Pregnancy Weight Gain Calculator

How is the total gain range chosen?

The calculator computes your pre-pregnancy BMI (weight in kg divided by height in metres squared) and picks a row from a table of 2009 Institute of Medicine ranges: under 18.5, 18.5 to 24.9, 25 to 29.9 and 30 or above. Singleton and twin pregnancies use different rows. The ranges are in kilograms.

How is “expected gain at this week” calculated?

It is a straight-line model written into the code, not an IOM chart. Up to week 13 it assumes 1 kg gained in proportion to the weeks elapsed (so about 0.8 kg at week 10). After week 13 it adds a fixed weekly rate for your BMI group, such as 0.40 kg per week for normal weight, 0.27 for overweight and 0.22 for obese in a singleton pregnancy.

What does “On track” mean here?

Your actual gain (current weight minus pre-pregnancy weight) is compared with the modelled figure. If the two differ by less than 0.5 kg you are shown On track; otherwise the tool shows how many kilograms above or below the model you are. That is a simple arithmetic comparison, and real pregnancies vary week to week, so it is not a judgement about your health.

Why does an underweight twin pregnancy show no range?

The 2009 IOM guidance gives provisional twin ranges for normal weight, overweight and obese women only. For an underweight starting BMI with twins the calculator returns your BMI and gain so far and says no IOM range exists, rather than making up numbers. Speak with your care provider for guidance in that situation.

What inputs are rejected?

Weights or height that are zero (which is what a cleared box becomes) or negative give an error, as does a week below 1 or above 42. The field hint says 1 to 40, but weeks 41 and 42 are accepted. The week is rounded to the nearest whole number.

Does it handle other situations such as triplets or a very short stature?

No. It only has Single and Twins options, and it applies the same adult BMI cut-offs to everyone. Teenage pregnancy, higher multiples, medical conditions and short stature are outside what the tool models, and a clinician may set different goals.

Is this medical advice?

No. It explains arithmetic against published ranges and is general information only. Your midwife or doctor weighs your history, tests and growth scans, and their advice should take priority over anything shown here.