20 weeks scan.. advice needed

Hi ladies...

So today I went for my 20 week scan. The lady doing my scan said baby is fine everything is normal. However the baby's stomach is measuring slightly small but she wasn't concerned about how small it was because she said take only slightly smaller than it should be, I'm petite she said your a small girl yourself so it's nothing to worry about but I'm super paranoid now if it was something to be concerned about would they tell me? In my report it said AC is measuring 137.9 and I'm 20 weeks anybody have any advise to put my mind at rest xx