Pregnancy Weight Gain Calculator - By Week, BMI & Trimester

Gaining weight during pregnancy is not just normal, it's necessary. But how much is the right amount? That depends on where you started, how far along you are, and whether you're carrying one baby or two. This calculator takes your pre-pregnancy weight, height, and current week of pregnancy and gives you a personalized weight gain target based on the Institute of Medicine (IOM) guidelines. Whether you're in your first trimester or heading into the final stretch, you'll get a clear picture of where you should be and what a healthy range looks like for your body. Scroll down for the full breakdown by BMI category, trimester, and week-by-week chart so you know exactly what to expect.

Enter Details

Age(age 18 to 55 only)

Gender

MaleFemale

IOM total gain guidelines (singleton).

Units

Height

Pre-pregnancy weight

Result

Uses pre-pregnancy BMI to suggest total weight-gain range.

Note — This result is an estimate. Talk to a healthcare provider for personalized guidance.

How to Use This Pregnancy Weight Gain Calculator

The calculator is straightforward, but getting accurate results means putting in accurate numbers. Here's what you'll need before you start.

Enter Your Pre-Pregnancy Weight & Height

Use your weight from before you got pregnant, not your current weight. If you don't remember the exact number, a close estimate works, but the more precise you are, the more useful your results will be. Enter your height in feet and inches (or centimeters if you prefer metric). The calculator uses these two values to figure out your pre-pregnancy BMI, which is the starting point for all the recommendations.

Don't use your current weight here. Your pre-pregnancy BMI is what determines which weight gain range applies to you, so swapping in today's number would throw off the whole calculation.

Select Your Current Week of Pregnancy

Enter how many weeks pregnant you are right now. If you're not sure, your first ultrasound dating scan is the most reliable reference. The calculator uses your current week to estimate how much weight you should have gained so far and how much you can expect to gain from here to your due date.

Keep in mind that week-by-week targets are averages. A pound or two off in either direction on any given week is completely normal. The trend over several weeks matters more than any single weigh-in.

Singleton vs. Twin Pregnancy - What Changes?

Carrying twins changes the math significantly. The IOM has separate weight gain guidelines for twin pregnancies, and the recommended ranges are notably higher across every BMI category. If you're expecting twins, make sure to select that option in the calculator so your results reflect the right targets.

For example, a woman with a normal pre-pregnancy BMI is advised to gain 25 to 35 pounds with a singleton pregnancy. With twins, that same woman's recommended range jumps to 37 to 54 pounds. The difference is real and it matters, so don't use singleton guidelines if you're carrying two.

How Much Weight Should You Gain During Pregnancy?

There's no one-size-fits-all answer, and anyone who gives you a single number without asking about your starting weight is oversimplifying things. The IOM guidelines, which most OB-GYNs and midwives follow, tie recommended weight gain directly to your pre-pregnancy BMI. The idea is that women who start pregnancy at different weights have different nutritional reserves and different risk profiles, so the targets are adjusted accordingly.

The weight you gain goes toward more than just the baby. It includes the placenta, amniotic fluid, increased blood volume, breast tissue, uterine growth, and fat stores your body builds to support labor and breastfeeding. All of that adds up, and it's supposed to.

Recommended Weight Gain by Pre-Pregnancy BMI (IOM Guidelines)

Pre-Pregnancy BMI Category	BMI Range	Recommended Weight Gain (Singleton)	Recommended Weight Gain (Twins)
Underweight	Below 18.5	28–40 lbs	Not established (consult provider)
Normal Weight	18.5–24.9	25–35 lbs	37–54 lbs
Overweight	25.0–29.9	15–25 lbs	31–50 lbs
Obese	30.0 and above	11–20 lbs	25–42 lbs

These ranges were updated by the IOM in 2009 and remain the standard reference used by most healthcare providers in the U.S. They're designed to support healthy outcomes for both mom and baby while accounting for the fact that different starting points call for different approaches.

Pregnancy Weight Gain for Underweight Women

If your pre-pregnancy BMI was below 18.5, you're in the underweight category and your recommended range is 28 to 40 pounds. That's the widest range of any BMI group, and it's intentionally generous. Women who start pregnancy underweight often have fewer nutritional reserves, so gaining enough weight becomes especially important for supporting fetal development.

If morning sickness or a low appetite is making it hard to gain weight early on, talk to your provider. Getting enough calories and nutrients in the first trimester can be tough, but it's worth addressing sooner rather than later.

Pregnancy Weight Gain for Normal Weight Women

A pre-pregnancy BMI between 18.5 and 24.9 puts you in the normal weight category. The recommended total weight gain is 25 to 35 pounds for a singleton pregnancy. This is the range most people think of as the standard, and it's based on decades of research tying it to healthy birth weights and lower rates of complications.

That said, "normal" is a range, not a single point. A woman at the lower end of this BMI category might naturally gain toward the higher end of the weight range, and vice versa. Your provider can help you figure out what's realistic for your body specifically.

Pregnancy Weight Gain for Overweight Women

With a pre-pregnancy BMI of 25 to 29.9, the recommended weight gain drops to 15 to 25 pounds. Women in this category already have additional fat stores that can help support the pregnancy, so the body doesn't need to build as much from scratch.

Staying within this range can reduce the risk of gestational diabetes, high blood pressure, and delivering a larger-than-average baby. None of that means you should be restricting calories aggressively. It just means the target is more modest, and focusing on nutrient-dense foods tends to make it easier to stay within the range without feeling deprived.

Pregnancy Weight Gain for Obese Women

If your pre-pregnancy BMI was 30 or above, the recommended total weight gain is 11 to 20 pounds. This is the most conservative range, and it reflects the reality that higher starting weight is associated with a greater risk of certain complications, including gestational hypertension, preeclampsia, and cesarean delivery.

Gaining less weight during pregnancy when you start at a higher BMI is not about appearance. It's about reducing risk. Some research has explored whether very limited weight gain or even a slight loss (under careful medical supervision) could be safe in certain cases, but that's a conversation to have directly with your OB or midwife, not something to attempt on your own.

Twin Pregnancy Weight Gain Guidelines

Twin pregnancies have their own set of guidelines because two babies, two placentas, and significantly more amniotic fluid mean your body has a lot more to support. The IOM recommendations for twins are higher across every BMI category, and hitting those targets early in pregnancy is especially important since twins are more likely to be born preterm.

Research from Dr. Barbara Luke and others has found that gaining weight earlier in a twin pregnancy, particularly in the second trimester, is linked to better birth weights and longer gestational age. So while pacing matters in any pregnancy, it's especially relevant when you're carrying multiples.

Normal weight (BMI 18.5–24.9): 37–54 lbs total
Overweight (BMI 25–29.9): 31–50 lbs total
Obese (BMI 30+): 25–42 lbs total

If you're underweight and carrying twins, official IOM guidelines don't specify a range. Your provider will set an individualized target based on your situation.

Pregnancy Weight Gain by Trimester

Total weight gain over nine months is only part of the picture. How that weight is distributed across the three trimesters also matters. Gaining too much too fast in the first trimester, or stalling out in the third, can signal issues worth discussing with your care team. The general pattern is slow and steady early on, then a more consistent weekly gain through the second and third trimesters.

First Trimester (Weeks 1-12)

Most women gain very little in the first trimester, and that's completely normal. The typical recommendation is about 1 to 4 pounds total over the first 12 weeks, though some women gain nothing at all, especially if nausea and food aversions are significant. A few women actually lose a pound or two in the first trimester due to morning sickness, and that's usually not a concern as long as they're staying hydrated and catching up later.

The baby itself is still very small at 12 weeks, so the weight you gain in this stage is mostly from increased blood volume and early fluid changes, not baby weight. Don't stress too much about the scale right now. Focus on getting through nausea, eating what you can tolerate, and staying hydrated.

Second Trimester (Weeks 13-26)

This is when steady, consistent weight gain kicks in. For most women, the recommendation is roughly 1 pound per week during the second trimester, though this varies slightly depending on your starting BMI. If you're underweight, aiming for a little more is fine. If you started with a higher BMI, a bit less per week might be appropriate.

By the end of the second trimester, many women have gained somewhere between 12 and 20 pounds in total, depending on their category. Appetite often picks up in the second trimester after the fog of first-trimester nausea lifts, which naturally supports this rate of gain. This trimester is also when the baby grows significantly and fat stores in your own body start to build in preparation for breastfeeding.

Third Trimester (Weeks 27-40)

Weight gain continues at roughly the same weekly rate through the third trimester, around 1 pound per week for normal-weight women. The baby is doing most of its growing now, and a significant chunk of the weight gained in this trimester goes directly toward fetal growth, amniotic fluid, and preparing your body for labor.

Some women notice their weight gain slows or even plateaus in the last few weeks before delivery. That's common and often not a problem, but mention it to your provider if you're unsure. Swelling can also affect the number on the scale during this trimester, so don't panic if you see a sudden jump. Water retention in the legs and feet is very common in late pregnancy and doesn't reflect fat gain.

Pregnancy Weight Gain by Week (Week-by-Week Chart)

The table below shows approximate cumulative weight gain targets by week for singleton pregnancies across BMI categories. These are averages based on IOM guidelines and typical patterns of fetal and maternal weight distribution. Individual variation is normal, and your actual gain may be slightly above or below these numbers at any given week.

Week	Underweight	Normal Weight	Overweight	Obese
4	0–1 lbs	0–1 lbs	0–1 lbs	0–1 lbs
8	1–2 lbs	1–2 lbs	0–1 lbs	0–1 lbs
12	2–4 lbs	1–4 lbs	1–3 lbs	0–2 lbs
16	6–8 lbs	4–7 lbs	3–5 lbs	2–4 lbs
20	10–13 lbs	8–11 lbs	5–8 lbs	4–6 lbs
24	14–18 lbs	12–15 lbs	8–11 lbs	6–9 lbs
28	18–23 lbs	15–19 lbs	11–14 lbs	8–11 lbs
32	22–28 lbs	18–23 lbs	13–17 lbs	10–14 lbs
36	26–34 lbs	22–29 lbs	15–21 lbs	11–17 lbs
40	28–40 lbs	25–35 lbs	15–25 lbs	11–20 lbs

Use this chart as a general reference point, not a strict grading rubric. Babies grow in spurts, your appetite fluctuates, and the scale doesn't tell the whole story. If your gain is consistently far outside the expected range, that's worth a conversation with your provider, but a week or two of variance is nothing to lose sleep over.

What If You Gain Too Much or Too Little?

Staying within the recommended range is a goal, not a guarantee, and plenty of healthy pregnancies involve gaining a bit more or less than the guidelines suggest. That said, consistently falling well outside the range in either direction does carry some real risks. Knowing what those are can help you understand why your care team pays attention to the scale at prenatal visits.

Risks of Gaining Too Much Weight

Gaining significantly more than recommended is associated with a higher likelihood of several complications. These include gestational diabetes, gestational hypertension and preeclampsia, the need for a cesarean delivery, and having a baby that's large for gestational age (LGA). A larger baby can make delivery more complicated and may increase the child's own risk of obesity later in life.

For the mother, excess weight gain also tends to be harder to lose after delivery, which can affect long-term health. Postpartum weight retention is one of the leading contributors to weight gain over a woman's lifetime, and starting that process with a significant overage makes it harder.

Higher risk of gestational diabetes
Increased likelihood of preeclampsia
Greater chance of C-section delivery
Larger-than-average baby (macrosomia)
More postpartum weight to lose

None of this is meant to be scary. It's just context. If you're gaining more than expected, your provider can work with you on nutrition strategies that support a healthier trajectory without putting you or the baby at risk.

Risks of Gaining Too Little Weight

Gaining too little weight during pregnancy is less talked about, but it carries its own set of concerns. Inadequate weight gain is linked to preterm birth, low birth weight, and poor fetal growth (intrauterine growth restriction, or IUGR). Babies born too small face higher risks of developmental delays, respiratory issues, and other health challenges in the newborn period and beyond.

For the mother, not gaining enough weight can mean insufficient nutrient stores for labor and recovery, as well as difficulty producing enough milk if you plan to breastfeed. Some women gain too little because of severe nausea and vomiting (hyperemesis gravidarum), food insecurity, underlying health conditions, or anxiety around weight and body image. Whatever the reason, it's worth addressing with your care team.

Increased risk of preterm birth
Low birth weight or growth restriction
Reduced nutrient reserves for labor and recovery
Potential challenges with milk supply postpartum

If you're struggling to gain enough weight, your provider or a registered dietitian who specializes in prenatal nutrition can help you find practical, sustainable ways to increase your intake without making mealtime feel like a chore.