How big will they be?
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.
Their name, birth date, time, weight, and length on cream matte paper. Hangs in the nursery, ages well.
Birth weight is moderately heritable — roughly 30–40% genetic, 60–70% intrauterine environment. We blend the strongest population-level predictors: maternal pre-pregnancy weight, gestational age at delivery, parity (first vs. subsequent), and fetal sex.
Reported error around any single prediction is ±400–500 g (~1 lb) — the population variance is real and you should treat any prediction as a center-of-gravity estimate, not a fixed number.
- Doesn't model gestational diabetes, twin pregnancy, or growth restriction.
- Maternal height, smoking status, and ethnicity influence birth weight but aren't captured here.
Free, attribution-only. One iframe — your readers stay on your page, the calculator works exactly the same, and the math runs on willthey.baby so we keep it accurate.