We added baby to out ITA once born.....jsut submit ITA as normal and let case officer know you are pregnant. Once baby born, get birth certificate and passport sorted out asap, then need to supply babies details to case officer and get baby medical done.....cost about £50 and took all of 5 minutes. Baby was less than a month old at time so all Dr did was same stuff you would normally get done at the babies 6 week check-up and if you have red book (which I didn't as had home birth and not seen health visitor to get one at time of medical) then Dr will proably have a quick look at that too. Only complication would be if baby was unlucky enough to be born some serious medical condition from birth.
Having medical done while pregnant pretty good really because save money on not havong x-ray and although they are still 'required' to work out BMI etc.....Dr just write in big letters next to it that I was pregnant and it was meaningless. Same for things like higher blood pressure and higher cholesterol etc as all common when pregnant. If you end up having medical done after having baby there is more pressure on you to get back into shape quickly to fulfill the BMI and waist measurement requirements.