Pregnancy Weight Gain Calculator (IOM Guidelines)

Category	BMI	Total gain	Weekly
Underweight	< 18.5	28–40 lb	0.97–1.28 lb
Normal weight	18.5–24.9	25–35 lb	0.77–1.10 lb
Overweight	25.0–29.9	15–25 lb	0.51–0.73 lb
Obese	≥ 30.0	11–20 lb	0.37–0.60 lb

How to use this tool

Enter your pre-pregnancy height and weight to calculate BMI.
Choose Single baby or Twins, then enter gestational week and current weight gain.
Review the BMI-based total range; singleton weekly comparison begins at week 14.

IOM/NAM weight gain guidelines

Pre-pregnancy BMI	Category	Total recommended gain	Weekly rate (2nd & 3rd trimester)
< 18.5	Underweight	28–40 lbs (12.5–18 kg)	1–1.3 lbs/week
18.5–24.9	Normal weight	25–35 lbs (11.5–16 kg)	0.8–1 lb/week
25.0–29.9	Overweight	15–25 lbs (7–11.5 kg)	0.5–0.7 lbs/week
≥ 30.0	Obese	11–20 lbs (5–9 kg)	0.4–0.6 lbs/week

How the estimate works

The calculator estimates pre-pregnancy BMI by dividing weight in kilograms by height in meters squared, then selects the matching IOM category. For singleton pregnancies, the week-by-week estimate starts at week 14: it adds that category's lower or upper weekly rate to the first-trimester baseline of 0.5–2 kg, up to the recommended total range. For example, a normal-BMI singleton pregnancy at week 20 has an estimated range of about 3.0–5.5 kg (6.5–12.1 lb) so far. This is a broad counseling estimate, not a prediction of fetal growth or a substitute for individual care.

Where the weight goes

For a normal-BMI pregnancy with a 25–35 lb total gain, the weight is approximately distributed as:

Baby: ~7.5 lbs
Placenta: ~1.5 lbs
Amniotic fluid: ~2 lbs
Uterus growth: ~2 lbs
Increased blood volume: ~4 lbs
Breast tissue: ~2 lbs
Fluid, fat, and protein stores: ~7 lbs

First trimester note

The estimate uses approximately 0.5–2 kg (1.1–4.4 lb) total gain by the end of week 13 as its first-trimester baseline. Most recommended gain occurs in the second and third trimesters. Morning sickness may temporarily reduce appetite and cause slight weight loss in the first weeks, so this tool does not assign a first-trimester weekly status.

Starting the weekly comparison at week 14 avoids treating the first-trimester total as a precise target for each individual week. The tool's expected band is calculated from the entered BMI category and gestational week, and it does not account for pregnancy-specific medical history or complications.

Twin and multiple pregnancy

Choose Twins to see published pregnancy-long totals for normal BMI (37–54 lbs), overweight BMI (31–50 lbs), or obese BMI (25–42 lbs). IOM does not establish a total range here for underweight twin pregnancies, and there are no IOM week-by-week twin targets, so this calculator does not classify twin progress by gestational week. Metric values in the calculator convert these whole-pound ranges. Higher multiples require individualized guidance from a healthcare provider.

Frequently asked questions

Who sets these guidelines?

The recommendations come from the IOM (Institute of Medicine, now the National Academy of Medicine) 2009 report "Weight Gain During Pregnancy: Reexamining the Guidelines".

What if I am carrying twins?

Select Twins to see total gain ranges for normal, overweight, and obese pre-pregnancy BMI categories. IOM does not establish a total range for underweight twin pregnancies or week-by-week twin targets, so talk with your OB-GYN or midwife about an individualized plan.

Is this calculator a substitute for medical advice?

No. Every pregnancy is different. Use these ranges as a starting point for discussion with your doctor or midwife.

Why is there no weekly first-trimester target?

Guidance describes first-trimester gain as a total of about 0.5–2 kg (1.1–4.4 lb) by the end of week 13, rather than a weekly target. This tool begins its singleton weekly comparison at week 14.