The quality of NHS maternity services varies considerably across England, a report from the Healthcare Commission claims.
The commission ranked 21% as "least performing", 22% as "fair performing" and 26% of trusts as "best performing".
Trusts in the north of England performed better than those in London, with 75% of northern trusts ranked as "best performing" compared with 70% of London trusts ranked as "least well performing".
Anna Walker, the Commission's Chief Executive, said: "The review raises real concerns about performance in London. There are a number of factors that may have influenced these results, such as lower staffing levels and the mobility and mix of the population.
"But London trusts need to rise to these challenges."
The Commission will publish individual trust reports, those performing badly will have to produce an "action plan" that strategic health authorities will help them to implement.
Anna Walker adds: "There have been serious concerns about maternity services.
"The care given to women during pregnancy, birth and the time just after giving birth is extremely important to both the mother and baby. It can have lasted negating effects if not handled correctly."