How the Pregnancy Weight Gain Calculator Works
This calculator uses the IOM 2009 guidelines (Institute of Medicine) to determine recommended weight gain during pregnancy. Recommendations are personalized based on your pre-pregnancy BMI and current gestational week.
IOM 2009 Recommendations
| BMI Category | Pre-pregnancy BMI | Total Gain |
|---|---|---|
| Underweight | < 18.5 | 12.5-18 kg (28-40 lbs) |
| Normal weight | 18.5-24.9 | 11.5-16 kg (25-35 lbs) |
| Overweight | 25-29.9 | 7-11.5 kg (15-25 lbs) |
| Obese | >= 30 | 5-9 kg (11-20 lbs) |
How Weight Gain Distributes
Weight gain isn't uniform during pregnancy:
- First trimester (weeks 1-13): minimal gain, about 0.5-2 kg total. Nausea may even cause initial weight loss.
- Second trimester (weeks 14-26): steady gain, about 0.3-0.5 kg per week for normal-weight women.
- Third trimester (weeks 27-40): similar to second trimester, with possible slowdown in the last weeks.
Where Does the Extra Weight Go?
For a normal-weight woman gaining 12.5 kg:
- Baby: ~3.4 kg | Placenta: ~0.7 kg | Amniotic fluid: ~0.8 kg
- Uterus: ~0.9 kg | Breasts: ~0.5 kg
- Blood volume increase: ~1.8 kg | Body fluids: ~1.8 kg | Fat reserves: ~2.6 kg
When to Be Concerned
Consult your OB/GYN if weight gain is significantly below or above recommendations. Regular monitoring during prenatal visits allows timely intervention with dietary modifications or diagnostic investigations.