The Mayor has launched a priority seating scheme for pregnant women on the Tube.
It aims to encourage passengers to give up their seats to mothers-to-be who are often forced to stand.
Ken Livingstone was announcing full details of how the scheme will operate at High Street Kensington Tube station.
It follows an earlier attempt by the Mayor to resolve the problem by giving pregnant women badges declaring their expectant status, in the hope that other passengers would make way for them.
A recent survey found that London is the least sympathetic place in Britain towards mothers-to-be. About a third of women said they had never been offered a seat while travelling during pregnancy.
The survey of more than 500 new mothers, carried out by London Underground, found those given a seat had to stand for an average of five stops beforehand.
One in five spent more than £100 on taxis during pregnancy to avoid using the Tube.
Karlene Davis, general secretary of the Royal College of Midwives, said: " Offering assistance to pregnant women is not just a matter of courtesy, it's a matter of health. Pregnancy is stressful."
Jo deBank of London TravelWatch said: "Anything encouraging people to be more aware of pregnant women is welcome."
