The number of newborn babies affected by a bacterial infection which can cause a fatal blood disease is underestimated, according to researchers.
Newborn babies are at risk from the infection
Group B streptococcal infection can lead to the potentially fatal blood poisoning condition sepsis.
But researchers say not all cases of Group B strep are detected by existing tests.
They estimate that there may be two to three times the number of cases being missed, meaning babies may develop sepsis unnecessarily.
Between 10 and 20% of premature babies who develop sepsis die. In full-term babies, the figure is around five to 10%.
We want every pregnant woman to be screened
Jane Plumb, Group B Strep Support
Researchers from St George's Hospital in London say doctors need to be aware that some cases of group B strep are not detected by the blood tests and cerebrospinal fluid assessment currently carried out.
Antibiotics 'not the answer'
They looked at the incidence of Group B strep in over 400 babies referred for screening for the condition within three days of being born.
The babies were classed as confirmed cases of early onset infection if a baby was younger than three days, tested positive for group B streptococcus, or had symptoms of sepsis, pneumonia, or meningitis.
They were classed as probable cases if they had a positive deep-ear swab for group B streptococcus, plus pneumonia, meningitis, or sepsis.
Based on their findings, the researchers estimated a rate of definite and probable infections of 3.6 per 1,000 live births, two to three times the current estimate.
Dr Alison Bedford-Russell, of the neonatal unit at St George's, who led the study, told BBC News Online: "In some babies, group B strep was there, but it didn't show up in the blood cultures or cerebral spinal fluid assessment."
But she said the answer was not to give all mothers antibiotics so their babies did not pick up group B strep
"In the US, group B strep is recognised to be a problem, and about 30% of mothers are given antibiotics in labour.
"There has been a reduction in group B strep, but now they have seen a rise in another organism that's even worse."
She added: "The answer to the problem is not to wham mothers with antibiotics just in case.
"It's to be astute and alert to what the babies might be suffering from."
Dr Jane Hawdon, a consultant neonatologist at University College London Hospital, said doctors and community staff needed to be aware poorly babies might have be infected with Group B strep.
"If babies show any sign at all of being poorly, you need to treat it as if it's an infection."
She said parents needed to be more aware of the risks of Group B strep.
"We often find a reluctance of parents to agree to infection screening or antibiotics treatment.
"Parents see a reasonably healthy baby in front of them, but if they've got a Group B strep infection, they could get sick very quickly.
"There also needs to be more awareness amongst parents."
Jane Plumb, chairman of the Group B Strep Support group said: "If the number of infections is higher than the accepted one in 1,000, that's even more support for a screening programme.
"We want every pregnant woman to be screened at 35 to 36 weeks. Those found to be carrying Group B strep, or who deliver before then, should be offered antibiotics during labour to prevent it being passed on to the baby."
The research is published in The Lancet.