Foetal Weight Chart: What to Expect Week by Week

What is a Foetal Weight Chart?

A foetal weight chart is a standardized reference guide that shows the expected range of a baby’s weight (and often size) at different weeks of pregnancy. Based on large datasets from healthy pregnancies and global growth standards, it helps doctors assess whether foetal growth is progressing as expected.

These charts rely on ultrasound-based measurements. In early pregnancy, growth is commonly assessed using measurements like crown–rump length (CRL), while later scans use foetal biometry (such as head, abdomen, and femur measurements) to calculate an estimated foetal weight (EFW). Many foetal weight charts present values in grams, making it easier to compare week-by-week trends.

During routine ultrasounds, your doctor compares your baby’s estimated fetal weight (EFW) against week-specific percentile ranges rather than a single "ideal" number. This helps flag growth concerns early and decide whether closer monitoring or additional evaluation is needed.

As per data from PLoS Medicine (PMC/PubMed), the WHO foetal weight (estimated foetal weight) reference charts are based on a multinational longitudinal ultrasound study of 1,387 low-risk singleton pregnancies across 10 countries, establishing international percentile curves for foetal weight and key biometric measurements used in clinical growth monitoring.

Why is Tracking Foetal Weight Important?

Monitoring foetal weight serves several crucial purposes for both you and your healthcare team:

• Early detection of growth issues: Identifying intrauterine growth restriction (IUGR) or excessive growth that might signal health risks
• Clinical decision guidance: Helping doctors determine optimal delivery timing and necessary interventions
• Parental reassurance: Providing peace of mind by comparing your baby's measurements to healthy averages
• Overall health assessment: Supporting evaluation of organ maturation and fat accumulation in later pregnancy stages
• Risk management: Identifying potential complications before they become serious problems

Foetal Weight by Week: A Detailed Breakdown (Approximate Averages)

Understanding week-by-week development helps you appreciate your baby's remarkable growth journey. Here's what the foetal weight chart kg and gram measurements typically show:

Weeks 1 to 4: Early Foetal Development

During the first month, your baby begins as a microscopic cluster of rapidly dividing cells. The embryo implants in your uterus, focusing on fundamental organ formation rather than measurable size.

By week 4, your baby measures approximately 2-4 millimetres long, with negligible weight under 1 gram. The neural tube forms during this critical period, laying the foundation for brain and spinal cord development. While weight tracking isn't feasible through ultrasound yet, crucial organogenesis occurs that will support future growth.

Week 5 to 12: Rapid Growth Begins

This period marks the beginning of measurable development on any foetal development weight chart:

• Week 5: 1–2 grams, 6–10 millimetres (heartbeat becomes detectable)
• Week 6-7: 1-10 grams, 10-20 millimetres (limbs and major organs form)
• Week 8: 1-20 grams, 16 millimetres (facial features develop)
• Week 9: 2-27 grams, 23 millimetres (fingers and toes appear)
• Week 10: 4-35 grams, 31 millimetres (vital organs function)
• Week 11: 7-45 grams, 41 millimetres (tooth buds form)
• Week 12: 14-58 grams, 54 millimetres (reflexes develop)

Week 13 to 20: Continued Development

Growth accelerates significantly during this phase, with your baby's weight typically doubling every few weeks. By week 20, the average weight reaches 300-331 grams, with length measuring 16-25.7 centimetres.

Your baby develops fingerprints, begins hearing sounds, and reaches early viability potential (around 24 weeks). Weight gain averages 50-100 grams weekly as bones harden and the first fat layers begin forming. The foetal weight chart shows consistent upward progression during this crucial development period.

Week 21 to 30: Foetal Weight Growth Acceleration

This phase shows dramatic increases on the foetal weight chart in grams:

• Week 21: 360-399 grams, 26.7-27.4 centimetres
• Week 22: 430-478 grams, 27.8-29 centimetres
• Week 23: 501-568 grams, 28.9-30.6 centimetres
• Week 24: 600-670 grams, 30-32.2 centimetres
• Week 25: 660-785 grams, 33.7-34.6 centimetres
• Week 26: 760-913 grams, 35.1-35.6 centimetres
• Week 27: 875-1055 grams (approximately 1 kg), 36.6 centimetres
• Week 28: 1005-1210 grams, 37.6 centimetres
• Week 29: 1153-1239 grams, 38.6-39.3 centimetres
• Week 30: 1319-1559 grams, approximately 40 centimetres

Week 31 to 40: Preparing for Birth

The final phase shows remarkable weight gain, with babies typically gaining 200-300 grams weekly. Fat deposits accumulate for temperature regulation after birth, while lungs mature and brain development accelerates rapidly.

By week 40, healthy babies typically weigh 3.3-3.6 kilograms and measure approximately 51 centimetres. The baby shifts to a head-down position, preparing for delivery while continuing rapid brain and lung development.

Factors Affecting Foetal Weight

Several maternal, foetal, and environmental factors influence how your baby’s measurements compare with the standard foetal weight chart:

• Maternal nutrition: Adequate intake of calories, protein, iron, calcium, and essential vitamins is crucial for steady foetal growth. Poor nutrition or deficiencies can slow weight gain and affect overall development.

• Genetic factors: A baby’s growth potential is partly determined by parental height, body build, and family growth patterns. These genetic influences explain why healthy babies may naturally be smaller or larger than average.

• Placental health: The placenta acts as the lifeline between mother and baby, supplying oxygen and nutrients. Placental insufficiency can limit foetal weight gain even when maternal health appears normal.

• Maternal medical conditions: Conditions such as gestational diabetes, high blood pressure, or thyroid disorders can alter nutrient delivery and foetal metabolism. These may lead to either excessive growth or restricted foetal weight.

• Lifestyle factors: Smoking, alcohol use, and exposure to high altitudes reduce oxygen availability to the foetus. This can interfere with normal growth and lower the expected foetal weight.

• Multiple pregnancies: In twin or triplet pregnancies, babies share the same uterine environment and maternal resources. As a result, individual foetal growth and weight gain are often lower compared to singleton pregnancies.

Tests to Check Foetal Health

Healthcare providers use various methods to assess foetal development beyond basic weight charts:

• Ultrasound biometry: Measures head circumference, abdominal circumference, and femur length for estimated foetal weight
• Doppler scanning: Assesses blood flow in the umbilical arteries and placental function
• Non-stress testing: Monitors foetal heart rate reactivity and movement patterns
• Biophysical profile: Evaluates amniotic fluid levels, foetal movement, and muscle tone
• Fundal height measurement: A simple clinical measurement comparing uterine size to gestational age

How Accurate is a Foetal Weight Chart?

Foetal weight charts provide valuable guidance, but they have inherent limitations you should understand. Ultrasound-based estimated foetal weight calculations typically have a 10–15% margin of error, with accuracy improving after 20 weeks of pregnancy.

The accuracy decreases for very small or very large babies, potentially reaching 20% error at term. These charts rely on mathematical formulas validated by WHO data, but individual variation means they guide rather than predict exactly.

Multiple factors affect accuracy, including maternal body size, foetal position, and amniotic fluid levels. Your healthcare provider interprets these measurements alongside other clinical findings for a comprehensive growth assessment.

How to Use a Foetal Weight Chart

Using a foetal weight chart effectively requires understanding percentiles and trends rather than focusing on single measurements. Your doctor compares ultrasound-derived estimated foetal weight to chart percentiles, typically looking at the 10th to 90th percentile range.

Tracking trends over multiple scans provides more valuable information than isolated readings. Your healthcare provider uses gestational age from your last menstrual period or early ultrasound dating to ensure accurate comparisons.

Professional interpretation remains crucial, as providers assess percentile risks and overall growth patterns. For example, measurements below the 10th percentile may indicate small-for-gestational-age concerns requiring additional monitoring.

Can Foetal Weight Be Affected by Pregnancy Complications?

Several pregnancy complications can significantly impact foetal growth patterns:

• Gestational diabetes: Often leads to macrosomia (birth weight exceeding 4 kilograms), increasing caesarean delivery risk (C-section risk)
• Preeclampsia and hypertension: Cause intrauterine growth restriction through poor placental blood flow
• Placental insufficiency: Reduces nutrient and oxygen delivery, limiting growth potential
• Maternal infections: Conditions like the Zika virus can disrupt normal development and weight gain
• Intrauterine growth restriction (IUGR): Results in weight below the 10th percentile, requiring intensive monitoring
• Oligohydramnios: Low amniotic fluid levels often correlate with slowed foetal growth

Conclusion

A foetal weight chart is a helpful way to understand week-by-week growth, but it should always be interpreted alongside ultrasound findings and your doctor’s guidance. If your baby’s weight seems higher or lower than expected, timely follow-ups and the right tests can clarify whether it’s a normal variation or needs closer monitoring.

For dependable pregnancy monitoring, Metropolis Healthcare supports expecting parents with 4,000+ diagnostic tests, specialized pregnancy panels, and full body checkups, along with home sample collection backed by 10,000 touchpoints.

You also get quick turnaround times and accurate results, plus easy booking via website, call, app, or WhatsApp, making it simpler to stay consistent with antenatal testing through every trimester.

FAQ

What is the average foetal weight at 30 weeks?

At 30 weeks of pregnancy, the average foetal weight is approximately 1.3 to 1.5 kg, though healthy variations exist.

How can I ensure my baby is growing properly in the womb?

• Attend regular antenatal check-ups
• Follow a balanced, nutrient-rich pregnancy diet
• Take prescribed supplements
• Undergo recommended screening tests
• Avoid smoking, alcohol, and unnecessary medications

How much weight does a baby gain in the last month of pregnancy?

During the final month, babies typically gain 200–300 grams per week, making this the fastest growth phase by weight.

What can cause slow growth in foetal weight?

• Placental insufficiency
• Maternal anaemia or malnutrition
• Chronic medical conditions
• Infections
• Genetic factors

Can foetal weight predict delivery complications?

Extremely low or high foetal weight may increase the risk of complications such as preterm birth, shoulder dystocia, or operative delivery, which is why growth monitoring is essential.

How accurate is a foetal weight chart?

Foetal weight charts provide reliable population-based estimates but are not exact. Clinical interpretation is always required.

What should I do if my baby’s growth seems slower than expected?

• Consult your obstetrician immediately
• Follow recommended investigations
• Do not rely on charts alone to draw conclusions

What is the ideal foetal weight at 40 weeks?

At full term (40 weeks), an ideal foetal weight typically ranges between 2.8 and 3.5 kg.

Does maternal diet affect foetal weight?

Yes. Adequate intake of protein, iron, calcium, folate, and overall calories directly supports healthy foetal growth.