Pregnancy Weight Gain Calculator

This calculator gives the healthy weight range for the week of pregnancy you are in, based on your height and your weight before you conceived. Someone 165 cm who started at 50 kg has a pre-pregnancy BMI of 18.4, which is underweight, so at week 18 the recommended range is 52.8 to 55.0 kg and by week 40 it is 62.7 to 68.1 kg. The chart plots the whole band from conception to delivery with your current weight marked on it, and the table underneath lists every week.

How the range is built

Everything follows from your pre-pregnancy BMI, because that is what the Institute of Medicine guidelines key off. Lower starting BMI means a larger recommended gain, and the reasoning is straightforward: a woman starting underweight has less stored energy to draw on, while a woman starting with more has some of the requirement already covered.

The IOM never published a twin range for women starting underweight, since too few pregnancies were available to set one; the figure shown for that row is the one usually quoted alongside their table, and it deserves more caution than the other three.

Gain is not spread evenly. The first trimester adds very little, roughly 1 to 4 pounds in total, because the fetus itself weighs almost nothing at that stage. From about week 13 the curve straightens out and runs at a steady weekly rate to term, which is why the band on the chart is nearly flat at the start and then climbs in a straight line. Around a pound a week through the second and third trimesters is the usual shape for a woman of normal weight.

Where the weight actually goes

Very little of it is the baby. A typical distribution at term:

The fat stores line is the one people find surprising and it is not a design flaw. Those stores exist to fuel the third trimester and breastfeeding, which is why trying to avoid them entirely tends to backfire.

How much extra to eat

Less than most people expect, and nothing at all at first. The common advice of an extra 300 calories a day is a rough average across the whole pregnancy. In practice the first trimester needs no increase, the second needs roughly 340 extra calories a day, and the third roughly 450. Eating for two in the literal sense overshoots by a wide margin.

To put 340 calories in food terms: a sandwich and a glass of milk, or a baked potato with a couple of ounces of chicken and an apple. The Calorie Calculator gives your baseline to add to.

Protein requirements rise as well, to around 71 grams a day, which is higher than the standard adult recommendation and worth checking against what you actually eat. The Protein Calculator covers that side.

The nutrients that matter most

• Folic acid, 600 mcg a day during pregnancy and 400 mcg before conceiving. It reduces neural tube defects, and because the neural tube closes in the first month, often before a pregnancy is confirmed, the pre-conception dose is the one that does the work. Found in leafy greens, citrus, beans and fortified grains.
• Iron, 27 mg a day, nearly double the usual requirement, because blood volume expands by 40% to 50%. Deficiency causes fatigue and raises the risk of preterm birth. Pair plant sources with vitamin C to absorb more, and keep tea and coffee away from iron-rich meals.
• Calcium, 1,000 mg a day, with vitamin D at 600 IU alongside it. If intake is short the baby draws on your bones rather than going without.
• Choline, 450 mg a day, important for brain development and missing from many prenatal vitamins. Eggs and lean meat are the main sources.
• DHA, 200 to 300 mg a day, an omega-3 fat that supports brain and eye development. Two portions of low-mercury fish a week covers it.

A prenatal vitamin covers most of these, but check the label rather than assuming, since choline in particular is often absent or present in token amounts.

What to avoid

The list is shorter than the internet suggests, and a few items on it matter a great deal:

• Alcohol. No amount has been shown to be safe. It raises the risk of miscarriage and stillbirth and causes fetal alcohol spectrum disorders.
• Smoking and vaping. Linked to preterm birth, low birthweight, placental problems and sudden infant death syndrome. Quitting at any stage helps.
• High-mercury fish. Shark, swordfish, king mackerel, tilefish and bigeye tuna. Salmon, shrimp, pollock, cod, tilapia and light canned tuna are fine, and two to three portions a week are actively encouraged.
• Raw or undercooked animal foods. Sushi, raw shellfish, undercooked meat and runny eggs, because of listeria, salmonella and toxoplasma. Pregnancy makes listeria infection far more dangerous than usual.
• Unpasteurised dairy and juice, plus soft cheeses made from unpasteurised milk. Raw sprouts should be cooked through.
• Caffeine above about 200 mg a day, which is roughly one 12 oz coffee. Herbal teas are poorly studied, so treat them with the same caution.

Wash fruit and vegetables properly, since soil-borne toxoplasma is a real if unglamorous risk.

When gain runs high or low

Both directions carry consequences, which is the reason for a range rather than a single number. Gaining too little is associated with preterm birth and with babies small for gestational age. Gaining too much raises the risk of a large baby and the delivery complications that follow, of caesarean birth, of gestational diabetes and preeclampsia, and of weight retained after the birth that never quite comes off.

That said, do not over-read a single week. Weight moves with fluid, salt, bowel habit and time of day, and the week-to-week noise easily exceeds the real change. Losing a little in the first trimester through nausea is common and rarely a problem. What matters is the trend across a month sitting roughly parallel to the band on the chart, and that conversation belongs with your midwife or doctor, who has your bloods and your history in front of them.

Weighing yourself sensibly

Weigh once a week rather than daily, at the same time of day, on the same scale, in similar clothing. First thing in the morning after using the bathroom is the most repeatable. Daily weighing during pregnancy tends to produce anxiety without producing information, because the swings from fluid and food easily outsize a week of genuine gain. Write the number down with the week beside it so you are comparing like with like at your next appointment.

Twins

Twin pregnancies need substantially more, and the guidance is provisional rather than settled. Switching the twins option changes the whole band, adding roughly 12 pounds to the recommended total for a woman of normal weight. Twin pregnancies also tend to deliver earlier, around 36 weeks, so the week-40 figure in the table is more of a theoretical endpoint than a date to plan around.

Reading the chart and table

The shaded band is the recommended range from conception to week 40, and the dot is your current weight at your current week. Sitting slightly outside the band is not an emergency; the useful signal is whether your line is climbing at roughly the same angle as the band, since the slope matters more than the position.

The second chart compares the recommended total across all four BMI categories with yours highlighted, which shows how much the starting point changes the target: an underweight woman is advised to gain more than twice what a woman starting in the obese category is. The table lists all 40 weeks with both the weight range and the gain it represents, your current week highlighted, so you can look ahead as easily as check today.

One last thing. These are population guidelines built for healthy pregnancies, and they cannot see your medical history, a multiple beyond twins, or any complication you are managing. Use the number as a reference point for a conversation with your care provider, not as a target to chase on your own. The Pregnancy Calculator and BMI Calculator cover the dates and the starting figure this page depends on.