Mothers and babies in England could be facing unnecessary risks in some maternity wards, a report claims.
It is the second critical report on maternity services this year
The King's Fund health think-tank said that most births are safe, but improvements were still needed.
The report says that some staff are overworked, need more training, or could be better supported by managers.
It comes after criticisms from the Healthcare Commission last month. Ministers said they were committed to providing the best possible services.
The review did not look at the quality or the efficiency of maternity services, but simply their safety during birth, the time at which the risks are highest for mother and baby.
Professor Onora O'Neill, who led the inquiry, said that death rates had stayed low despite a rising birth rate and a rise in more complex deliveries, for example those involving IVF twins or older mothers.
She said: "This is something to build on, so that all births are as safe as possible."
However, the inquiry, after visiting maternity units and hearing from women with experience of childbirth, claimed to have found lingering problems which could, if left unchecked, affect safety.
The report accused some NHS trust boards of "not giving high enough priority" to safety, and said that maternity staff were overloaded with guidelines from government and other national bodies.
It also cited examples of poor leadership, and conflict between midwives and doctors. "This can lead to poor team working that puts patient safety at risk," it said.
Finally, it said that low levels of staffing remained a problem in some units, with some not meeting the recommended level of consultant cover.
King's Fund chief executive Niall Dickson said: "The report should provide some reassurance to prospective mothers and their families, but it also clear there is much still to be done to make maternity care in England as safe as it could and should be."
The review follows hot on the heels of the Healthcare Commission's own detailed look at the quality of maternity services.
A commission spokesman said that the new review provided "another important piece of the picture".
"It echoes many of the findings from our work on maternity services.
"Our recent review of maternity services across England did not find any NHS trust to be offering an unsafe service, but it did show that changes are needed in the way maternity services are provided."
The government has already announced an extra investment of £330m in maternity services, and this week said that some of this would be spent recruiting 4,000 extra midwives by 2012.
Health Minister Ann Keen said: "We are committed to building on our excellent safety record and ensuring we provide the best possible care for babies and expectant mothers."
According to Louise Silverton, of the Royal College of Midwives, "For too long maternity care has been left behind when it comes to resources including staffing, and this should galvanise trusts to pull their fingers out."
Professor Sabaratnam Arulkumaran, the president of the Royal College of Obstetricians and Gynaecologists, said that he was working to remove barriers between midwives and labour ward doctors.