Pregnancy Weight Gain Calculator
Based on 2009 IOM guidelines — the standard used by obstetricians worldwide. Enter your pre-pregnancy BMI and current week to get your personalised recommended weight gain range, trimester breakdown, and weekly rate.
Enter your pre-pregnancy details and current gestational week. The calculator applies IOM 2009 guidelines to show your recommended total gain, where you should be right now, and your weekly target rate.
Weeks 1–13
Weeks 14–27
Weeks 28–40
How Much Weight Should You Gain During Pregnancy?
Pregnancy weight gain isn't one-size-fits-all. The right amount depends on your weight before pregnancy — specifically your pre-pregnancy BMI (Body Mass Index). Gaining too little can affect your baby's growth and increase the risk of preterm birth. Gaining too much is associated with gestational diabetes, pre-eclampsia, a higher chance of caesarean delivery, and difficulty losing weight after birth.
The evidence-based standard used by obstetricians and midwives worldwide is the 2009 Institute of Medicine (IOM) gestational weight gain guideline — updated and endorsed by the American College of Obstetricians and Gynecologists (ACOG). These guidelines give a recommended total weight gain range and a weekly rate of gain for each BMI category. This calculator applies those guidelines to your pre-pregnancy BMI and current gestational week to give you a personalised result.
IOM 2009 Pregnancy Weight Gain Guidelines — the Full Table
The IOM guidelines are based on pre-pregnancy BMI, which is calculated as weight (kg) ÷ height (m)². The table below shows the recommended total weight gain and weekly rate for singleton pregnancies, and the provisional guidelines for twin pregnancies.
| BMI category | Pre-pregnancy BMI | Singleton total gain | Twins total gain | Weekly rate (2nd/3rd tri) |
|---|---|---|---|---|
| Underweight | Below 18.5 | 12.5–18 kg (28–40 lb) | No guideline* | ~0.44–0.58 kg/week |
| Normal weight | 18.5–24.9 | 11.5–16 kg (25–35 lb) | 16.8–24.5 kg (37–54 lb) | ~0.35–0.50 kg/week |
| Overweight | 25.0–29.9 | 7–11.5 kg (15–25 lb) | 14.1–22.7 kg (31–50 lb) | ~0.23–0.33 kg/week |
| Obese | 30.0 and above | 5–9 kg (11–20 lb) | 11.3–19.1 kg (25–42 lb) | ~0.17–0.27 kg/week |
*The IOM did not establish twin guidelines for underweight women due to insufficient data. Consult your healthcare provider.
Source: National Academies of Sciences — Weight Gain During Pregnancy: Reexamining the Guidelines (2009)
Where Does Pregnancy Weight Gain Actually Go?
A common misconception is that pregnancy weight gain is mostly fat. In reality, most of the recommended gain has a specific physiological purpose. Here's the approximate breakdown for a normal-weight woman gaining the recommended 11.5–16 kg:
- Baby: approximately 3.0–3.5 kg at full term
- Placenta: approximately 0.7 kg
- Amniotic fluid: approximately 0.8 kg
- Uterus growth: approximately 0.9 kg
- Increased breast tissue: approximately 0.9 kg
- Increased blood volume: approximately 1.5 kg
- Fluid retention: approximately 1.5–2.0 kg
- Maternal fat stores: approximately 2.5–4.0 kg (energy reserve for breastfeeding)
This is why even "lean" recommended gains of 11.5 kg are well above just the baby's weight — the body needs those extra kilograms to support the pregnancy and prepare for breastfeeding. According to the World Health Organization, appropriate gestational weight gain is a vital sign of maternal and fetal nutritional status.
What This Calculator Shows You
Enter your height and pre-pregnancy weight and the calculator works out your pre-pregnancy BMI and BMI category. Select your current gestational week and — optionally — your current weight. The results show:
- Your BMI category and which IOM guideline applies to you
- Recommended total weight gain range for your full pregnancy
- Expected gain by your current week based on the IOM rate of gain
- Where you stand right now — on track, below range, or above range (if you enter your current weight)
- Weekly rate of gain for the second and third trimesters
- Trimester breakdown showing how gain is distributed across all three trimesters
Why Staying Within the IOM Range Matters
Research consistently shows that staying within the IOM-recommended range is associated with the best outcomes for both mother and baby. A large study published in the American Journal of Obstetrics & Gynecology found that women who gained excessively (particularly 20+ lbs above guideline) had significantly higher rates of pre-eclampsia, eclampsia, and caesarean delivery. Women who gained below the recommendations had lower risk of some obstetric complications but higher rates of small-for-gestational-age babies.
The message isn't that the number on the scale determines outcomes — many factors contribute — but that staying reasonably within the recommended range gives both mother and baby the best statistical chance of a healthy pregnancy and birth.
Pregnancy Weight Gain by Trimester
First Trimester (Weeks 1–13)
The first trimester accounts for very little of total weight gain — typically 0.5–2 kg (1–4.5 lbs) for all BMI categories. The baby is tiny (about 28g at 12 weeks), and your body is focused on building the placenta, increasing blood volume, and laying hormonal groundwork. Morning sickness often suppresses appetite and may cause weight loss in this period, which is normal.
Second Trimester (Weeks 14–27)
This is when steady, consistent weight gain begins. The baby is growing rapidly, your blood volume continues to expand, and energy stores start building. For normal-weight women, this typically means gaining around 0.35–0.50 kg per week. Total second-trimester gain is usually 4–6 kg.
Third Trimester (Weeks 28–40)
The baby doubles in weight in the final trimester, and fluid retention increases. Weekly gain rates are similar to the second trimester. Total third-trimester gain is often 4–6 kg. Some women notice weight gain slowing or plateauing in the final weeks — this is normal.
Twin Pregnancies — Higher Recommendations
Twin pregnancies require more total weight gain than singletons because of the additional baby, placenta, and amniotic fluid. The IOM's provisional guidelines for twin pregnancies are: 16.8–24.5 kg (37–54 lbs) for normal-weight women, 14.1–22.7 kg (31–50 lbs) for overweight, and 11.3–19.1 kg (25–42 lbs) for obese. There are no specific guidelines for underweight women carrying twins due to insufficient research data, as noted by the National Academies. Twin pregnancies are always managed more closely by a maternal-fetal medicine specialist — always follow their specific guidance.