Janey,
I too had a huge night out when I was pregnant with my son James and didn't know it... I was around people who were smoking more than just cigarettes, and I was in a smoke filled nightclub, before I left to walk home with my then SIL, I even fell over !!!
My son was born a healthy 4.29kgs and is 12 years old now, and a very normal little man
I am certain everything will be fine at your next scan... until then, breathe easy knowing that you're not the first to have had a big night out before finding out about a bub.
Lisa