Birth weight is mostly genetics, gestational age, and how the pregnancy went — not magic. Here's an honest range based on the factors that actually move the number.
Birth weight prediction is, honestly, more art than science. We're using a regression that captures the biggest factors — gestational age, parental size, parity, sex — but the ±1 lb range exists because we genuinely cannot do better from the inputs you can give us in a form.
Tall parents, gestational diabetes, placental insufficiency, fetal growth restriction, and a dozen other things your OB tracks can shift the actual number meaningfully. Treat the prediction as a midpoint with wide uncertainty, not a target.