Expectant mothers have faced being turned away from more than four out of 10 maternity wards due to lack of beds and staff, according to new data obtained by Labour.
Hospitals in England temporarily closed their maternity wards to new admissions on 382 occasions last year, compared to 375 times in 2015 and 225 occasions in 2014, with capacity issues and staff shortages among the main causes, the party said.
The figures were uncovered through freedom of information requests to 136 hospital trusts with maternity units in England, where 42 (44%) out of the 96 trusts that responded to Labour's requests said they had temporarily closed their doors on at least one occasion in 2016.
Some examples included:
:: The maternity unit at Royal Berkshire NHS Foundation Trust had to close 30 times due to an "insufficient midwifery staffing for workload" in 2016.
:: The maternity unit at St Helens and Knowsley Teaching Hospitals NHS Trust had to close for more than 30 hours on one occasion in February 2016 because of bed capacity and high activity.
:: The maternity unit at Epsom and St Helier University Hospitals Trust closed on two occasions in 2014 because the lift broke down and there were "no lifts in the maternity unit to transfer patients".
Shadow health secretary Jonathan Ashworth said: "These findings show the devastating impact which Tory underfunding is having for mothers and children across the country.
"It is staggering that almost half of maternity units in England had to close to new mothers at some point in 2016.
"The uncertainty for so many women just when they need the NHS most is unthinkable.
"Under this Government, maternity units are understaffed and under pressure.
"It's shameful that pregnant women are being turned away due to staff shortages, and shortages of beds and cots in maternity units."
Midwifery leaders called for action to tackle "significant pressures" on maternity services across England, which face a shortage of around 3,500 full-time midwives.
Sean O'Sullivan, head of health and social policy at the Royal College of Midwives (RCM), said: "This latest research from the Labour Party comes as no surprise to the RCM and really further proves just how badly England's maternity services are struggling due to understaffing.
"The RCM respects and supports decisions made to close maternity units when failing to do this will compromise the safety of the service and the women and babies already being cared for.
"Nevertheless, if units are regularly and persistently having to close their doors it suggests there is an underlying problem around capacity and staffing levels that needs immediate attention."
He added: "The RCM has warned time and time again that persistent understaffing does compromise safety and it's about time the Government listened to those best placed to advise."
A Department of Health spokeswoman said: "We want the NHS to be one of the safest places in the world to have a baby and patients should be reassured we continue to have enough midwives in the NHS.
"Temporary closures in NHS maternity units are well rehearsed safety measures which we expect trusts to use to safely manage peaks in admissions.
"To use these figures as an indication of safe staffing issues, particularly when a number of them could have been for a matter of hours, is misleading because maternity services are unable to plan the exact time and place of birth for all women in their care."