Hey mumma bear,

You poor thing, i know it is easy to say but try not to worry too much. Maybe you had a junior sonographer that was learning, so it took them alot longer to find everything? It is also true that it can be difficult to see certain orgains if the baby is lying in one position and wont move.

About the BPD, (Diameter of baby's head). The difference between 18w and 19w is about 42mm and 45mm +/-2mm. (According to ASUM, Aus society for US in med) with the variation of 2mm it could be well within the normal limits for your 19w baby to have an 18w-ish BPD.
Between 18w and 19w the BPD increases 3mm in one week, so a difference of 0.6mm is really insignificant. It doesnt sound like anything is too
So try not to worry too much as it sounds like it is fairly normal.

My girlfriend has a US at one hospital and they said her bubs kidneys were too big and weren't functionaing properly, and that he had a funny shaped head and a small chin. So her OB sent her in a panic to another US place that are the specialist in baby US, and it was all fine, 3 months later she gave birth to a perfectly healthy baby boy! He has a normal shaped head that it is a bit longer than the average, just like his dad!!! lol.

I hope i have been able to help a little bit, and you are not worrying too much.
I'm thinking of you, and have my fingers crossed for the next scan