Babies born in NHS hospitals at weekends are more likely to be stillborn or die in the first week of life than those delivered Monday to Friday, research suggests.
Experts estimate there are 770 more baby deaths every year and 470 more infections among new mothers than would happen if performance was consistent across the week.
The team also found babies born at weekends were more likely to be readmitted to hospital in an emergency.
The findings, published in the British Medical Journal (BMJ), will feed into the row over the "weekend effect", relating to research that suggests worse outcomes for patients admitted to hospitals on weekends.
A study 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.
Researchers behind the study said a lack of consultant cover and "reduced" services could be contributing to higher death rates.
But they argued it would be "rash and misleading" to conclude that an exact number of deaths could have been avoided.
Health Secretary Jeremy Hunt has used the figures as part of his drive to create a seven-day NHS.
The latest study looks at the "weekend effect" on outcomes in NHS obstetric care for 1.3 million births in England between April 2010 and March 2012.
The result showed that "babies born at the weekend had an increased risk of being stillborn or dying in hospital within the first seven days", researchers 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."
The experts, from Imperial College London, said they found statistically significant associations in four out of seven indicators "all of which were consistent with a lower standard of care for women admitted and babies born at weekends".
The largest effects were seen in the higher rates of death among babies, either as a stillbirth or death within the first seven days of birth (7% higher than weekdays), infections after childbirth (6% higher) and the baby suffering an injury during childbirth (6% higher).
The death rate among babies was 7.3 per 1,000 babies delivered at weekends - 0.9 per 1,000 higher than for weekdays.
The team estimated that 770 more baby deaths per year, from the annual total of 4,500 deaths among 675,000 births, occurred above what we would expect if mortality was always the same as for babies delivered on Tuesday - the midweek reference day.
They also found 470 maternal infections above what would be expected from performance seen for women admitted on the reference day.
However, there was no consistent link between outcomes and staffing levels on wards.
The authors said it was an observational study so no definitive conclusions could be drawn about cause and effect.
Health minister Ben Gummer said: "This is 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."
Dr David Richmond, president of the Royal College of Obstetricians and Gynaecologists (RCOG), said: "Although no definitive conclusions can be drawn from these results, they emphasise 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.
"Appropriate numbers of maternity staff underpin a safe and quality service and adequate 'out-of-hours' senior staffing remains a key issue in maternity care.
"This includes appropriate staff training and supervision, ensuring good outcomes outside normal working hours and effective planning and risk management.
"More robust evidence on the quality of care afforded by different models of labour ward staffing is also required."
Andrew Whitelaw, emeritus professor of neonatal medicine at the University of Bristol, said key data for up to 10% of babies was missing from the study, including on weight and twin births.
He added: "Ninety-six per cent of the elective Caesarean sections were carried out during weekdays.
"They carry a very low risk of the baby dying in the first week. The operation is planned and prepared for. All necessary trained staff know what is required and are present in good time.
"While elective Caesarean has its downsides, it virtually eliminates the risk of critical lack of oxygen to the foetus and of physical trauma, which are possible with spontaneous labour.
"I cannot see that the authors' sophisticated statistical analysis has allowed for this big contribution of low risk babies during weekdays.
"While a comparison of weekends versus weekdays does show a statistically significant difference, the figure shows the highest perinatal mortality for an individual day was on Thursday not Saturday or Sunday."
Louise Silverton, director for midwifery at the Royal College of Midwives (RCM), said: "The study itself does not give a reason for why these 'effects' occur.
"Midwifery and maternity staffing levels are the same on weekends as they are on weekdays. Midwives work across 24 hours, 365 days per year."
She said maternity units were already working at full capacity with greater demand for care with increasing complications, and added: "We need to make sure that we identify the 'possible causes' so we can continue to ensure women receive the best possible care for both them and their baby."