The Pregnancy Weight Gain Calculator

Pregnancy weight gain calculator at a glance#

How much weight should I gain during pregnancy?#

It depends on your BMI before you were pregnant. For one baby, the 2009 Institute of Medicine (IOM) guidelines recommend a total of 28 to 40 lb if your BMI was under 18.5, and 25 to 35 lb if it was 18.5 to 24.9. From 25.0 to 29.9 the range is 15 to 25 lb, and at 30.0 or higher it's 11 to 20 lb. The CDC uses the same ranges. Twins call for more.

How much weight should I have gained at 20 weeks?#

For a BMI in the normal range and one baby, the calculator's recommended gain by week 20 is 7.3 to 12.3 lb. Take a woman who is 5 ft 5 in (165.1 cm) and weighed 140 lb (63.5 kg) before pregnancy. Her BMI is 23.3, so the calculator puts her week 20 range at 147.3 to 152.3 lb, and a weight of 150 lb that week is inside it.

How does the pregnancy weight gain calculator work?#

It finds your BMI from your height and your weight before pregnancy, matches it to an IOM category, and builds a week-by-week range that ends at the IOM total for one baby in week 40. Enter your current week, height, current weight and weight before pregnancy, and tick the box if you're carrying twins. If you're not sure which week you're in, the pregnancy calculator works it out from your last period or due date. It shows your recommended weight range for this week and at delivery, your BMI before pregnancy, a chart and a table for all 40 weeks.

Start with your BMI before pregnancy#

BMI is weight in pounds times 703, divided by height in inches squared. In metric it's kilograms divided by meters squared. For the example, 5 ft 5 in is 65 inches, so 140 × 703 ÷ (65 × 65) = 98,420 ÷ 4,225 = 23.3. In metric, 63.5 ÷ (1.651 × 1.651) is also 23.3.

Use your weight from before you got pregnant, not today's. The calculator takes your current weight only to plot where you are on its chart. If you just want your BMI, the BMI calculator works it out from height and weight in pounds or kilograms.

The 2009 guidelines use the World Health Organization's BMI cutoffs: underweight below 18.5, normal weight 18.5 to 24.9, overweight 25.0 to 29.9 and obese 30.0 or higher. This table shows where those cutoffs fall for common heights.

IOM weight gain recommendations by BMI#

The table gives the IOM's total gain for a full-term pregnancy and its average weekly gain in the second and third trimesters, with the range in parentheses. The weekly rates assume a gain of 1.1 to 4.4 lb (0.5 to 2 kg) in the first trimester.

The lower your BMI before pregnancy, the more you're advised to gain. In the example, a BMI of 23.3 puts the target at 25 to 35 lb, so the 140 lb starting weight leads to about 165 to 175 lb at delivery.

The IOM says these guidelines are meant for women in the United States and may apply in other developed countries. They aren't meant for places where women are much shorter or thinner than American women, or where good obstetric care isn't available.

How the calculator spreads the gain over 40 weeks#

The calculator draws its weekly range in two parts.

• Weeks 1 to 13. The range rises from nothing to 1.1 to 4.4 lb by the end of week 13, the IOM's first-trimester assumption. It's the same for every BMI group.
• Weeks 14 to 40. It adds an equal amount each week so that week 40 lands on the IOM total for your BMI. For a normal-weight BMI, that's 0.89 to 1.13 lb a week.

For the example woman, the range is 141.1 to 144.4 lb at week 13, 147.3 to 152.3 lb at week 20 and 165.1 to 174.9 lb at week 40. Those last figures are a gain of 25.1 to 34.9 lb, the IOM's 25 to 35 lb to within a tenth of a pound.

The weekly steps aren't the IOM's own weekly rates. For a normal-weight BMI, the IOM gives an average of 1 lb a week, with a range of 0.8 to 1 lb. The calculator's steps run a little higher because they're set to reach the total, so treat the week 40 range as the target and the weekly figures as a guide.

One caution at the edges: if the calculator shows your BMI as 18.5, 24.9 or 29.9, it may have used the neighboring category. The IOM puts 18.5 and 24.9 in normal weight and 29.9 in overweight, so check your range against the table.

In kilograms, the calculator shows 66.8 to 69.1 kg at week 20 and 74.9 to 79.3 kg at delivery, a gain of 11.4 to 15.8 kg. The IOM table lists 11.5 to 16 kg, which isn't an exact conversion of 25 to 35 lb.

This gives an estimate, not medical advice. A doctor can tell you what the number means for you.

Weight gain with twins#

The IOM gave provisional ranges for twins. A woman of normal weight should gain 37 to 54 lb (17 to 25 kg), overweight 31 to 50 lb (14 to 23 kg) and obese 25 to 42 lb (11 to 19 kg) by term. It found too little information to set even a provisional range for underweight women carrying twins.

The CDC lists 50 to 62 lb for that group, a figure it takes from a separate 2003 study rather than from the IOM. The calculator's twin result for a BMI under 18.5 is about 38 to 41 lb at week 40, which matches neither, so if that's you, ask your doctor for a target. For triplets or more, the CDC says to talk to your health care provider about weight gain goals.

What to do with your result#

The range is a guide, not a pass or fail. The IOM wants its guidelines used alongside a doctor's judgment and a conversation about diet and exercise. If your gain is outside the range, it says clinicians should look at other evidence, including how the baby is growing, before suggesting you change anything.

It's also common to land outside the range. The CDC cites a study that found only about 1 in 3 women gained the recommended amount, while about 1 in 5 gained less and 1 in 2 gained more. If your number sits above or below the band, bring it to your next appointment.

Sources#

• Institute of Medicine and National Research Council, Weight Gain During Pregnancy: Reexamining the Guidelines (2009), Summary and Table S-1
• CDC, Weight Gain During Pregnancy

Frequently asked questions#

How much weight should I gain in the first trimester?#

The IOM's weekly rates assume 1.1 to 4.4 lb (0.5 to 2 kg) over the first trimester, whatever your starting BMI. The calculator follows that and reaches 1.1 to 4.4 lb at the end of week 13.

How much weight should I gain per week?#

In the second and third trimesters, the IOM's average is about 1 lb a week if you were underweight or normal weight, 0.6 lb if overweight and 0.5 lb if obese. In kilograms that's 0.51, 0.42, 0.28 and 0.22 kg a week.

Is the recommended weight gain different for twins?#

Yes, it's higher. The IOM's provisional twin ranges are 37 to 54 lb for normal weight, 31 to 50 lb for overweight and 25 to 42 lb for obese. There's no IOM range for underweight women with twins.

Should I use my current weight or my weight before pregnancy?#

Your weight before pregnancy. The BMI category, and with it your target, comes from that number. The calculator uses your current weight only to place you on the chart.

Does the calculator work in kilograms?#

Yes. Use the switch next to the Calculate button to change between pounds with feet and inches, and kilograms with centimeters.

What BMI counts as obese for pregnancy weight gain?#

30.0 or higher. The IOM uses a single range for all classes of obesity, 11 to 20 lb (5 to 9 kg) for one baby.

What if I've gained more or less than the range?#

Talk to your doctor before changing how you eat or exercise. The IOM asks clinicians to check other factors, such as fluid retention and how the baby is growing, before suggesting a change in weight gain.

What should my weight be before getting pregnant?#

The IOM says women should enter pregnancy with a BMI in the normal weight range, 18.5 to 24.9. The ideal weight calculator shows the healthy BMI weight range for your height.