Great question - this depends on various factors including age, risk, and multiples. With my first, we had one at 8 weeks (viability), 20 weeks (growth scan) and 30 weeks (I pushed hard for this one; there were concerns she was small because I wasn’t gaining much weight). Two is pretty standard. With my current pregnancy, I’ve had 7, 9, and 11 because of an earlier miscarriage at 12 weeks. So it depends on a few things!
I think it also depends on where you are. In Canada, or at least my experience, we have the standard minimum of 11-13 weeks (NT scan) and 20 weeks, but I've had extras at 8-9 weeks (dating), and the 30ish week one too for growth, I think due to my age and miscarriage history. I also had extras tacked on during the first trimester due to bleeding in both my pregnancies... So I would say I had 5 done last pregnancy and am on track for about the same this time around.
As others have said, it often depends on your risk factors and pregnancy details, as well as your provider’s preferences and location. I was told at my first appt that the office typically schedules 3 ultrasounds for a standard pregnancy (one in each trimester.) I’ve already had 2 in my first trimester though, because I’m classified as high risk. Hope this helps!
Echoing others. It can also depend on OB and heads of department. For DS1 we did 8 weeks, 20 weeks then 30 and somewhere after that thinking he was too big. DS2 8, 20, and then I think 24 due to placenta size. and 30. But with this little guy we've already had 2 because there is a new MFM who does an additional before the AS to check for neck folds (something the old MFM, i've had the same OB for all 3 and delivery at same hospital never did) so probably a 20 and then 30 week one too.
Re: For ultrasounds