There is no single right number. The recommended range depends on your pre-pregnancy BMI, and the official guidance is narrower than most people expect — here are the ranges, the weekly pace, and what the weight is made of.
These come from the Institute of Medicine (IOM, 2009) guidelines, still the standard reference in most of the English-speaking world. Everything keys off your pre-pregnancy BMI.
Pre-pregnancy BMI
One baby
Twins
Underweight (< 18.5)
28–40 lb (12.7–18.1 kg)
No official range*
Normal (18.5–24.9)
25–35 lb (11.3–15.9 kg)
37–54 lb (16.8–24.5 kg)
Overweight (25.0–29.9)
15–25 lb (6.8–11.3 kg)
31–50 lb (14.1–22.7 kg)
Obese (≥ 30.0)
11–20 lb (5.0–9.1 kg)
25–42 lb (11.3–19.1 kg)
*The IOM published no twin range for an underweight BMI — care there is individualised.
The starting point matters more than the number: the ranges are built on pre-pregnancy BMI, so knowing that figure is the first step. If you are unsure of your pre-pregnancy weight, your first antenatal appointment usually records it.
The weekly pace
Gain is not spread evenly. The first trimester adds very little — typically 1 to 4 lb in total — and some people lose weight through nausea, which is not usually a concern.
A common assumption is that the gain is mostly baby. In a typical 30 lb gain it is closer to a quarter:
Roughly, in a 30 lb gain
Baby7.5 lb
Fat stores7 lb
Blood volume4 lb
Body fluids4 lb
Uterus2 lb
Amniotic fluid2 lb
Breast tissue2 lb
Placenta1.5 lb
Which is also why the scale does not drop by 30 lb at delivery. Blood volume, fluid and tissue changes resolve over the following weeks rather than immediately.
Reading your own trend
Weigh consistently — same time of day, similar clothing. Day-to-day fluid shifts of 2 to 3 lb are normal and mean nothing on their own.
Watch the trend, not the reading. A single high week is noise; four weeks of steep gain is a pattern worth mentioning.
Sudden swelling with rapid gain — particularly in the face and hands, or more than about 4 lb in a week in the third trimester — is worth contacting your provider about promptly.
These are general guidelines, not medical advice, and they do not replace your own care. Individual circumstances — multiple pregnancies, existing conditions, nausea, prior history — change what is appropriate. Your midwife or doctor is the right person to interpret your numbers.
It depends on your pre-pregnancy BMI. The Institute of Medicine recommends 28–40 lb if you were underweight, 25–35 lb for a normal BMI, 15–25 lb if overweight, and 11–20 lb if obese. Twin pregnancies have separate, higher ranges.
How much weight should I gain each week?
Very little in the first trimester — roughly 1 to 4 lb in total. From the second trimester onward the guideline rate is about 1 lb a week for a normal pre-pregnancy BMI, 0.6 lb if overweight, and 0.5 lb if obese.
Where does pregnancy weight actually go?
Only a quarter or so is the baby. In a typical 30 lb gain, roughly 7.5 lb is the baby, about 4 lb is extra blood volume, 4 lb is other fluid, 2 lb each for the uterus and breast tissue, 1.5 lb placenta, 2 lb amniotic fluid, and around 7 lb of fat stores.
What if I gain more or less than the guideline?
It is common, and a range is not a rule. Gaining outside the recommended band is associated with slightly different risks, so it is worth raising with your midwife or doctor — but a single week's reading means very little, and the trend over time is what clinicians actually watch.
Should I diet during pregnancy?
Active weight loss is not generally recommended during pregnancy, including at higher BMIs, unless a clinician is specifically supervising it. The guidance for a higher BMI is a smaller gain, not a loss. Any change to eating or exercise while pregnant is worth discussing with your provider first.
Sources
Primary references used for the figures and rules on this page.
