Excuses for not drinking?

Hi everyone , just found out we're pregnant last weekend, we have decided that we don't want to tell anyone yet as we want to announce to our families at Christmas (when I will be about 13 weeks).

We are going out for a friend's birthday on Friday, obviously I won't be drinking. Normally I do like a drink so I know that our friends will question me.

I had said that I was going to drive because we have a dog and our friends had booked a table for 7pm so said that we might have to leave early as the dog would have been on his own most of the day and we wouldn't have much time between getting home from work and leaving to go out... Our friend has now offered to put the booking back to 8pm so that we can get a taxi with them... I don't know what excuse I can give now! I think they will guess straight away if I use the antibiotics excuse. And they will think it's odd if I get my own drinks as we alway go in rounds..... Help! !