Which guidelines does this calculator use?

It uses the 2009 Institute of Medicine (IOM) guidelines for singleton pregnancies, which are widely adopted by clinical authorities.

Are the results medical advice?

No. This tool is for education only and cannot replace personalized guidance from your healthcare provider.

Does the tool support twins or multiples?

This version focuses on singleton pregnancies. Twin guidance varies and depends on the person’s health history—ask your clinician.

What if my weight gain is outside the range?

Ranges are estimates. Many healthy pregnancies fall slightly outside them. Discuss trends, diet, and symptoms with your provider.

Why does the range change by week?

We interpolate cumulative gain from typical first-trimester totals to full-term targets to give a clear week-by-week expectation.

How accurate is BMI during pregnancy?

BMI is based on pre-pregnancy measurements and is only a rough index. Body composition and health conditions also matter.

Pregnancy Weight Gain Calculator

This evidence-based calculator estimates how much weight gain is recommended during pregnancy based on your pre-pregnancy BMI and current gestational week. It’s designed for educational use in singleton pregnancies and to support informed conversations with your healthcare provider.

Enter your details

Metric (kg, cm)

US (lb, ft/in)

Height *

Pre-pregnancy weight *

Current weight *

Gestational week (0–42) *

Educational use only. Not a diagnosis or medical advice.

Results

Pre-pregnancy BMI —

BMI category —

Recommended total gain (to term) —

Recommended gain by week —

Your gain so far —

Status —

Recommended remaining (to term) —

Data Source and Methodology

Authoritative Data Source: Institute of Medicine (US) and National Research Council. Weight Gain During Pregnancy: Reexamining the Guidelines. Washington (DC): National Academies Press (US); 2009. Available at: National Academies Press.

All calculations are strictly based on the formulas and data provided by this source.

This tool models cumulative recommended gain for singleton pregnancies using first-trimester totals and full-term targets from IOM (2009). Weekly recommendations are presented as an interpolated cumulative range for clarity.

The Formulas Explained

BMI (pre‑pregnancy), Metric:

\( \mathrm{BMI} = \dfrac{W_{\mathrm{kg}}}{(H_{\mathrm{m}})^2} \)

BMI (pre‑pregnancy), US:

\( \mathrm{BMI} = 703 \times \dfrac{W_{\mathrm{lb}}}{(H_{\mathrm{in}})^2} \)

Cumulative recommended gain by week w (singleton):

Let \( w \in [0, 42] \), first trimester end at \( w=13 \).
First‑trimester total range (all BMI): \( [F_{\min}, F_{\max}] = [0.5,\, 2.0] \,\mathrm{kg} \).
Total full‑term range by BMI: \( [T_{\min}, T_{\max}] \) (see table below).

If \( 0 \le w \le 13 \):
\( G_{\min}(w) = F_{\min} \cdot \dfrac{w}{13}, \quad G_{\max}(w) = F_{\max} \cdot \dfrac{w}{13} \)

If \( 13 < w < 40 \): linear interpolation to term
\( t = \dfrac{w-13}{27} \)
\( G_{\min}(w) = F_{\min} + t \cdot (T_{\min} - F_{\min}) \)
\( G_{\max}(w) = F_{\max} + t \cdot (T_{\max} - F_{\max}) \)

If \( w \ge 40 \):
\( G_{\min}(w) = T_{\min}, \quad G_{\max}(w) = T_{\max} \)

Glossary of Variables

BMI: Body Mass Index from pre-pregnancy height and weight.
BMI category: Underweight (<18.5), Normal (18.5–24.9), Overweight (25–29.9), Obese (≥30).
Gestational week (w): Number of weeks pregnant at the time of measurement.
Recommended total gain: IOM full-term target range for singleton pregnancy based on BMI.
Recommended gain by week: Interpolated cumulative range up to week w.
Your gain so far: Current weight minus pre-pregnancy weight.
Status: Whether your gain is below, within, or above the recommended range for week w.

How It Works: A Step‑By‑Step Example

Inputs

Units: Metric
Height: 165 cm (1.65 m)
Pre‑pregnancy weight: 62 kg
Current weight: 68 kg
Gestational week: 24

Steps

Compute BMI: \( \mathrm{BMI} = 62 / 1.65^2 \approx 22.8 \Rightarrow \) Normal.
IOM total range (Normal): \( T_{\min}=11.5\,\mathrm{kg}, \; T_{\max}=16.0\,\mathrm{kg} \).
First‑trimester total: \( F_{\min}=0.5, \; F_{\max}=2.0\,\mathrm{kg} \).
Week 24 lies in the interpolation window. \( t=\frac{24-13}{27}\approx 0.407 \).
\( G_{\min}(24)=0.5+0.407\,(11.5-0.5)\approx 5.0\,\mathrm{kg} \)
\( G_{\max}(24)=2.0+0.407\,(16.0-2.0)\approx 7.7\,\mathrm{kg} \).
Your gain so far: \( 68-62=6.0\,\mathrm{kg} \) which falls within the 5.0–7.7 kg range.

Result: Within the recommended range at week 24. Estimated remaining to term: about 5.5–10.0 kg.

Frequently Asked Questions (FAQ)

Do the recommendations change if I’m very short or very tall?

The ranges are based on BMI categories, which already account for height. Individual factors still matter—ask your provider for tailored guidance.

Can I use this tool before confirming pregnancy?

It will compute projections by week even at early gestation, but decisions should be guided by a clinician who knows your medical history.

What if I don’t know my exact pre‑pregnancy weight?

Use your best recent estimate. If you later obtain a more accurate value, update it—the BMI category may shift slightly.

Why does my provider’s recommendation differ?

Clinicians personalize advice based on health conditions, nutrition, edema, and fetal growth—going beyond the IOM population guidelines.

Is weight loss during pregnancy ever normal?

Early in the first trimester, some people lose weight due to nausea. Report significant or persistent weight loss to your clinician.

Should I target the middle of the range?

Many do. However, your provider may recommend the lower or upper part of the range depending on your health and fetal growth trends.

Authorship and Review

Tool developed by Ugo Candido.
Content verified by CalcDomain Editorial Team.
Last reviewed for accuracy on: September 14, 2025.