I had one at 12 weeks (didn't need an earlier dating scan as I knew my dates), one at 18-19 weeks for all of the 'big' stuff (also found out the gender!) and I will have another one around 32-33 weeks because the placenta was lying low at the two earlier ones.

A few other people with low lying placentas have also said they get one done around 32 weeks to check that it has moved out of the way.

I thought you could tell position of baby without having to do an u/s? My baby has been head down facing outwards for over a month and my ob told me just from feeling and finding the hb. I can tell from the type/position of movement as well.