It means the weight of the baby. Nhs probably has something similar but this is off Canada gov website since I’m in Canada where you can look up what it means in grams for your baby whether it’s a girl or a boy https://www.canada.ca/en/public-health/services/injury-prevention/health-surveillance-epidemiology-division/maternal-infant-health/birth-weight-gestational.html
Im 28 weeks and my baby is in 3rd percentile I don't know what it means maybe someone here knows ? but I know my baby is smaller than it should it weights 886grams and I have ctg monitoring and doppler scan every week and growth scan every 2 weeks
50 is bang in the middle. Means half of the babies would be bigger at this stage and half would be smaller. Mine was 56 percentile so quite similar !