Pregnancy Weight Gain Calculator

Why the target depends on your starting BMI

There is no single pregnancy weight target. The range comes from your body mass index before conception, because a person who starts underweight has more to gain and a person who starts with obesity has less room before the risks climb.

BMI is weight in pounds times 703, divided by height in inches squared. At 5 foot 6 and 140 lb that is 703 x 140 / 4,356, or 22.6, which sits in the normal band. The four bands and their totals:

• Under 18.5: 28 to 40 lb
• 18.5 to 24.9: 25 to 35 lb
• 25.0 to 29.9: 15 to 25 lb
• 30.0 and up: 11 to 20 lb

These are the 2009 Institute of Medicine recommendations, the set most US obstetric practices still use. They are ranges rather than targets because two people with the same BMI can land 10 lb apart and both be fine.

The weekly column is derived, not a separate guideline. Each range is spread across the 27 weeks from week 14 to week 40, so the pace and the total agree. That is why the calculator can show a week 20 expectation that adds up to the week 40 total instead of contradicting it.

Worked example: 5 foot 6, 140 lb, week 20

Start at 140 lb with a BMI of 22.6, which is the normal band, so the total range is 25 to 35 lb. The first trimester is expected to add 1.1 to 4.4 lb, and the remaining 20.6 to 30.6 lb is spread over weeks 14 to 40 at 0.9 to 1.1 lb a week.

At week 20 the arithmetic is 1.1 + 7 x 0.9 = 7.3 lb on the low end, and 4.4 + 7 x 1.1 = 12.3 lb on the high end. A scale reading of 152 lb means a gain of 12.0 lb, inside the range and near the top of it. The remaining distance to the total is 13 to 23 lb.

Two readings of that table are useful. The gap between the low and high columns is about 2 lb at week 8 and about 10 lb at week 40, so the range widens as pregnancy goes on. And the pace is not linear in the first trimester: week 8 expects 0.7 to 2.7 lb, which is a fraction of the weekly pace later on.

If your own numbers sit 2 lb outside the band at one visit, that is not a crisis. A single weigh-in reflects water, food and time of day. The pattern across four to six visits is what your clinician reads.

First trimester and the rest of pregnancy move at different speeds

The first 13 weeks are slow on purpose. The embryo is small, the placenta is still forming, and the total gain is 1 to 4 lb for most people. Nausea, food aversions and fatigue can push it below that, and a loss of 2 to 5 lb in the first trimester is common enough that clinicians rarely worry unless it continues.

From week 14 the pace picks up. A normal BMI gain runs 0.9 to 1.1 lb a week, which is about 1 lb every seven days for 27 weeks. That single number explains most of the total: 27 weeks at 1 lb is 27 lb, plus the first trimester, and you are at the 25 to 35 lb range.

The rate differs by band because the total differs. Overweight runs 0.5 to 0.8 lb a week, obese 0.4 to 0.7, and underweight 1.0 to 1.3. A person with obesity who gains 1 lb a week from week 14 will pass 20 lb by week 33, which is why the weekly pace is worth watching even when the total looks generous.

Weeks 28 to 40 are when the baby gains most of its weight, roughly 5 to 6 lb in that stretch, and when fluid and blood volume peak. A plateau of two weeks in the third trimester is common and usually not a problem if the baby is growing on schedule.

Twins widen every range

A twin pregnancy needs more weight, and the increase is larger than most people expect: 37 to 54 lb at a normal BMI, against 25 to 35 lb for one baby. That is 12 to 19 lb more, and it is not optional padding. Two placentas, two babies and a larger blood volume all cost weight.

The pattern holds across the bands. Overweight with twins is 31 to 50 lb, obese with twins is 25 to 42 lb, and the weekly pace after week 13 runs 1.3 to 1.8 lb at a normal BMI. The underweight twin range is the one gap in the evidence: the 2009 report did not issue a separate recommendation, so most practices use the normal-weight range of 37 to 54 lb.

Twins also change the timeline. The average twin pregnancy delivers around 35 to 36 weeks, so the 40 week total is a ceiling that many people never reach. The calculator still shows the week 40 range because it is the reference point, but read the week 30 to 34 numbers as the practical target.

What the gain is made of

Weight gained in pregnancy is not mostly fat, and that is why the range is generous. At delivery, a 26 lb gain breaks down into the pieces below.

Only the last line, about 7 lb of fat and other stores, is the part that has to be lost afterward. The baby, placenta, fluid and blood leave within the first two weeks. That is why the scale often drops 12 to 15 lb by the first postpartum check.

It also explains why a 35 lb gain is not 35 lb of fat. A person who gains at the top of the normal range may leave the hospital 15 lb lighter and be back within 5 lb of their starting weight by six months without doing anything dramatic.

When the scale does not follow the curve

Real pregnancies do not track a line. Water moves the number by 2 to 5 lb in a day, and the third trimester brings swelling in the feet and hands that can add several pounds in a week. A single high reading is information, not a verdict.

Some patterns do deserve a call. A sudden gain of 3 to 5 lb in a week, especially with swelling in the hands or face, a headache that will not go away, or changes in vision, is worth reporting the same day, because those can be signs of preeclampsia. Losing weight after week 14, or gaining nothing for three visits, is also worth raising.

At the other end, gaining faster than the range is common and treatable without dieting. The usual approach is to keep activity at 150 minutes a week, add protein and vegetables, and cut liquid calories, not to restrict food. Deliberate weight loss in pregnancy is not recommended at any BMI, and the ACOG guidance on obesity and pregnancy is clear that the focus should be on nutrition and activity rather than the scale.

What to do with the number

Weigh yourself at the same time of day, on the same scale, once a week. Daily weighing adds noise without adding information, and the week to week trend is what your clinician will ask about anyway. Bring the numbers to your prenatal visits rather than trying to interpret each one alone.

Food quality does more than portion control. The second trimester needs about 340 extra calories a day and the third about 450, which is a sandwich and a glass of milk, not a second dinner. Protein, iron, folate, calcium and vitamin D are the nutrients that matter most, and a prenatal vitamin covers the gaps that diet does not.

Activity stays at 150 minutes of moderate movement a week, which is the same target as before pregnancy. Walking at 3 mph, swimming and prenatal yoga all count. The NICHD pregnancy overview covers the visit schedule and the tests that go with it, and the Institute of Medicine report on weight gain is the source for every range on this page.

This is an estimate for planning, not medical advice. Your clinician may set a different target based on your health, the baby's growth and how the pregnancy is progressing.