Baby measuring small all of a sudden.

Anyone else have this issue? My son has stayed perfectly on track this whole pregnancy up until Wednesday when I had my last us. Hadn't had one for 6 wks and baby was measuring wks behind. They are taking him next Friday and I'll be 37 wks but as of now baby only weighs 4.2 😦 so scared of going through another nicu stay. My 1st born was 10wks early and had to stay for 6 long, miserable weeks!