IOM Weight Gain Guidelines
The Institute of Medicine (IOM), now the National Academy of Medicine, published comprehensive guidelines for pregnancy weight gain in 2009. These guidelines are based on pre-pregnancy BMI and are endorsed by the American College of Obstetricians and Gynecologists (ACOG).
| Pre-Pregnancy BMI Category | BMI Range | Total Weight Gain (kg) | Weekly Rate (2nd/3rd Tri) |
|---|---|---|---|
| Underweight | < 18.5 | 12.5 – 18.0 kg | ~0.51 kg/week |
| Normal Weight | 18.5 – 24.9 | 11.5 – 16.0 kg | ~0.42 kg/week |
| Overweight | 25.0 – 29.9 | 7.0 – 11.5 kg | ~0.28 kg/week |
| Obese | ≥ 30.0 | 5.0 – 9.0 kg | ~0.22 kg/week |
Weight Gain by Trimester
First Trimester (Weeks 1–13)
During the first trimester, weight gain is typically modest — approximately 0.5 to 2 kg total. Some women may even lose weight due to morning sickness. The fetus at this stage weighs very little, and most of the gain is from increased blood volume and uterine growth.
Second Trimester (Weeks 14–27)
Weight gain accelerates during the second trimester. This is when the baby grows rapidly and the body stores additional fat reserves for breastfeeding. Most women gain approximately 0.3 to 0.5 kg per week during this period.
Third Trimester (Weeks 28–40)
Weight gain continues at a similar or slightly slower pace in the third trimester. The baby gains most of its birth weight during this period. Some women experience a slight slowdown or plateau in the final 2 to 3 weeks before delivery.
Pre-Pregnancy BMI Categories
Your pre-pregnancy BMI determines which weight gain range is recommended for you. The BMI calculation is the same as for non-pregnant individuals:
It is important to use your pre-pregnancy weight, not your current weight during pregnancy, for this calculation. If you did not know your exact weight before pregnancy, use the weight from your first prenatal visit (ideally before 12 weeks).
Weight Gain Curve Diagram
Twin Pregnancy Guidelines
The IOM also provides recommendations for twin pregnancies, which require greater weight gain:
| Pre-Pregnancy BMI | Total Gain for Twins (kg) |
|---|---|
| Normal Weight (18.5–24.9) | 16.8 – 24.5 kg |
| Overweight (25.0–29.9) | 14.1 – 22.7 kg |
| Obese (≥ 30.0) | 11.3 – 19.1 kg |
Note: There are insufficient data for underweight women carrying twins to establish firm guidelines. Consult your healthcare provider for personalized advice.
Where Does the Weight Go?
The total weight gained during pregnancy is distributed across several components. For a woman who gains approximately 13.5 kg during pregnancy with a single baby:
| Component | Approximate Weight |
|---|---|
| Baby | 3.2 – 3.6 kg |
| Placenta | 0.7 kg |
| Amniotic fluid | 0.9 kg |
| Uterine enlargement | 0.9 kg |
| Breast tissue growth | 0.5 – 1.4 kg |
| Increased blood volume | 1.4 – 1.8 kg |
| Extra fluid (edema) | 0.9 – 1.4 kg |
| Maternal fat stores | 2.7 – 3.6 kg |
Risks of Too Much or Too Little Gain
Excessive Weight Gain
- Gestational diabetes
- Preeclampsia (pregnancy-related high blood pressure)
- Increased likelihood of cesarean delivery
- Large-for-gestational-age baby (macrosomia)
- Postpartum weight retention
- Increased risk of childhood obesity for the baby
Insufficient Weight Gain
- Preterm birth (delivery before 37 weeks)
- Low birth weight baby (< 2,500 g)
- Small-for-gestational-age baby
- Failure to initiate breastfeeding
- Nutritional deficiencies for mother and baby
Frequently Asked Questions
I gained too much weight in the first trimester. Should I diet now?
No. Dieting during pregnancy is not recommended. Instead, focus on nutrient-dense foods, appropriate portion sizes, and regular physical activity (as approved by your healthcare provider). The goal is to slow the rate of gain to the recommended weekly rate for the remainder of the pregnancy.
Does it matter when I gain the weight?
Yes. The pattern of weight gain matters. Very rapid weight gain in any trimester, or sudden jumps in weight, should be discussed with your healthcare provider as they could indicate fluid retention or other concerns. Steady, gradual weight gain is ideal.
What if I am obese? Should I try to lose weight during pregnancy?
The current IOM guidelines recommend that even obese women gain 5 to 9 kg during pregnancy. Some research suggests that very modest weight gain or even weight maintenance may be appropriate for women with very high BMI (≥ 40), but this should only be pursued under close medical supervision.
How is weight gain different for twins?
Twin pregnancies require more weight gain because you are growing two babies, two placentas, and more amniotic fluid. The IOM guidelines for twins are approximately 50% higher than for singleton pregnancies. Most twin pregnancies also deliver earlier (around 36 to 37 weeks), so the rate of gain needs to be faster.