Tests for diabetes in pregnancy – which affects the developing baby – are taking place too late, warn scientists.
Untreated, the condition can increase the risk of a stillbirth and other complications.
Most screening takes place at 28 weeks, but a University of Cambridge study of 4,069 women showed the foetus was already affected by then.
Charities said gestational diabetes was involved in a “significant number” of potentially avoidable stillbirths.
Gestational diabetes is common and affects up to 18 in every 100 pregnancies.
The extra sugar in the bloodstream acts as “baby fuel” leading to rapid growth inside the womb.
Most babies are normal and healthy but the condition increases the likelihood of a large baby, which can be difficult to deliver, suffering bone fractures.
The babies can also be at higher risks of obesity and diabetes later in life.
The study, published in the journal Diabetes Care, showed excessive foetal growth had already started by the usual time of screening.
Women testing positive for gestational diabetes at that 28-week stage were twice as likely as other mothers to have an abnormally large foetus.
Mothers who were obese as well as having gestational diabetes had five times the risk of a large foetus.
Prof Gordon Smith, one of the researchers, told the BBC News website: “The recommendations are that screening should take place at some point between 24 and 28 weeks, but in practice a lot screen at 28 weeks.
“Our findings indicate that it should be brought forward to 24 weeks and that would still be consistent with existing guidelines.
“And we should possibly be doing a second, earlier, screening test for early onset of the disease – but that needs further research.”
There were no signs of large babies at 20 weeks.
‘Early is key’
Dr Daghni Rajasingam, from the Royal College of Obstetricians and Gynaecologists, said: “This study emphasises the importance of early detection and diagnosis.
“There is growing awareness for the need to screen earlier, but further research should assess the ideal timing of screening and the impact this has on the child’s health.
“It is important to emphasise that immediate changes to lifestyle, including a healthy diet and moderate levels of exercise, can have significantly positive effects on a woman and her baby’s health.”
Janet Scott, from the stillbirth charity Sands, said: “We know from recent enquiries that failure to screen for gestational diabetes currently plays a part in a significant number of potentially avoidable stillbirths at term.
“Good risk assessment is crucial to avoiding harm to mothers and babies and we welcome these important findings which have real potential to inform better antenatal care for these high-risk pregnancies.”
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Read more: www.bbc.co.uk