Pregnancy Weight Gain Calculator
Find out how much weight gain is recommended during your pregnancy based on your pre-pregnancy BMI, following IOM guidelines. Enter your current week to see expected gain progress.
Reviewed by Richard Ross · Last updated April 2026
How Pregnancy Weight Gain Calculator works
IOM pregnancy weight gain guidelines
The Institute of Medicine (IOM) guidelines are the most widely used reference for pregnancy weight gain. They base recommended total gain on pre-pregnancy BMI: underweight women should gain more (12.5–18 kg for singletons); normal weight 11.5–16 kg; overweight 7–11.5 kg; obese 5–9 kg. For twin pregnancies, all recommendations are higher. These ranges reflect both maternal and infant health outcomes.
Where does pregnancy weight gain go?
A full-term pregnancy weight gain of 12–13 kg is typically distributed as: baby (3–4 kg), placenta (~0.7 kg), amniotic fluid (~0.8 kg), uterus enlargement (~1 kg), breast tissue (~0.5 kg), increased blood volume (~1.5 kg), fluid retention (~2 kg), and stored fat for energy and breastfeeding (~2–3 kg). Most of this weight is lost in the weeks following delivery.
Pattern of gain through pregnancy
Weight gain typically follows a pattern: minimal gain in the first trimester (0.5–2 kg) followed by more consistent gain from the second trimester onwards. This calculator uses the IOM approximate model: roughly 1.5 kg in the first trimester, then steady weekly gain based on the recommended total. Significant deviation from this pattern — especially sudden large gains — can indicate complications such as pre-eclampsia and should be discussed with a midwife.
What if I gain outside the recommended range?
Gaining below the recommended range is associated with low birth weight and preterm birth. Gaining above the recommended range increases risks of gestational diabetes, large-for-gestational-age babies, and difficulty losing weight postpartum. However, these are population-level averages — individual variation is normal, and your midwife or obstetrician is best placed to advise on your specific situation.
Frequently asked questions
How much weight should I gain during pregnancy?
It depends on your pre-pregnancy BMI. IOM guidelines: underweight (BMI < 18.5): 12.5–18 kg; normal (18.5–24.9): 11.5–16 kg; overweight (25–29.9): 7–11.5 kg; obese (BMI ≥ 30): 5–9 kg. These are for single pregnancies; twin pregnancies require more gain.
Is it safe to diet during pregnancy?
Intentional calorie restriction is not recommended during pregnancy, even if you are overweight or obese. The focus should be on nutrient-dense food rather than calorie restriction. If you are concerned about weight gain, discuss this with your midwife, who can refer you to a dietitian if needed.
How much weight is normal to gain in the first trimester?
Weight gain in the first trimester is typically minimal — around 0.5–2 kg in total. Some women lose weight in the first trimester due to morning sickness, which is generally not harmful to the baby if it resolves by the second trimester. Most weight gain occurs from the second trimester onwards.
Can I use this calculator for twins?
Yes — select "Twins" to see the higher recommended gain ranges for twin pregnancies based on IOM guidelines. Twin pregnancies are managed differently from singleton pregnancies and require closer monitoring. Discuss your weight gain targets with your obstetrician.
How much weight gain is normal during pregnancy?
Recommended weight gain depends on pre-pregnancy BMI. For a normal BMI (18.5-24.9): 11.5-16kg total. For underweight (BMI below 18.5): 12.5-18kg. For overweight (BMI 25-29.9): 7-11.5kg. For obese (BMI 30+): 5-9kg. These Institute of Medicine guidelines are based on optimal pregnancy outcomes. Gaining outside these ranges is associated with complications — excessive gain with gestational diabetes, pre-eclampsia, large-for-gestational-age babies, and difficult labour; insufficient gain with preterm birth and low birth weight.
When does pregnancy weight gain happen?
In the first trimester (weeks 1-12), weight gain is minimal — typically 1-2kg total, and some women lose weight due to nausea and vomiting. Most weight is gained in the second and third trimesters. Average weekly gain in the second trimester (weeks 13-28) is around 0.5kg/week. Third trimester gain continues at a similar rate. The foetus contributes approximately 3-3.5kg at term; the placenta 0.7kg; amniotic fluid 0.8kg; increased blood volume 1.8kg; breast tissue 0.4kg; and maternal fat and fluid the remainder.
Is dieting safe during pregnancy?
Deliberate calorie restriction during pregnancy is not recommended and can harm foetal development. Even women with obesity should not attempt to lose weight during pregnancy — the goal is to gain within the lower end of the recommended range, not to lose. Nutritional quality matters more than quantity: adequate folate, iron, calcium, vitamin D, iodine, and omega-3 DHA are important for foetal neurological and skeletal development. Speak to your midwife or GP if you are concerned about excessive weight gain.
How much weight is retained after birth?
Most women retain some weight 6 months postpartum. Studies show average retention of 1-3kg at 6 months, with higher retention in those who gained above the recommended range. Breastfeeding can help mobilise fat stores but typically provides only modest weight loss in itself. Most retained weight is lost within 12-18 months with normal activity levels, though hormonal changes and sleep deprivation (which affects appetite hormones) make postpartum weight management challenging.
Does pregnancy weight gain affect the baby's size?
Yes, but the relationship is complex. Excessive maternal weight gain is associated with large-for-gestational-age (LGA) babies, which increases the risk of birth complications including shoulder dystocia, C-section, and birth trauma. LGA babies also have higher rates of childhood obesity and metabolic syndrome. Insufficient weight gain is associated with small-for-gestational-age (SGA) babies with increased risk of preterm birth, low birth weight, and developmental problems. Neither extreme is without risk.
What exercise is safe during pregnancy?
NHS and RCOG guidance supports regular moderate-intensity exercise throughout an uncomplicated pregnancy. Suitable activities include walking, swimming, stationary cycling, yoga, and low-impact aerobics. Exercise has benefits for maternal weight gain, blood pressure, gestational diabetes prevention, and mood. Activities to avoid include contact sports, activities with fall risk (skiing, horse riding), scuba diving, and vigorous exercise lying flat on the back after the first trimester. Always inform your midwife or GP about your exercise habits.
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This calculator provides estimates for informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider.