Why Does Pre-Pregnancy Weight Matter?
The amount of weight you should gain during pregnancy is directly tied to your Body Mass Index (BMI) before you became pregnant. The official guidelines set by the Institute of Medicine (IOM) and endorsed by the American College of Obstetricians and Gynecologists (ACOG) stipulate that women who begin their pregnancy underweight need to gain more weight to support the baby's growth, while women who are overweight or obese should gain less.
Where Does the Weight Go?
It is a common misconception that all weight gained during pregnancy is body fat. In reality, maternal fat stores only account for a fraction of the total weight gained. For a typical healthy pregnancy with a total gain of 30 lbs (13.5 kg), the weight is distributed roughly as follows:
- Baby: 7 to 8 lbs (3.2 to 3.6 kg)
- Placenta: 1.5 lbs (0.7 kg)
- Amniotic Fluid: 2 lbs (0.9 kg)
- Uterus Expansion: 2 lbs (0.9 kg)
- Maternal Breast Tissue: 2 lbs (0.9 kg)
- Maternal Blood Volume: 4 lbs (1.8 kg)
- Fluids in Maternal Tissue: 4 lbs (1.8 kg)
- Maternal Fat Stores: 7 lbs (3.2 kg)
Risks of Gaining Too Much or Too Little
Gaining less than the recommended amount of weight increases the risk of delivering a premature baby or a baby that is "Small for Gestational Age" (SGA), which can lead to complications initiating breastfeeding, fighting off infections, and hitting developmental milestones.
Conversely, gaining significantly more than the recommended guidelines increases the mother's risk of developing gestational diabetes, hypertension (preeclampsia), and delivery complications due to the baby being "Large for Gestational Age" (macrosomia).
Expecting Twins?
If you are expecting twins, your body naturally needs to provide more blood, fluid, and nutrients. The IOM guidelines recommend significantly higher weight gain ranges for twin pregnancies, which our calculator automatically accounts for if you select "Yes" in the dropdown menu above.