Weight gain in pregnancy: a range, not a race
Gaining weight in pregnancy is not a side effect — it is part of the job. The baby, placenta, amniotic fluid, increased blood volume, fluid retention and energy stores for breastfeeding together account for most of it, and gaining within the recommended range is associated with healthier outcomes for both mother and baby, while gaining far above or below it raises risks on both sides. This calculator uses the widely cited Institute of Medicine (IOM) recommendations, which set the range from your pre-pregnancy BMI: people who start pregnancy lighter are advised to gain more, and people who start heavier are advised to gain less. It then works out how much of that total would typically have arrived by the week you have reached. This page is general information only and is not medical advice — your own clinician's guidance always comes first.
The IOM ranges and the weekly rates
Gain is not linear. The IOM model assumes a small gain across the whole first trimester — about 1.1 to 4.4 lb in total over weeks 1 to 13 — and then a steadier weekly rate through the second and third trimesters: about 1.0–1.3 lb a week if you started underweight, 0.8–1.0 for the normal class, 0.5–0.7 for overweight and 0.4–0.6 for obese. This calculator follows that model exactly: before week 14 it scales the first-trimester total by the week you have reached, and afterwards it adds the weekly rate for each week past 13. Pounds are the IOM's native unit; kilograms are shown alongside throughout (total ranges are roughly 12.5–18 kg, 11.5–16 kg, 7–11.5 kg and 5–9 kg).
How to use the "expected so far" range
Read the expected-so-far figure as a corridor, not a target line. Real gain arrives in spurts — a flat fortnight followed by a jump is completely normal — and home scales add noise from fluid, time of day and clothing that can easily exceed a week's true gain. Weighing once a week under similar conditions and watching the trend over three or four weeks gives a far truer picture than daily numbers. Being modestly outside the corridor at one check is common and usually self-corrects; being persistently and substantially outside it is a conversation for your midwife or doctor, who can look at the whole pregnancy rather than one arithmetic model. Note the model here is for a single baby: recommended totals for twins are higher, and your care team will give you the right ranges.
What the gained weight actually is
It helps morale to know the anatomy of the total. In a typical pregnancy gaining around 30 lb, roughly 7 to 8 lb is the baby at term, 1.5 lb the placenta, about 2 lb amniotic fluid, 2 lb the grown uterus, 1 lb breast tissue, around 4 lb extra blood, 4 lb or so of fluid in tissues, and the remainder — often 7 lb or more — maternal energy stores that support late pregnancy and breastfeeding. Very little of a recommended gain is "extra" in any meaningful sense, and much of it leaves with the birth itself. That breakdown is also why crash thinking has no place here: restricting intake to chase a cosmetic number risks the baby's growth, and any concern about your gain — too fast, too slow, or sudden swelling with a rapid jump, which should be checked promptly — belongs in a conversation with your care team, not a diet. This calculator is not medical advice and cannot assess you or your baby.
Worked example
Pre-pregnancy weight 65 kg, height 165 cm: BMI = 65 ÷ 1.65² = 23.87, in the normal class. Recommended total gain: 25–35 lb (about 11.5–16 kg), at 0.8–1.0 lb a week after the first trimester. At week 20: expected so far = 1.1 + 0.8 × 7 = 6.7 lb at the low end, and 4.4 + 1.0 × 7 = 11.4 lb at the high end (about 3.0–5.2 kg).
How to use this calculator
- Enter your weight before pregnancy in kilograms — the class depends on where you started, not where you are now.
- Enter your height in centimetres so the calculator can find your pre-pregnancy BMI.
- Enter your current week of pregnancy, from 1 to 42.
- Read the expected gain so far in the headline panel, in pounds and kilograms.
- Compare with the recommended total for the whole pregnancy, and raise any persistent large gap with your care team.
If your gain is outside the range
First, check the boring explanations before worrying: a different scale, heavier clothing, a later time of day, or a week entered slightly off can each shift the comparison by more than a week of genuine gain. If the gap is real and persistent, small course corrections beat dramatic ones — regular meals built on nutrient-dense food, daily walking as your clinician advises, and attention to drinks and snacks that add calories without nutrition. Gaining too little invites the same conversation from the other side: appetite loss, nausea and food aversions are common and treatable contributors your team can help with.
Two situations deserve prompt contact rather than calculator-watching: a sudden large gain over a few days, especially with swelling of the face or hands, headaches or visual changes, and any gain pattern paired with reduced baby movements later in pregnancy. Both can signal conditions that need timely assessment. Otherwise, hold the perspective this page is built on: the ranges are population guidance refined by your clinician for you personally, and a healthy pregnancy tolerates ordinary variation around them well. This is general information, not medical advice.
Important note
For information only — not medical advice. This calculator uses standard published formulas to estimate values for general education and planning. It does not diagnose, treat, or replace the advice of a doctor or other qualified health professional. If you have a health concern, are pregnant, or are making decisions about treatment, medication, or a major diet or exercise change, please talk to a healthcare professional first.