I think they just go based off of how you're gaining weight and how you're measuring? :/ I'm honestly not sure. I get jealous seeing all these ladies like oh my baby weighs this much and I'm measuring ahead and stuff. I'm like I want to know!
I just had my last one and they said you have to have a certain ultrasound which of course isn't covered by my insurance to determine weight. They guess she'll be around 6lbs when she's born based off how big I am but, can't tell me how big they think she is now :(