Several mothers undergoing pregnancy are not getting the best possible care during childbirth.
The warning comes after new research uncovers huge variations in maternity care practice.
It shows that the number of mothers-to-be who have to undergo induced labour or instrumental deliveries is twice as high in some hospitals compared with others.
The Royal College of Obstetricians and Gynaecologists (RCOG) and the London School of Hygiene and Tropical Medicine analysed information on English hospitals in 2011/12.
They discovered that there was a two-fold difference between hospitals with the highest and lowest levels of induction of labour among women giving birth for the first time.
The highest had 38% of new mothers being induced. The lowest had 17%.
The statistics also show that in some areas one in five first-time mothers had to have an emergency caesarean section after induction of labour, compared with double that rate in other areas.
Researchers also said that the number of new mothers who had instrumental deliveries fluctuated from 16% to 32%.
The RCOG said the "concerning" disparities could indicate that NHS resources are not being utilised in the best way.
RCOG vice president Dr David Richmond said the research emphasised that specialist-delivered care must grow so that care for women with complicated childbirth needs can be provided by trained clinicians all the time.
Dr Richmond said: "We cannot be sure that every woman is getting the best possible care."
Copyright Press Association 2013