With DS1 I had a scan at 12 weeks to check dates, position and for multiples, and another at 20 weeks.

With DS2 I was bleeding so had a scan at 7 weeks, then my anomoly scan at 20 weeks and a position scan at 39 weeks (to check for breech).

With the twins I decided that as my dates were spot on, and I was having no pain or bleeding, just to wait until 21 weeks for a scan. I then had one more scan at 36 weeks to check everything was ok for a normal birth.