The short answer. How much you should gain depends on your pre-pregnancy BMI, not your current weight. For a single baby the IOM 2009 ranges are 12.5–18 kg (underweight), 11.5–16 kg (normal), 7–11.5 kg (overweight) and 5–9 kg (obese). The first trimester accounts for only 0.5–2 kg of that in every band; the rest accrues steadily from week 14. One weigh-in tells you very little — the line through several weeks is the thing worth looking at.
The ranges, by pre-pregnancy BMI
| Pre-pregnancy BMI | Total gain | Weekly rate from week 14 |
|---|---|---|
| Under 18.5 — underweight | 12.5–18 kg | 0.44–0.58 kg |
| 18.5–24.9 — normal | 11.5–16 kg | 0.35–0.50 kg |
| 25–29.9 — overweight | 7–11.5 kg | 0.23–0.33 kg |
| 30 and over — obese | 5–9 kg | 0.17–0.27 kg |
Every band shares the same first-trimester allowance: 0.5–2 kg across weeks 1–13. That is the part people most often get wrong — twelve weeks of pregnancy are expected to add about as much as a heavy lunch bag, and gaining nothing at all in the first trimester is common, particularly with morning sickness.
Where the weight goes
By the end of pregnancy, roughly:
- Baby — 3–3.5 kg
- Placenta — about 0.7 kg
- Amniotic fluid — about 0.8 kg
- Larger uterus — about 1 kg
- Breast tissue — 0.5–1 kg
- Extra blood volume — 1.2–1.5 kg
- Extra fluid — 1.5–2 kg
- Fat stores — 2.5–4 kg, laid down for feeding after birth
Less than a third of it is the baby, which is worth remembering on a week when the number moves and nothing else seems to have changed.
How to weigh yourself so the number means something
Once a week is enough
Daily weighing measures fluid, not growth. Body weight swings by a kilogram or more within a single day, which is enough to make a perfectly normal week look alarming in either direction.
Same conditions every time
Same scales, same time of day, similar clothing. Most of the difference between two readings taken carelessly is the method, not you.
Look at the line, not the point
One reading outside the range means very little. Three in a row heading away from it is a conversation with your midwife — and the reason the app plots the curve rather than showing a single figure.
Contact your doctor or midwife promptly if:
- You gain more than about 2–3 kg in a single week, particularly with swelling of the face or hands, headache or vision changes — this combination can signal pre-eclampsia
- You are losing weight after the first trimester
- You cannot keep food down, or sickness is severe enough to stop you eating or drinking
- Sudden swelling appears in the hands or face rather than the ankles
What this calculator will not do
- Give you a target. The ranges are population guidance. Your own history, medical conditions and clinician's advice all sit above them.
- Handle twins. The IOM ranges for twins are higher and separate; this tool does not use them.
- Tell you to diet. Pregnancy is not the time to lose weight, and no output here should be read that way.
- Replace being weighed at your appointments. It is a way to understand the number, not a substitute for the person recording it.
For the full picture — trimester-by-trimester targets, what to do about a fast or slow week, and twin guidance — read the pregnancy weight gain guide.
Inside Baby Novum: every weigh-in plotted inside your personal BMI corridor, so you see the trend rather than one week — plus 240 foods rated by trimester, a hydration goal calculated from your weight, and the full week-by-week guide.
Related guides
FAQ — Pregnancy weight gain
How much weight should I gain during pregnancy?
It depends on your pre-pregnancy BMI, not on your current weight. The IOM 2009 ranges for a single baby are: 12.5–18 kg if you were underweight (BMI under 18.5), 11.5–16 kg if normal (18.5–24.9), 7–11.5 kg if overweight (25–29.9) and 5–9 kg if obese (30+).
How is the weekly range calculated?
The first trimester is treated as a single block — 0.5–2 kg in total across weeks 1–13, the same for every BMI band. From week 14 a steady weekly rate applies: about 0.44–0.58 kg/week if underweight, 0.35–0.5 if normal, 0.23–0.33 if overweight and 0.17–0.27 if obese. The calculator adds the first-trimester block to the weeks since 13.
I have gained nothing in the first trimester. Is that a problem?
Usually not. The whole first-trimester allowance is only 0.5–2 kg, and with morning sickness many people gain nothing or lose a little. Mention it at your next appointment, but it is a common starting point rather than a warning sign on its own.
Where does the weight actually go?
Only a part of it is the baby. At term a rough breakdown is: baby about 3–3.5 kg, placenta 0.7, amniotic fluid 0.8, larger uterus 1, breast tissue 0.5–1, extra blood volume 1.2–1.5, extra fluid 1.5–2, and fat stores 2.5–4 for feeding after birth.
Should I diet if I am gaining too fast?
No. Pregnancy is not the time to lose weight, and restrictive dieting can cut nutrients the baby needs. Talk to your doctor or midwife — the usual advice is to change what you eat rather than how much, and the ranges are population guidance, not a personal target.
Does the calculator work for twins?
No. The IOM publishes separate, higher ranges for twin pregnancies and this calculator does not use them. If you are expecting twins, your own clinician's numbers are the ones to follow.
How often should I weigh myself?
Once a week is plenty, on the same scales, at the same time of day, in similar clothes. Day-to-day weight swings by a kilogram or more with fluid alone, which is exactly why one reading tells you far less than a line through several.
Pregnancy weight gain — at a glance
The recommended range is set by your pre-pregnancy BMI and nothing else: 12.5–18 kg if you started underweight, 11.5–16 kg at a normal BMI, 7–11.5 kg if overweight, 5–9 kg if obese — all for a single baby, all from the IOM 2009 guidelines. The first trimester contributes only 0.5–2 kg of that in every band, which is why gaining nothing in the first twelve weeks is normal rather than worrying. From week 14 the gain becomes steady, between roughly 0.17 and 0.58 kg a week depending on the band. Weigh once a week, on the same scales at the same time, and read the line rather than the point — a single reading moves by a kilogram on fluid alone. Being outside the range once is not a problem to solve; a trend heading away from it is a conversation to have. And two things always warrant a prompt call: a jump of 2–3 kg in one week alongside swelling, headache or vision changes, or losing weight after the first trimester.
Baby Novum is an informational and tracking tool, not a medical device, and does not provide medical diagnosis or treatment. Always consult your doctor or midwife about your pregnancy.