During the final trimester of pregnancy (roughly weeks 28 through 40), babies typically gain about half a pound (0.2 to 0.25 kg) per week on average, though this varies by baby and by week.
How the pace changes over the trimester:
- Weight gain tends to accelerate through weeks 28–36, sometimes reaching close to 200–250 grams (about 7–9 ounces) per week during the peak growth period around 32–36 weeks.
- In the final 2–3 weeks before a full-term birth (around 37–40 weeks), the rate of gain often slows slightly as the baby has less room to grow and prepares for delivery.
Typical overall numbers:
- At the start of the third trimester (~28 weeks), a fetus weighs roughly 2.2–2.9 lbs (1–1.3 kg).
- By full term (~40 weeks), average birth weight is about 7–7.5 lbs (3.2–3.4 kg), though a healthy range spans roughly 5.5–10 lbs (2.5–4.5 kg).
- This means a baby may gain somewhere between 4 and 5 pounds total over the final trimester.
Important caveats:
- These are population averages from ultrasound growth studies; individual babies vary considerably based on genetics, placental function, maternal nutrition, and health conditions like gestational diabetes or hypertension.
- Growth is usually tracked via ultrasound-estimated fetal weight percentiles rather than a strict weekly number, since ultrasound weight estimates carry a margin of error (often ±10-15%).
- A single measurement showing faster or slower gain than "average" isn't necessarily concerning — clinicians look at trends over multiple visits and compare against growth curves specific to gestational age.
Because growth patterns and what's considered "normal" can differ based on individual pregnancy circumstances, anyone with concerns about fetal growth should discuss their specific ultrasound or growth chart results with their obstetrician or midwife rather than relying on general averages.