Baby measuring small at 34.2 weeks.

I went today for my 2 weeks appointment and his measuring at 29 weeks, up until today his been measuring right on target. They seemed pretty concerned and have booked me for an ultrasound at the hospital this afternoon. Has this happened with anyone else? I'm freaking out my mum passed away a few years ago have no aunts or sisters so I'm in the dark with this. TIA