Pregnancy Weight Gain Calculator
See the weight range recommended for the week you are in, where you sit against it today, and the full week-by-week path to delivery. The ranges come from the Institute of Medicine guidelines, which start from your BMI before pregnancy rather than from your weight now.
| Week | Recommended weight range | Recommended weight gain |
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Your current week is highlighted. Scroll for the rest.
The number that never moves
Every range on this page is anchored to one figure: your BMI before you were pregnant. Not your weight today, not your weight last month. Enter the same pregnancy at week 10 and week 30 and the recommended total is identical, because it was set the moment the pregnancy started.
That catches people out, and it is worth understanding rather than working around. The guidance exists because outcomes for both mother and baby differ by starting point — someone beginning at a lower BMI has a different optimal gain from someone beginning higher, and the ranges reflect that. Once the pregnancy is underway, the starting point is fixed, so the target is fixed too.
It also means the calculator needs an accurate pre-pregnancy weight to be worth anything. A guess that is ten pounds out can push you into a neighbouring category and shift the whole recommendation by several pounds. If you are unsure, your maternity notes usually record it, and a rough figure from a recent check-up beats a number from memory.
Why the curve is flat then steep
Look at the chart and the band barely rises for the first three months, then climbs in a straight line to delivery. That shape is deliberate, and it follows how a pregnancy actually develops.
weeks 1–13 0 → about 1 to 4 lbs in total
weeks 13–40 from there, straight to the recommended total
The recommended totals, by pre-pregnancy BMI
under 18.5 28 – 40 lbs (twins 37 – 54)
18.5–24.9 25 – 35 lbs (twins 37 – 54)
25.0–29.9 15 – 25 lbs (twins 31 – 50)
30 and over 11 – 20 lbs (twins 25 – 42)
Worked through — 134 lbs before, BMI 23, week 20
total = 25 to 35 lbs, and week 20 is 7 of the 27 weeks past 13
low = 1.1 + (25 − 1.1) × 7 ÷ 27 = 7.3 lbs
high = 4.4 + (35 − 4.4) × 7 ÷ 27 = 12.3 lbs
In the first trimester the embryo weighs almost nothing — a few grams by the end of it — so there is very little to gain weight for. The total expected across those three months is one to four pounds, which is small enough that morning sickness routinely wipes it out. Losing a little early is common and not usually a concern.
From then on the baby, the placenta and the fluid around it all grow steadily, and the recommended gain becomes roughly a pound a week. That is the part people notice, and it is why a woman who gained nothing by week 12 has not fallen behind in any meaningful way.
Where the weight actually goes
A useful corrective to the anxiety around this number: most of the gain is not fat, and most of it is not the baby either.
At term, a baby is typically around seven or eight pounds. The placenta adds a pound and a half, the amniotic fluid about two, the uterus itself around two, breast tissue one to three, and the extra blood and fluid volume your body carries account for another five to seven. Add those up and a large share of a twenty-five pound gain is accounted for before any fat is stored at all.
The fat that does accumulate — commonly six to eight pounds — is not incidental either. It is an energy reserve laid down for the late pregnancy and for breastfeeding, which is metabolically expensive. It is supposed to be there.
All of which explains why the scale keeps climbing in the last weeks even if nothing about how you eat has changed, and why the weight comes off in stages rather than at once after delivery.
Too little and too much both matter
Most of the public conversation about pregnancy weight runs in one direction. The guidance runs in both, and the risks at the low end are real.
Gaining too little is associated with a higher chance of the baby being born small for its gestational age, and with preterm birth. It is the reason the recommendation for someone starting at a high BMI is still a positive number rather than zero, and why pregnancy is not a time to diet even if losing weight would otherwise be sensible.
Gaining well above the range is associated with a larger baby, which raises the likelihood of a difficult delivery and of a caesarean, and with weight that is harder to lose afterwards. It also tracks with a higher chance of gestational diabetes and raised blood pressure, though the direction of cause there is tangled.
Two things are worth holding onto. These are population-level associations, not predictions about you — plenty of pregnancies outside the range are entirely uneventful. And the ranges are wide on purpose: the gap between the bottom and top of a normal-BMI recommendation is ten pounds, which is a lot of room.
Twins are a different sum
Answer yes to the twins question and the totals jump considerably — from 25 to 35 pounds up to 37 to 54 for someone starting at a normal BMI. That is more than double the baby weight, because everything else scales too: two placentas, more fluid, more blood volume.
The twin guidance is also less firmly established than the singleton figures. There is no published range for an underweight start with twins, which is why this calculator uses the normal-weight range in that case — a reasonable default rather than a recommendation in its own right. Anyone carrying twins should be having this conversation with their obstetric team regardless, since twin pregnancies are monitored more closely for other reasons anyway.
The scale is a poor day-to-day guide
The week-by-week table invites daily weighing, and daily weighing is mostly noise.
Body weight swings by a couple of pounds within a single day from food, fluid and salt, and pregnancy adds fluid retention that varies with heat, activity and how long you have been on your feet. Swelling in the third trimester can add several pounds that come and go. Against that background, a single reading tells you very little about whether you are on track.
What is informative is the trend across a few weeks, measured the same way each time: same day of the week, first thing, after the bathroom, before eating, in the same clothes. If the line is roughly parallel to the band on the chart, the gap between you and the middle of the range is not the point — the slope is.
When these ranges do not apply
The Institute of Medicine ranges were drawn up for singleton and twin pregnancies in generally healthy adults, and several situations fall outside them.
Triplets and higher-order pregnancies have no equivalent published range and are managed individually. Adolescent pregnancy is treated differently, since the mother is still growing herself. So is a pregnancy following bariatric surgery, where absorption is altered and gain is monitored closely. Gestational diabetes, hyperemesis severe enough to cause real weight loss, and pre-existing conditions affecting weight all change the picture in ways a general range cannot capture.
And the ranges assume the pregnancy is progressing normally. If your weight is moving sharply in either direction, if you have sudden swelling in your face or hands, or if you are unable to keep food down, those are reasons to contact your midwife or doctor rather than to consult a table.
Frequently asked questions
How much weight should I gain during pregnancy?
It depends on your BMI before you were pregnant. Starting at a normal BMI the recommended total is 25 to 35 pounds; underweight is 28 to 40, overweight 15 to 25, and 30 or above 11 to 20. Almost all of that comes after the first trimester, at roughly a pound a week.
Why does the calculator use my weight before pregnancy?
Because the guidance is built around your starting point, which does not change once you are pregnant. Outcomes differ by pre-pregnancy BMI, so the recommended total is set from it. That is also why the range stays the same whether you check at week 10 or week 30.
I have not gained anything in the first trimester. Is that a problem?
Usually not. The expected gain across the whole first trimester is only about one to four pounds, and nausea commonly cancels it out. Some early loss is common too. It is worth mentioning at your next appointment, but on its own it is rarely a concern.
Is it safe to lose weight while pregnant?
Deliberate dieting is not recommended, even starting from a high BMI. The recommendation for a BMI of 30 or above is still a gain of 11 to 20 pounds rather than zero, because gaining too little is associated with small-for-age babies and preterm birth. Any plan to restrict intake during pregnancy should come from your doctor or midwife.
How much of pregnancy weight is the baby?
Less than most people expect. A baby at term is typically seven or eight pounds out of a twenty-five pound gain. The placenta, amniotic fluid, a larger uterus, breast tissue and a significantly increased blood and fluid volume make up most of the rest, with six to eight pounds of fat stored as an energy reserve for late pregnancy and breastfeeding.
What if I am carrying twins?
The totals are substantially higher — 37 to 54 pounds from a normal starting BMI, against 25 to 35 for one baby. The twin guidance is less well established than the singleton figures, and there is no published range for an underweight start, so anyone expecting twins should follow their obstetric team's advice rather than a general table.
How often should I weigh myself?
Weekly at most, and the trend matters far more than any single reading. Weight swings a couple of pounds within a day from food and fluid alone, and pregnancy adds retention that varies with heat and activity. Weigh at the same time of day, in the same conditions, and read the slope rather than the number.
These ranges come from Institute of Medicine guidance for singleton and twin pregnancies and are general information, not personal medical advice. They do not apply to triplets or higher-order pregnancies, to adolescent pregnancy, after bariatric surgery, or where gestational diabetes, severe sickness or other conditions affect weight. Your midwife or doctor knows your history and how your pregnancy is progressing; this calculator knows four numbers. Contact them rather than a table if your weight is changing sharply in either direction, if you have sudden swelling of the face or hands, if you cannot keep food down, or about any concern regarding your pregnancy. Learn more about how we build our calculators and read our privacy policy.