Worried FTM-to-be

Hi po! Based on my LMP, I am 23 weeks pregnant. Kanina nagpa-ultrasound ako (my third time), and sabi ng OB, 20 weeks pa si baby based on his weight and other stats. Sino po sa inyo ang may ganitong experience? Worried lang ako kasi underweight at maliit si baby. Thanks po sa makakasagot.