Around three in four labour wards do not have on-site overnight cover from consultants, figures suggest.
A survey of 165 maternity units found that in 2014/2015 around 27% of labour wards have consultants physically present overnight on weekdays, falling to 15% at the weekend.
The census by the Royal College of Obstetricians and Gynaecologists (RCOG) also found the number of consultants may have reduced since figures were first recorded in 2013 and some recommendations on early pregnancy care had not been implemented.
Edward Morris, vice-president of clinical quality at the body, said: "The RCOG recommends that trusts should ensure the adequate provision of consultant cover to deliver high quality safe care to women.
"It is important to highlight that all consultant-led maternity units currently have 24-hour access to consultant obstetricians on call, some with resident working where needed.
"Ultimately, local trusts need to look carefully at the mix of their patient load, risk profile and staffing to decide whether their particular unit needs more frequent consultant presence."
The RCOG said slightly fewer units had provided responses to the latest census, so the data had to be interpreted with "some caution".
The number of consultants had reduced since the survey was first carried out, however the body said this was likely to be a reflection of a fall in responses.
However it found recommendations to increase the low number of weekend early pregnancy services had not been put into effect.
"It is not clear why this remains the case as in the interest of patient care, this would be considered one of the first services that could be provided seven days a week," the RCOG said.
Some studies have suggested that round-the-clock cover by consultants is only necessary on the busiest labour wards.
An NHS England spokeswoman said: "Having a baby is now safer than it has ever been and the vast majority of mothers report that they get great NHS maternity care.
"Researchers at Oxford University have shown that overnight consultant obstetrician presence isn't proven to improve care, and a national diktat to that effect would mean the closure of many smaller units, which is another reason why it wouldn't necessarily be a good idea."