With Eva, I had one to date the pregnancy, because my dates and their dates were off. [I was right!] I had the standard 20 week ultrasound. [because it was standard!] and I had to have a few more due to her small size and my developing pre-e.
My sister had the standard one at 20 weeks, but had to have another at 30 weeks because she was measuring small. Turns out, the baby was measuring right on time, but he had a serious heart condition! Sometimes the condition isn't even noticed until after birth, when the baby goes into distress.
So I may be too late to date the pregnancy by the time my appointment rolls around, so I'll probably just have the 20 week one, but towarsd the end, I'm going to get another one, to check for any birth defects.
I'm a worrier now that it happend so close to home, ya know?