Gestational Age

Hello mommies! Currently on my 23rd week and 6 days. Took CAS yesterday and thankfully normal naman lahat ng organs and body parts ni baby. We also found out that we are carrying a baby BOY ? The only thing that bothers me is his gestational age. Kung pagbababsehan kasi ang last menstrual cycle ko im already 23 weeks pero base on my ultrasound 20 weeks lang ang gestational age ni baby :(( My ob told me this morning to get another ultrasound next month. Im worried because i dont if my baby is weeks behind his age or baka late lang ang ovulation ko accdg to my ob. Tho regular naman ang menstrual cycle ko ever since. I'm confused if i will rely on my LMP or UTZ. Has anyone experienced this also? Let me know your thoughts