UK's Diana was in labour for 16 hours with Prince William

The world might have to wait for some more hours for the news of the arrival Kate Middleton's first child, as babies, even royal babies, don't always keep to schedule.
The biggest determinant of when a baby will appear is whether it is your first child. If it is, then giving birth generally takes longer, experts say.
The late Diana, Princess of Wales, was in labour for 16 hours with William. However, Harry was born just nine hours after Diana arrived in hospital.
Kate is planning to have a hypnobirth, in which relaxation techniques are used to limit the need for pain medication. There is little research into the efficacy of this technique, and none showing conclusively that it speeds up the process, The Times reported.

After being admitted to hospital at 5.30am (local time), if Kate follows what would be expected of a first-time mother, it is more likely than not that she will have given birth to the would-be British monarch by early evening.