Hi Kath
Congratulations! Don't worry, it's perfectly possible to combine pregnancy and PhDs. In fact, one of my friends had 2 children during her studentship! It's not going to be the easiest time of your life but don't panic! Things to consider; are you research council funded? If so, have a look on the appropriate website and you should be able to find some info about maternity leave/payment. Otherwise, consult your research degrees/student handbook.
Assuming you are a science student, do you work with (or in close proximity to others who work with) chemicals that are hazardous/teratogenic? If so, you should tell your supervisors sooner rather than later as you will obviously need to stop working with these (the Uni should do a risk assessment for you even if you don't work with any particularly high hazard chemicals). You will probably find that your registration as a PhD student can be temporarily suspended while you are on mat. leave, that way, you don't lose any crucial thesis completion time! Good luck.