Almost 18 weeks - baby isn't growing well

Went for an ultrasound (gender scan) yesterday and, after discussing with my partner, was referred to the VU because the baby's growth isn't looking good. I'm almost 18 weeks pregnant and the score was P1 (extremely low). They're going to do all sorts of tests on Monday (ultrasound, blood test, and possibly amniocentesis). Help, does anyone have experience with this because I'm really worried!
Negative stories are welcome too, I'd like to prepare myself. I don't have any other pregnancy symptoms except for fatigue.
Thanks in advance!