Pregnancy Weight Gain Calculator
Estimate how much weight you should gain in pregnancy based on your pre-pregnancy BMI, for a single baby or twins. The calculator uses the Institute of Medicine (IOM/NASEM) ranges, shows whether your gain is on track for your current week, projects a healthy weight target for this week, and gives a week-by-week schedule.
Formula
Worked example
A 165 cm woman weighing 64 kg before pregnancy has a BMI of 23.5 (normal weight), so the IOM singleton range is 25-35 lb total. By week 20, an on-track gain is roughly 7-11 lb, putting her projected weight near 67-69 kg. If she is carrying twins, the total range rises to 37-54 lb.
How much weight should you gain in pregnancy?
The amount of weight that is healthy to gain during pregnancy depends mainly on your weight before you conceived, expressed as your body mass index (BMI), and on whether you are carrying one baby or twins. The Institute of Medicine (now the National Academies of Sciences, Engineering, and Medicine) published ranges in 2009 that are still endorsed by the American College of Obstetricians and Gynecologists (ACOG). Women who start at a lower BMI are advised to gain more, while women who start at a higher BMI are advised to gain less. This calculator computes your pre-pregnancy BMI, looks up the matching range for a singleton or twin pregnancy, and shows the total pounds you should aim to gain.
Twin pregnancies need more
A twin pregnancy calls for substantially more weight gain than a singleton because there are two babies, two placentas and more amniotic fluid. The IOM twin ranges are 37 to 54 lb for normal-weight women, 31 to 50 lb for overweight women and 25 to 42 lb for women with obesity. There is no firm twin range for underweight women, so the calculator notes this and your provider will set an individual goal. Switch the pregnancy type to twins and every output, the on-track range, the projected weight, the chart and the schedule, updates to the twin guidance.
When the weight is gained matters
Gain is not spread evenly across the nine months. In the first trimester, most women gain only about one to four and a half pounds, because the baby is still tiny and much of the early change is fluid and breast tissue. The bulk of healthy gain happens in the second and third trimesters at a fairly steady weekly rate, roughly half a pound to just over a pound per week for a singleton, depending on your BMI category. The on-track range and the week-by-week schedule apply that first-trimester allowance and then the appropriate weekly rate, so you can see whether your gain so far is reasonable and what a healthy weight looks like for how far along you are.
Why staying in range helps
Gaining within the recommended range is linked to better outcomes for both parent and baby. Gaining too little raises the chance of a small or preterm baby, while gaining too much raises the risk of gestational diabetes, high blood pressure, a large baby, cesarean delivery, and difficulty losing the weight afterward. The ranges are guides, not rigid rules: a single week above or below is rarely a problem, and what matters most is the overall trend. Your obstetric provider will weigh you at visits and can personalise the target if you are carrying twins, started at an unusual BMI, or have other health conditions.
IOM/NASEM recommended weight gain by pre-pregnancy BMI
| Pre-pregnancy BMI | Category | Singleton total | Twin total | Rate (2nd/3rd trimester) |
|---|---|---|---|---|
| Below 18.5 | Underweight | 28-40 lb (13-18 kg) | No firm range | ~1.0-1.3 lb/week |
| 18.5-24.9 | Normal weight | 25-35 lb (11-16 kg) | 37-54 lb (17-25 kg) | ~0.8-1.0 lb/week |
| 25.0-29.9 | Overweight | 15-25 lb (7-11 kg) | 31-50 lb (14-23 kg) | ~0.5-0.7 lb/week |
| 30.0 and above | Obese | 11-20 lb (5-9 kg) | 25-42 lb (11-19 kg) | ~0.4-0.6 lb/week |
Total recommended gain for a singleton and for twins, plus the singleton second/third-trimester weekly rate. About 1-4.4 lb is expected in the first trimester. The IOM gives no firm twin range for underweight women.
Frequently asked questions
How much weight should I gain if I am pregnant with twins?
Switch the pregnancy type to twins. The IOM twin ranges are about 37-54 lb for normal-weight women, 31-50 lb for overweight women, and 25-42 lb for women with obesity, with no firm range for underweight women. The calculator applies these to your BMI and projects an on-track range and a healthy weight for your week.
What weight should I be at this week of pregnancy?
Enter your current week and the calculator projects a healthy weight range by adding the on-track gain for that week to your pre-pregnancy weight. The week-by-week schedule lists both the cumulative gain and the projected weight every two weeks, in your chosen units.
I gained more than the range early on. Is that a problem?
Not necessarily. The total range and weekly rates are guides, and weight naturally fluctuates with fluid, sodium, and meals. What matters is the overall trend rather than any single weigh-in. Bring it up at your next prenatal visit, where your provider can look at the full picture.
Does this calculator replace medical advice?
No. It applies general IOM/NASEM guidelines for a healthy singleton or twin pregnancy and cannot account for your full medical history. Always follow the advice of your obstetrician, midwife, or other prenatal care provider, who can tailor a weight-gain goal to your situation.