Ranges come from the Institute of Medicine (now the National Academy
of Medicine), Weight Gain During Pregnancy: Reexamining the
Guidelines (2009) — still endorsed by ACOG Committee Opinion 548
and republished by the CDC. Pre-pregnancy BMI sets the band:
underweight, normal, overweight, or obese (IOM did not subdivide
obesity).
First-trimester gain is small and band-independent (about 1.1–4.4 lb /
0.5–2.0 kg total). The rest of the total is distributed across
weeks 14–40 as a weekly rate. "Expected so far" is
the first-trimester portion plus the weekly rate × weeks since 13,
shown as a band.
IOM did not publish a weight-gain range for underweight twin
pregnancies; this calculator refuses to invent one. Higher-order
multiples (triplets+) are also out of scope.