Baby not growing at normal rate???!?!!

Amber • 20 years old, mumma to my beautiful boy Oliver and expecting my second in Jan 2018 👶🏼
So my midwife informed me today that baby isn't growing at a normal rate, and that his head and legs are smaller than normal. They've booked me in to have another ultrasound next Friday to see if he's still growing slower than he's supposed to and if he is then the midwife said I'll need weekly scans to keep track of his growth, as the placenta may not be feeding him properly and he may need to be delivered at 36 weeks opposed to waiting to go full term. Has anyone else had this happen to them? This is my first baby and even tho the midwife told me to leave all the worrying to them, I can't help but think this could be worse than what they're making it out to be 😁😁😁