Fetal Weight Calculator: What Your Baby Should Weigh This Week
The fetal weight calculator compares your baby’s estimated weight to the standard growth chart for your gestational week — find your exact week and trimester first with the Pregnancy Trimester Calculator.
Fetal weight estimates are based on population growth charts and are for informational reference only. All concerns about fetal growth should be discussed with your midwife or obstetrician.
Why Fetal Weight Matters More Than Most People Realize
Most parents track their pregnancy week by week, but far fewer understand what their baby should actually weigh at each stage — and that gap means growth problems can go unnoticed until they become serious.
According to the World Health Organization, intrauterine growth restriction — a condition where a baby grows more slowly than expected, called IUGR — affects approximately 10% of pregnancies and is associated with a 6-fold increase in neonatal complications compared to normally grown babies. Tracking fetal weight by week against standardized benchmarks is how providers identify this pattern before it escalates.
Early detection of growth restriction allows for closer monitoring and early delivery planning that prevents the most serious outcomes. Undetected IUGR that results in a premature or compromised birth can require NICU care at $3,000 to $7,000 per day. Most cases identified before 34 weeks with regular monitoring do not require NICU admission at all.
Fetal Weight Explained in Plain English
Fetal weight is the estimated mass of your baby at a given point in pregnancy. It increases from less than 14 grams at 12 weeks to an average of 3,400 grams — just over 3 kg — at 40 weeks. The change is not steady: fetal weight roughly doubles between weeks 28 and 34, making the third trimester the most critical period for growth monitoring.
An estimated fetal weight is produced by ultrasound measurements — the size of the baby’s head, abdomen, and thigh bone — fed into a standardized formula that converts those measurements into a weight estimate. Because no formula is perfectly accurate, fetal weight is always expressed as a range, and your result’s significance comes from comparing it against the expected range for that specific gestational week.
The Fetal Weight Formula — Step by Step
Fetal Weight Formula: Gestational Week → Average Benchmark Weight → Growth Trajectory Comparison
Gestational Week is the foundation — the number of weeks since your last menstrual period, which determines which benchmark applies. A week-28 baby should weigh approximately 1,000 grams. A week-32 baby should weigh approximately 1,700 grams. A week-36 baby should weigh approximately 2,600 grams. Using the wrong week shifts the entire comparison and can make a normally growing baby appear small or a large baby appear normal.
Average Benchmark Weight is the expected weight for that week based on population-level growth studies. At week 20, the benchmark is approximately 300 grams. At week 28, it is approximately 1,000 grams. At week 36, it is approximately 2,600 grams. The normal range extends 15% to 20% above and below each benchmark — a week-28 baby weighing 850 grams falls within normal range, while one weighing 650 grams falls below it.
Growth Rate Trajectory is the rate at which weight increases week by week — which accelerates significantly in the third trimester. Baby weight during pregnancy increases by approximately 25 to 35 grams per day between weeks 32 and 40, compared to just 5 to 10 grams per day at week 20. A single weight reading tells you less than two readings 2 to 4 weeks apart, because trajectory — whether growth is accelerating or flattening — is what providers use to assess wellbeing. To confirm your gestational age accurately before entering it into the calculator, use the Pregnancy Due Date Calculator.
Worked Example: A Parent Checks Week-28 Growth Before a Scan
Maya is 31 and is attending her 28-week growth scan. Her ultrasound report shows an estimated fetal weight of 950 grams. She wants to know whether this is within normal range before discussing it with her midwife.
Week-28 average benchmark: approximately 1,000 grams. Normal range at this week: approximately 850 to 1,150 grams (±15%). Maya’s baby at 950 grams falls within this window — 50 grams below the midpoint but 100 grams above the lower boundary.
Her result of 950 grams at week 28 is within the normal range. The midpoint for week 28 is 1,000 grams, so her baby is tracking slightly below the average but not below the lower threshold.
Maya shares the number with her midwife, who confirms 950 grams is within acceptable range, and books a follow-up growth scan at week 32 to confirm the trajectory is continuing normally. The midwife notes that trend matters more than any single reading.
What to Do with Your Fetal Weight Calculator Result
- Run the fetal weight calculator above after every growth scan to build a week-by-week trend record — a single reading in isolation is less useful than 3 readings showing a consistent trajectory.
- Do not treat a single below-average reading as cause for alarm. Counter-intuitively, the trend across multiple scans is far more predictive than any one reading — a baby consistently tracking at the 20th percentile is growing normally, while a baby that drops from the 50th to 10th percentile between two scans warrants immediate follow-up.
- After delivery, track your baby’s ongoing growth using the Baby Growth Percentile Calculator — fetal weight charts apply only in the womb; the growth percentile system used after birth uses different measurements and different reference data for the same tracking purpose.
- Flag any estimated fetal weight below 85% of the week benchmark to your provider within 48 hours — this falls below the lower edge of the normal range and typically warrants a repeat scan within 1 to 2 weeks rather than waiting for the next scheduled appointment.
Fetal Weight: 5 Common Questions Answered
Q: How accurate is an estimated fetal weight from ultrasound? A: Fetal weight estimates from ultrasound are accurate to within 10% to 15% of actual birth weight in most cases. A 10% margin on a week-32 estimate of 1,700 grams means the real weight is likely between 1,530 and 1,870 grams. Accuracy decreases slightly in late pregnancy as babies run out of room for the measurements to be taken clearly.
Q: What is a normal fetal weight at 36 weeks? A: The average fetal weight at 36 weeks is approximately 2,600 grams — about 2.6 kg. The normal range at this week spans approximately 2,200 to 3,000 grams. Most babies born at 36 weeks who fall within this range do not require specialist care, though 36-week deliveries are still classified as late preterm.
Q: Is a bigger baby healthier? A: Not necessarily. Babies weighing above 4,000 grams at birth — a condition called macrosomia — face increased risks of birth injury, shoulder dystocia during delivery, and neonatal blood sugar problems. A baby tracking above the 90th percentile for weight may trigger additional monitoring and delivery planning discussion regardless of how healthy the weight sounds in absolute terms.
Q: At what week does fetal weight gain accelerate most? A: Fetal weight gain accelerates significantly between weeks 28 and 36 — the body adds approximately 200 to 250 grams per week during this window. This is why third-trimester growth scans are spaced 4 weeks apart rather than further — changes in trajectory that predict problems become visible over this timeframe.
Q: Does maternal weight gain directly affect fetal weight? A: Yes, with a moderate correlation. Gaining within the recommended range for your pre-pregnancy BMI is associated with normal fetal growth. Severe calorie restriction can restrict fetal weight, while excessive gestational weight gain is associated with above-average fetal weight. The relationship is real but not one-to-one — genetics, placental function, and blood sugar control all contribute independently.
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Related: Pregnancy Trimester Calculator | Pregnancy Due Date Calculator