I think take any measurements with a grain of salt unless they specifically mention then to you. Whenever I've had any of my scans (I've had 4 so far, including doing the 20 week one twice since they couldn't do everything they wanted the first time), I've always watched them take the measurements really closely and they do it so fast, that I can't see how they can be all that precise. For us, the cerebral measurement was a little on the low side, but when I looked it up, even 0.5 mm would have made a huge difference (that's so small!). If they are rushed or just can't get a clear picture, it's so easy to be way off. Neither the sonographer (who was also a midwife) or my usual midwife were concerned about it at all. If there is something that raises a red flag (I also have a low placenta), they'll definitely talk it over with you. It must not be something they're worried about, so try not to overthink it. I panicked about our low measurement for a few days thinking something was wrong, until I researched the normal measurements and found we really were pretty close to normal, and more than likely it's just a matter of how the measurement is taken that can throw you above or below average.
