What this calculator does
The Institute of Medicine sets recommended weight gain ranges for pregnancy that depend on pre-pregnancy BMI. The ranges move in the opposite direction to BMI: a lower starting BMI calls for more gain, a higher one for less.
The difference is substantial. An underweight starting BMI carries a 12.5 to 18 kg recommendation, while an obese starting BMI carries 5 to 9 kg, with the expected weekly rate falling from 0.51 to 0.22 kg across the same range.
The formula
Pre-pregnancy BMI is calculated from weight and height and matched to one of four IOM categories, each with its own total range and second and third trimester weekly rate.
| Term | Meaning |
|---|---|
| IOM guidelines | The 2009 Institute of Medicine recommendations this calculator applies. |
| Pre-pregnancy BMI | BMI before conception, which determines which range applies. |
| Weekly rate | The expected gain per week during the second and third trimesters. |
The inputs explained
| Field | What to enter |
|---|---|
| Pre-pregnancy weight (kg) | Weight before pregnancy, in kilograms. |
| Height (cm) | Height in centimetres. |
| Weeks pregnant (weeks) | How many weeks pregnant, used for the rough gain-to-date figure. |
When to use it
Understanding a recommended range
The range depends entirely on starting BMI, which is not always explained.
Checking progress against the guideline
The weekly rate gives a rough expectation for the second and third trimesters.
Preparing for an antenatal appointment
Knowing which category applies makes the conversation easier to follow.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does starting BMI change the guideline?
Three pre-pregnancy weights and the ranges they fall into.
| Pre-pregnancy weight | Recommended total gain | Typical rate, 2nd/3rd trimester |
|---|---|---|
| 50 kg | 12.5 - 18 kg | 0.51 kg/week |
| 65 kg | 11.5 - 16 kg | 0.42 kg/week |
| 85 kg | 5 - 9 kg | 0.22 kg/week |
Questions
Are these ranges a target I must hit?
No. They are population guidelines, and individual circumstances vary a great deal. Your midwife or obstetrician is the right person to discuss what is appropriate for your pregnancy, and they will consider far more than BMI.
Do they apply to twins?
No. The IOM publishes separate and higher ranges for twin pregnancies. This calculator covers singleton pregnancies only.
What if I am gaining outside the range?
Raise it at an antenatal appointment rather than adjusting your eating on your own. Both faster and slower gain have many possible explanations, and pregnancy is not a time to restrict intake without medical guidance.
Why is the gain-to-date figure only rough?
Because it assumes a steady rate from week 13 plus a nominal first trimester gain, and real pregnancies do not follow a straight line. It gives a rough expectation, not a figure to measure yourself against.
For dating an early pregnancy from a scan, see the crown-rump length calculator. For the BMI that sets the category, see the BMI calculator.