"Doctor, is my baby gaining enough weight?" is one of the most common questions women ask during ultrasound visits. Weight gain through pregnancy is gradual and follows a rough week-by-week pattern — from around 300 grams at 20 weeks to about 3.2–3.4 kg by 40 weeks — picking up speed to roughly 150–200 grams a week in the third trimester, which is when most of the final weight is actually laid down. A single scan number matters far less than the trend across several scans.
Fetal weight, week by week (approximate)
| Week | Approximate weight |
|---|---|
| 20 weeks | ~300 g |
| 22 weeks | ~430 g |
| 24 weeks | ~600 g |
| 26 weeks | ~760 g |
| 28 weeks | ~1 kg |
| 30 weeks | ~1.3 kg |
| 32 weeks | ~1.7 kg |
| 34 weeks | ~2.1 kg |
| 36 weeks | ~2.6 kg |
| 38 weeks | ~3 kg |
| 40 weeks | ~3.2–3.4 kg |
These are averages, not a pass/fail line — every baby grows along their own curve.
Why doctors read a trend, not a single scan
One scan is a snapshot. Doctors assess fetal growth across multiple ultrasounds, watching the trend rather than reacting to one number in isolation. A reading that looks a little off on one visit often settles into a perfectly normal curve on the next — which is exactly why growth monitoring is scheduled as a series of checkpoints through pregnancy, not a single verdict.
What actually shapes the final number
The last trimester is when your baby gains the most weight and builds the fat stores needed for life outside the womb, and several things influence how that plays out:
- Your nutrition — your baby depends on a steady supply of protein, iron, calcium, and healthy fats; skipping meals or crash dieting at this stage can directly affect growth.
- Placental function — how efficiently the placenta delivers nutrients and oxygen to your baby.
- Genetics — your baby's growth pattern is also shaped by family genetics, the same way height and build are.
- Your own health conditions — anaemia, gestational diabetes, or high blood pressure can all affect fetal weight, which is exactly why timely management of these matters.
- Rest and stress — poor sleep and chronic stress can influence hormonal balance and indirectly affect fetal development.
- Regular growth monitoring — fundal height checks and scheduled ultrasounds are how your doctor confirms the trend is on track, rather than guessing from the outside.
A single scan is a snapshot. What tells your baby's real growth story is the trend across multiple scans — not one number, and not a comparison to someone else's pregnancy.
Healthy, guided weight gain in these final weeks supports a good birth weight and lowers complications — it isn't something to fight against once your doctor has confirmed your own gain is on track. For what that looks like on your plate, see what to eat in the third trimester for healthy weight gain, and for how your own weight gain is tracked against your BMI, see pregnancy weight gain by trimester and BMI.
This is distinct from a fetal growth restriction finding, which is a specific Doppler/scan-trend diagnosis your doctor would discuss with you directly — not something you'd infer from a single weight figure on a report.
Frequently asked
My scan showed a slightly different weight than the "average" for my week — should I worry?
Not on its own. These figures are averages across a wide range of healthy pregnancies, and a small variation at one scan is common. Your doctor interprets any single number in the context of your whole pregnancy and the trend across your scans, not against a fixed target.
Which trimester matters most for birth weight?
The third trimester. This is when babies typically gain the most weight — often 150–200 grams a week — and build the fat stores needed after birth, which is why nutrition and monitoring both matter more, not less, as you approach your due date.
Can I do anything to help my baby gain weight well in the third trimester?
Focus on what's within your control: steady, balanced meals with enough protein, iron, calcium, and healthy fats; not skipping meals; keeping up with your scheduled scans and check-ups; and managing any existing condition like anaemia, gestational diabetes, or high blood pressure as your doctor advises. There's no special food or shortcut beyond consistent, guided care.
Is a bigger baby always a better sign?
No — doctors look at whether growth is appropriate for your baby and your pregnancy, not simply "bigger is better." Both unusually low and unusually high growth trends are discussed with you directly if they come up, which is exactly why the scan series exists.