I had a chat with DP about it further and we decided just to fib all together and claim they can't give us a definite date due to being busy etc etc and try make out i went naturally which fingers crossed i end up doing!
Why do you need to fib at all? Just don't mention it. If anyone asks, just say baby will be here when it's ready then change the subject. Don't mention it at all on FB. When you go into labour, turn your phone & computer OFF. Both of you!

The only people we told were my parents & DH's parents, both times. DH's dad is his boss so we had to tell them as he needed the day off work. My parent's were looking after DS for DD's birth. They both came earlier on their own anyway.