Obesity and diabetes during pregnancy can cause "abnormally large" babies, putting the mother at risk during childbirth, according to a new study.
The condition also increases the likelihood the child will develop obesity and diabetes during later life, the study showed.
Research led by the University of Cambridge found that obese women who develop diabetes while pregnant - known as gestational diabetes - were five times more likely to be overly large by the sixth month of pregnancy.
Guidelines in the UK recommend that mothers at risk of developing gestational diabetes be offered a test between 24 and 28 weeks into pregnancy.
In practice, however, most women are screened at the 28 week mark, by which point, according to the study's findings, it would be too late.
"Our study suggests that the babies of women subsequently diagnosed with gestational diabetes are already abnormally large by the time their mothers are tested for the disease," Dr Ulla Sovio, the study's lead author, said.
The study looked at data on 4,069 women, 4.2% of whom were diagnosed with gestational diabetes at or after 28 weeks of pregnancy.
Researchers found the foetuses of women subsequently diagnosed with the condition grew excessively ahead of the diagnosis, between the 20 and 28 weeks mark.
Janet Scott, from the stillbirth charity Sands, which financially supported the research, welcomed the study's findings, saying the report's recommendations would improve antenatal care.
Gestational diabetes can usually be controlled through a combination of diet and exercise, as well as through medication.