Babies born at weekends in NHS hospitals are more likely to die in the first week of life than those delivered on weekdays, new research suggests.
Experts estimated 770 more babies die annually and 470 more infections occur among new mothers than would happen if performance was consistent across the week.
The findings, published in the British Medical Journal (BMJ), were based on a study of 1.3 million births in England between April 2010 and March 2012.
The death rate among babies was 7.3 per 1,000 delivered at weekends - 0.9 higher than for weekdays.
However, there was no consistent link between death rates and staffing level.
Nevertheless, the study is likely to feed the debate over the state of weekend NHS services.
A separate study published in the BMJ in September showed that around 11,000 more people die every year within 30 days of admission to hospital on Friday, Saturday, Sunday or Monday compared with other days of the week. Health Secretary Jeremy Hunt used those figures as part of his drive to create a seven-day NHS.
The latest results showed that "babies born at the weekend had an increased risk of being stillborn or dying in hospital within the first seven days", researchers from Imperial College London said.
They added: "Moreover, the results also suggest increases in the rates of other complications for both women admitted and babies born at weekends, with higher rates of puerperal infection (infection following childbirth), injury to neonate, and three day neonatal emergency readmissions."
Health minister Ben Gummer said it was further evidence that standards of care "are not uniform across the week".
"We are determined to tackle this so that every new baby and mother receives the high quality care they deserve, 24 hours a day, seven days a week," said Mr Gummer.
Dr David Richmond, president of the Royal College of Obstetricians and Gynaecologists, said the results emphasised the need to "identify the possible causes in order to ensure that women are receiving high-quality care at any given day of the week".
"The UK is a safe place for women to give birth, however pressure on maternity services is growing as women are having more complex pregnancies due to the rise in older mothers and maternal obesity," said Dr Richmond.