um...thoughts?

Hello all, so according to my 1st day of period, im 18 weeks. my 1st US at 10 weeks the baby was measuring 9 weeks. my dr said she will keep my due date from last period. yesterday at my visit she said i was 17 weeks. I will probably have a csection again, but should i be worried about csection happening too early? can the baby grow to match my current due date? just worried and confused.