How Much Weight Should You Gain During Pregnancy?
The amount of weight you should gain during pregnancy depends on your pre-pregnancy BMI. The most widely used clinical reference is the 2009 Institute of Medicine (IOM) — now the National Academies of Sciences, Engineering, and Medicine — guidelines. These remain the primary evidence-based standard used by OB-GYNs and midwives across the United States and many countries worldwide.
For a singleton pregnancy, the recommended total weight gain ranges from as little as 5–9 kg (11–20 lbs) for women with a BMI of 30 or above, up to 12.5–18 kg (28–40 lbs) for women who are underweight before pregnancy. The range accounts for the weight of the baby, placenta, amniotic fluid, increased blood volume, breast tissue, and fat stores your body builds as an energy reserve for labour and breastfeeding.
Twin pregnancies require more weight gain overall, though IOM guidelines for twins only cover normal weight, overweight, and obese categories — there is no published recommendation for underweight women carrying twins, so a healthcare provider consultation is essential in that case. For triplets or higher-order multiples, IOM guidelines do not apply, and individualised clinical guidance is necessary.
Why Pregnancy Weight Gain Recommendations Vary by BMI
Not all pregnancies start from the same baseline, which is why a one-size-fits-all weight-gain target does not work. The IOM guidelines account for this with four distinct BMI categories:
- Underweight (BMI < 18.5): Women who begin pregnancy underweight have fewer energy reserves and generally need to gain the most — up to 18 kg (40 lbs) — to support both their own health and the baby's growth.
- Normal weight (BMI 18.5–24.9): The middle range of 11.5–16 kg (25–35 lbs) covers the typical energy needs of pregnancy without unnecessary excess that can be hard to lose postpartum.
- Overweight (BMI 25.0–29.9): Women in this range are advised to gain less — 7–11.5 kg (15–25 lbs) — because existing fat stores can supply some of the additional energy pregnancy demands.
- Obese (BMI ≥ 30.0): The recommended range of 5–9 kg (11–20 lbs) is the most conservative. Excess weight gain during pregnancy with obesity is associated with higher risks of gestational diabetes, pre-eclampsia, and caesarean delivery. However, it is still important to gain some weight — near-zero or negative gains are not recommended.
These ranges are not arbitrary. They were derived from large population studies examining outcomes for both mother and baby, balancing foetal growth, birth weight, and maternal health risks.
Trimester-by-Trimester Weight Gain Guide
Pregnancy weight gain is not linear — most of it happens in the second and third trimesters, not evenly spread across all 40 weeks.
First Trimester (Weeks 1–12)
Most women gain very little in the first trimester — typically only 0.5–2 kg (1–4 lbs) in total. Morning sickness, food aversions, and fatigue are common during these weeks and can actually lead to weight loss for some women. This is generally not a cause for concern as long as you are staying hydrated and taking a prenatal vitamin. The baby is tiny at this stage, and the bulk of the weight gain later comes from the placenta, fluid, and maternal tissue changes rather than foetal size alone.
Second Trimester (Weeks 13–27)
This is typically when appetite returns and weight gain accelerates. For most women, the recommended pace is steady and consistent throughout the second trimester. A normal-weight woman, for example, should aim for roughly 0.35–0.50 kg (0.77–1.10 lbs) per week. Overweight women should aim for a slightly slower pace of 0.23–0.33 kg per week, while underweight women may need closer to 0.44–0.58 kg per week. The baby grows rapidly during these weeks — most of the organ development is complete and growth in length and weight accelerates.
Third Trimester (Weeks 28–40)
Weight gain continues at a similar weekly pace in the third trimester, but the composition changes. Much of the additional weight in the final weeks is the baby themselves, as well as the increased amniotic fluid and blood volume needed to support delivery. Many women also experience some fluid retention toward the end of pregnancy, which can cause temporary weight fluctuations from day to day. Tracking weekly averages rather than daily weigh-ins gives a more accurate picture of progress.
Remember that these guidelines are population-level recommendations. Individual variation is normal and expected. Your midwife or OB-GYN will monitor your weight gain at each antenatal appointment and can advise whether your pattern is appropriate for your specific circumstances.