How this calculator works
The IOM 2009 second- and third-trimester singleton rate range is selected using pre-pregnancy BMI. Both scenario lines start from your actual measured baseline, which can be recorded at 13 completed weeks or later. Each following row adds one week at the lower or upper published rate. The table and chart use kg; scalar outputs also offer lb. This is a constant-rate scenario, not a published week-specific reference curve or an assessment of excessive gain. The separate whole-pregnancy total range is not used to clamp the lines: extending a rate can lead outside that range. The tool does not model first-trimester gain, twins, fluid changes or fetal growth. The result is an estimate for information only and is not medical advice.
Formula
Scenario weight at week w = measured baseline kg + (w - baseline week) × selected rate in kg/week. Scenario gain = scenario weight - pre-pregnancy kg. IOM singleton weekly ranges by pre-pregnancy BMI are 0.44 to 0.58, 0.35 to 0.50, 0.23 to 0.33 and 0.17 to 0.27 kg/week for the underweight, normal, overweight and obese bands respectively.
Worked example
A person who was 60 kg at 160 cm before pregnancy has BMI 23.44. Starting at a measured 62 kg at week 13, seven weeks at 0.35 to 0.50 kg/week give 64.45 to 65.50 kg at week 20, or 4.45 to 5.50 kg above pre-pregnancy weight.
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Frequently asked questions
Is the shaded space between rates a normal range for my week?
No. The lines are simple extensions of your entered baseline. They are not gestational centiles or a validated test of whether your gain is appropriate.
Why must I enter a measured baseline?
The first trimester varies and a total gain guideline does not specify a unique starting weight at each week. Your measured value avoids inventing that starting point.
Can I use the table to change my diet or assess twins?
Use it as a discussion aid with your care team. It does not prescribe intake or assess fetal wellbeing, and its weekly rates are for singleton pregnancies. The related twin tool supplies provisional term totals only.