ReckonBoxHow the pregnancy weight gain calculator works
Recommended weight gain in pregnancy is set by your pre-pregnancy BMI, not your current weight. Enter your height and what you weighed before pregnancy, and the tool returns the Institute of Medicine range for your category, the weekly rate that belongs to that category, and how far along the projection you'd be at a given week.
The 2009 guidelines use the WHO and NHLBI BMI cut-offs, which is what the revision changed from the 1990 version.
| Pre-pregnancy BMI | Total gain, singleton | Weekly rate, 2nd and 3rd trimester |
|---|---|---|
| Under 18.5 | 28 to 40 lb | 1.0 to 1.3 lb |
| 18.5 to 24.9 | 25 to 35 lb | 0.8 to 1.0 lb |
| 25.0 to 29.9 | 15 to 25 lb | 0.5 to 0.7 lb |
| 30.0 or above | 11 to 20 lb | 0.4 to 0.6 lb |
The weekly rate is not one number for everyone
That right-hand column is where the consumer calculators flatten something real. calculator.net gives 1 to 4 lb across the first trimester and then about 1 lb a week for everyone. The American Pregnancy Association gives 1 to 2 lb a week for everyone.
The source table doesn't say that. The published rate falls as pre-pregnancy BMI rises, from 1.0 to 1.3 lb a week at the underweight end down to 0.4 to 0.6 lb with obesity. Someone starting at a BMI of 32 who follows a generic "1 to 2 lb a week" figure is tracking against a rate roughly triple the one written for them.
First-trimester gain is small and the guidelines treat it separately, around 1 to 4.5 lb in total.
Twins, including where no guideline exists
The Institute of Medicine publishes provisional twin ranges for three of the four BMI categories. Healthy weight gains 37 to 54 lb, overweight 31 to 50, and obesity 25 to 42.
For an underweight pre-pregnancy BMI there is no published twin guideline. This tool says so and shows the singleton range, because manufacturing a number to complete the table would be worse than admitting the gap.
Coverage across the field is patchy here. calculator.net carries the normal-weight twin figure. The American Pregnancy Association's weight gain guide omits multiples entirely.
Where the weight actually goes
A roughly 30 lb gain breaks down, per the American Pregnancy Association, as:
| Component | Weight |
|---|---|
| Baby | 7 to 8 lb |
| Maternal fat stores | 6 to 8 lb |
| Increased blood volume | 3 to 4 lb |
| Extra fluid | 2 to 3 lb |
| Uterus | 2 lb |
| Amniotic fluid | 2 lb |
| Placenta | 1.5 lb |
| Breast tissue | 1 to 3 lb |
The baby is a quarter of it. Most of the rest is the physical infrastructure of carrying one, which is worth knowing before reading any single number on a scale as a verdict.
What this calculator does not do
It doesn't tell you whether your gain is a problem. A number sitting outside a population range is information, and gain moves with first-trimester nausea, fluid retention, and a good deal else no calculator can see. Health Canada states directly that a pregnancy weight gain calculator should not replace individualised medical advice.
Nutritional targets sit outside this page as well, since the guidelines set a weight range and say nothing about what to eat. And nothing here speaks to a specific pregnancy, a specific complication, or whether any of it applies to you.
Results are estimates and not medical advice. Weight in pregnancy is a conversation for your doctor, midwife or a registered dietitian. To date the pregnancy itself, the due date calculator handles gestational age, and the BMI calculator covers the pre-pregnancy figure this page depends on.
Frequently asked questions
How much weight should I gain in pregnancy? It depends on your pre-pregnancy BMI. The 2009 Institute of Medicine guidelines give 28 to 40 lb for a BMI under 18.5, 25 to 35 lb for 18.5 to 24.9, 15 to 25 lb for 25.0 to 29.9, and 11 to 20 lb for 30 or above. Those figures are population guidance, and an individual target belongs with your doctor or midwife.
How fast should the weight come on? The published rate differs by BMI category, which is the part most calculators flatten. In the second and third trimesters the Institute of Medicine gives 1.0 to 1.3 lb a week for an underweight start, 0.8 to 1.0 for a healthy weight, 0.5 to 0.7 for overweight and 0.4 to 0.6 for obesity. First-trimester gain is small, around 1 to 4.5 lb in total.
What are the guidelines for twins? The Institute of Medicine publishes provisional twin ranges of 37 to 54 lb for a healthy pre-pregnancy BMI, 31 to 50 lb for overweight and 25 to 42 lb for obesity. No twin guideline exists for an underweight pre-pregnancy BMI, so this tool says so instead of showing an invented number.
Which BMI cut-offs do these guidelines use? The 2009 revision adopted the WHO and NHLBI cut-offs: under 18.5 is underweight, 18.5 to 24.9 is healthy weight, 25.0 to 29.9 is overweight and 30.0 or above is obesity. The BMI that governs the range comes from before pregnancy, not your current weight.
Where does the weight actually go? The American Pregnancy Association breaks a roughly 30 lb gain down as baby 7 to 8 lb, increased blood volume 3 to 4 lb, maternal fat stores 6 to 8 lb, uterus 2 lb, amniotic fluid 2 lb, placenta 1.5 lb and breast tissue 1 to 3 lb. Only a minority of the total is the baby.
What if my gain is outside the range? Being outside a population range is information, not a diagnosis. Gain varies with nausea in the first trimester, fluid retention, and plenty else that a calculator cannot see. Health Canada notes directly that a weight gain calculator should not replace individualised medical advice, and that is the right reading of any number on this page.