The Institute for Fiscal Studies study found a largely constant number of maternity cases since 2010 but weekend closures of maternity unit were 30% more likely on a Thursday, Friday and Saturday.
Researchers also concluded closures are also more likely to occur in the summer months and in December, the principal holiday periods.
The study warned closures were 50% more likely in June compared with January, for example.
“Given that the number of beds is fixed across the week, the most likely explanation for variation in closures by day of the week and by month is the availability of staff," the IFS said.
“However, using the data available, it is not possible to say whether these patterns are attributable to difficulty in filling planned rosters, as many staff balance other caring commitments, or to ineffective planning and management.”
The research, funded by the Economic and Social Research Council, examined sources of short-run and long-run pressures on maternity units in England
It showed giving birth was the single largest cause of admission to NHS hospitals in England.
“In common with many NHS services, providers of maternity care are reporting pressures from increased demand, staffing shortages and programmes to improve the quality of care," the report said.
“How units respond to these challenges carries important implications for the health of mothers and babies, and the finances of NHS acute trusts.”
The case mix of maternity unit admissions continued to change, according to the IFS’s analysis, with women giving birth later in life and with more complex health conditions. This can increase pressure on staff and resources, the report noted.
The IFS found that between 2006 and 2014 the number of C-sections performed by NHS hospitals in England each year increased by 23,000. Although, there was no evidence of a change in medical practice to explain this.
Researchers said this surge in the procedure was explained by more older women giving birth.
A Department of Health spokesman said: “The NHS is one of the safest places in the world to give birth and as this report says, maternity closures are not common.
“However, we want maternity care to be even better, which is why we have 2,119 more midwives on our wards today than in 2010 and another 6,500 in training.”
