1st Sonogram worries.

So I just recently went to get my first sonogram done last week, I am now 21 weeks. My doctor told me they usually don't ask to see you back unless they see something they need to take a look at later on in your pregnancy and well that's what is happening to me... My doctor told me they need to see me back at 28 weeks to see if something has changed. I'm not making scenes because I'm confused on what he was even talking about. He said something about something that could affect the way the baby grows. I don't know if anyone knows anything that could possibly cause that..? I'm just lost and confused on what it is he was talking about... is anything similar happening/happened to any of you moms?