How this calculator works
Height and pre-pregnancy weight determine the BMI band used in the IOM 2009 gain guidance. If you enter a known BMI, current BMI is scaled by the current-to-pre-pregnancy weight ratio. Current pregnancy BMI is shown as arithmetic only, without an adult weight category: the added weight is not a direct measure of body fat. Total gain references describe the whole pregnancy and cannot assess progress at a particular week. Twin guidance is provisional, and IOM did not issue a range for underweight twin pregnancies. An individual care plan can differ from these population references. The result is an estimate for information only and is not medical advice.
Formula
Pre-pregnancy BMI = pre-pregnancy kg / height in metres². Current BMI = pre-pregnancy BMI × current kg / pre-pregnancy kg. Weight change = current kg - pre-pregnancy kg. Reference end weights = pre-pregnancy kg + published total gain endpoints.
Worked example
At 160 cm and 60 kg before pregnancy, BMI is 23.44. A current weight of 72 kg gives a numerical BMI of 28.13 and a 12 kg gain. The reference band remains the normal pre-pregnancy band, whose singleton total gain range is 11.5 to 16 kg; it is not reclassified using 72 kg.
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Frequently asked questions
Should I use my current weight to select the pregnancy gain range?
Use weight before pregnancy. Using current weight could move the lookup into another band simply because the pregnancy is progressing.
Does a higher current BMI mean I should lose weight?
This calculation cannot set a weight-loss goal in pregnancy. Ask your maternity care team to interpret weight change alongside gestational age, fetal growth and your health.
Why is there no target for an underweight twin pregnancy?
IOM 2009 found insufficient evidence for that group. This tool leaves the endpoints blank instead of borrowing a range from another BMI band.