Debs,

They will want you there when your waters break so they can start the drip on you asap. Having your water broken for a certain amount of time can also increase the step B problem. Only a small percent of mums labour start with the waters breaking though - waters allowed to break naturally often break at second stage (pushing) but Obs will often break them sooner to speed the progress of the labour - which often brings the stronger contractions on fairly quickly, as that baby is pressing firmly on your cervix and no longer has that bag of waters buffering it.