of the babies that are not growing well - and we need to make people aware that here is something that can be done about a problem that is potentially avoidable. If we see a baby is not following the curve, we can do further tests, and if necessary, we can choose to deliver it early. We believe this could have a major impact on the incidence of stillbirth."

Peter Soothill, professor of maternal and fetal medicine at the university of Bristol, says: "Monitoring the growth of unborn babies is a very important part of antenatal care. It does assume all foetuses grow at the same rate, which is clearly not correct. However, when adjusting for an individual woman's circumstances, still one in 10 normal pregnancies could possibly be identified as having fetal growth restriction and we have to be careful not to generate unnecessary anxiety.

"Babies who are stillborn are much more often lighter than expected and so detecting and monitoring fetal growth is important. However, there are many different causes of being small, including those with inadequate placental function and others who are completely normal small babies. Size alone should probably not be regarded as a cause of stillbirth, but it is a feature which may help us understand the causes of stillbirth and help prevent it in future."

Colette Murphy's next pregnancy was carefully supervised, and her daughter Bronte, now four, was born at 8lb 13oz. But she recalls, "When I went back to the hospital, the consultant looked at the notes for both my son and my daughter. She said, 'Your son's life could have been saved. If you'd been monitored more closely he'd be here with you today.'"

· Perinatal Institute: www.perinatal.nhs.uk Customised growth chart downloadable from www.preg.info Stillbirth and Neonatal Death Society (Sands) Helpline: 020 7436 5881. Website: www.uk-sands.org