The variation in care women get when giving birth in hospital in England is concerning, experts say.
The Royal College of Obstetricians and Gynaecologists’ warning follows a review of more than 550,000 births.
It found “substantial variation” in practice between maternity units, and said this may suggest not all women get the best possible care.
Ministers said the NHS was a safe place to give birth but the report would help it improve.
The review – carried out with the London School of Hygiene and Tropical Medicine – looked at official data on births in 2013-14, excluding non-standard deliveries such as twins, triplets and pre-term babies.
It highlighted different rates of emergency caesarian sections, instrumental deliveries and episiotomies, which is when the area between the vagina and anus is cut.
In some maternity units 8% of mothers needed emergency c-sections, but in others the figure was 15%.
Just under one in five first-time mothers needed help with instruments while giving birth naturally in some units – but in other units this figure rose to almost three in 10.
The number of women who needed an episiotomy ranged from 29% to 44%.
RCOG president Dr David Richmond said: “We are concerned about the amount of variation identified in this report.
“Although the exact causes are difficult to establish, it is paramount that maternity units have information about their services, as well as the ability to compare themselves to the national average and to their peers.”
To help with that, the college has collated the results on an interactive website.
Elizabeth Duff of the National Childbirth Trust said: “Pregnant women should not have to endure a postcode lottery, and it is unacceptable these variations in maternity care are reported year after year.
“We urge NHS trusts to use this data to examine their practices and ensure the best possible outcomes for mothers and babies.”
Health minister Ben Gummer said: “Mothers and their babies deserve the very highest standards of care regardless of where they live.
“The NHS is already a safe place to give birth and by being open and honest about variations in care, RCOG’s report will help the NHS to improve.”
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