Hi
You can not retract the job offer to the pregnant lady now that you know she is pregnant, she could take you to a tribunal even though she is not employed yet (I know crazy isn't it!). Hopefully you can keep her under the NI level then she will have difficulty claiming allowances from you directly for leave etc, if that is what you are worried about.
Depending on your payroll expenses if she does claim allowances you should be able to claim most of it back, my advice would be to seek advice from fsb or acas about her rights, my knowledge on maternity benefits are minimal I'm afraid! Sounds to me like she already knew she was pregnant but just didn't mention it! Doesn't bode well really, as after all sounds like you would have employed her anyway.
Fox