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👶 Pregnancy & Baby

Baby Weight by Week

See the average baby weight and length for every week of pregnancy from week 8 to 40. Based on the Hadlock and INTERGROWTH-21st fetal growth charts.

(8–40 weeks)
Week 20
Tip: Use the arrow buttons or enter a week number to explore fetal growth by week.

Understanding Fetal Growth by Week

Fetal Growth Overview

Fetal growth follows a predictable pattern during pregnancy. The first trimester is characterized by rapid cell division and organ formation. During the second trimester, the baby grows in length and begins to put on weight. The third trimester is the period of most rapid weight gain.

  • First Trimester (weeks 1–12): The embryo develops into a fetus, with all major organs forming. By week 12, the baby weighs about 14g and is 5.4cm long.
  • Second Trimester (weeks 13–26): The baby grows rapidly in length. Weight increases from about 43g at week 14 to 760g at week 26. This is often when mothers first feel fetal movements.
  • Third Trimester (weeks 27–40): The baby gains most of its weight during this period. Weight increases from about 1,000g at week 28 to 3,500g at week 40. The baby also puts on body fat and matures organ systems.

Key Growth Milestones

  • Week 8: ~1g, 1.6cm — embryo becomes a fetus
  • Week 12: ~14g, 5.4cm — all organs formed, gender may be visible on ultrasound
  • Week 16: ~100g, 11.6cm — baby can make sucking motions
  • Week 20: ~300g, 16.4cm — halfway point, quickening (first movements)
  • Week 24: ~600g, 21.0cm — viability (survival outside the womb with medical support)
  • Week 28: ~1,000g, 25.4cm — third trimester begins, baby opens eyes
  • Week 32: ~1,700g, 30.0cm — rapid weight gain, baby settles into head-down position
  • Week 36: ~2,600g, 34.0cm — full term approaching, lungs maturing
  • Week 40: ~3,500g, 37.5cm — full term, ready for birth

How to Use This Chart

  • Find your current week: Enter your gestational age to see the average weight and length.
  • Compare percentiles: The 10th, 50th, and 90th percentiles show the range of normal growth.
  • Track changes: Use the table to see how weight gain accelerates in the third trimester.
  • Discuss with your provider: Use this chart as a conversation starter with your obstetrician or midwife.

💡 Clinical Note: The Hadlock growth chart is based on ultrasound measurements from a US population. The INTERGROWTH-21st standard, developed from data in 8 countries, is now recommended by the WHO as a global growth standard. Both are widely used in clinical practice.

Percentiles Explained

  • 10th percentile: 10% of babies at this gestational age weigh less than this value.
  • 50th percentile (average): Half of babies weigh more and half weigh less at this age.
  • 90th percentile: 90% of babies at this gestational age weigh less than this value.
  • Babies between the 10th and 90th percentiles are typically considered within the normal range.
  • Below 10th percentile: May indicate small for gestational age (SGA).
  • Above 90th percentile: May indicate large for gestational age (LGA).

Factors That Influence Fetal Size

  • Maternal height and weight: Taller and heavier mothers tend to have larger babies.
  • Maternal weight gain: Adequate weight gain during pregnancy supports fetal growth.
  • Nutrition: Protein, iron, folic acid, and overall caloric intake are important.
  • Placental function: The placenta must deliver adequate nutrients and oxygen.
  • Maternal health: Diabetes (especially gestational diabetes) can increase fetal size.
  • Smoking and alcohol: Both are associated with reduced fetal growth.
  • Multiple pregnancy: Twins and triplets tend to weigh less than singletons.
  • Ethnicity: Some ethnic groups have different average birth weights.

When to Consult Your Healthcare Provider

  • If you have concerns about your baby's size or growth.
  • If you have risk factors for fetal growth restriction (e.g., hypertension, smoking, multiple pregnancy).
  • If you have risk factors for fetal macrosomia (e.g., gestational diabetes, large baby previously).
  • If you notice a significant change in fetal movement patterns.
  • At each routine prenatal visit, discuss your baby's growth with your provider.

References & Further Reading

  • Hadlock, F.P., Harrist, R.B., & Martinez-Poyer, J. (1991). In utero analysis of fetal growth: a sonographic weight standard. Radiology, 181(1), 129-133.
  • Hadlock, F.P. et al. (1985). Estimation of fetal weight with the use of head, body, and femur measurements. American Journal of Obstetrics & Gynecology, 151(3), 333-337.
  • INTERGROWTH-21st Consortium. (2014). International standards for fetal growth based on serial ultrasound measurements. The Lancet, 384(9946), 869-879.
  • Papageorghiou, A.T. et al. (2014). International standards for fetal growth: the INTERGROWTH-21st project. American Journal of Obstetrics & Gynecology, 210(3), 239-244.
  • American College of Obstetricians and Gynecologists. (2021). Fetal Growth Restriction. Practice Bulletin No. 227.

Frequently Asked Questions

At 20 weeks of pregnancy, the average baby weighs approximately 300 grams (about 10.5 ounces) and measures about 16.4 cm (6.5 inches) from crown to rump. This is based on the Hadlock growth chart, which represents the 50th percentile. Individual babies may vary significantly.

At 30 weeks of pregnancy, the average baby weighs approximately 1,300 grams (about 2.9 pounds) and measures about 27.8 cm (10.9 inches) from crown to rump. By 40 weeks, the average weight is about 3,500 grams (7.7 pounds) and length is about 37.5 cm (14.8 inches).

In the third trimester (weeks 27–40), a baby gains approximately 150–250 grams (5–9 ounces) per week:

  • Weeks 28–32: ~150–200g per week
  • Weeks 32–36: ~200–250g per week
  • Weeks 36–38: ~200–230g per week
  • Weeks 38–40: ~150–200g per week (slowing slightly)

This rapid weight gain is primarily due to increased body fat and muscle development.

Crown-rump length (CRL): Measures the baby from the top of the head to the bottom of the buttocks. Used in the first and second trimesters (weeks 8–20+). This is the measurement used in this chart.

Crown-heel length (CHL): Measures the baby from the top of the head to the heel. Used in the third trimester and for newborns. It is longer than CRL by the length of the legs.

Newborns are typically measured in crown-heel length.

Percentiles show how a baby's weight compares to other babies at the same gestational age:

  • 10th percentile: Only 10% of babies weigh less at this age.
  • 50th percentile: The average or median weight.
  • 90th percentile: 90% of babies weigh less at this age.

Babies between the 10th and 90th percentiles are typically considered within the normal range. Those below the 10th percentile may be small for gestational age (SGA), and those above the 90th percentile may be large for gestational age (LGA).

The INTERGROWTH-21st charts were developed using data from multiple countries (Brazil, China, India, Italy, Kenya, Oman, UK, USA) and are considered applicable across different populations.

The Hadlock charts were developed in the US and may have some population-specific biases.

The INTERGROWTH-21st standard is now recommended by the WHO as a global growth standard and is increasingly used in clinical practice worldwide.

Many factors influence birth weight:

  • Maternal weight and height: Larger mothers tend to have larger babies.
  • Maternal weight gain: Adequate gain supports fetal growth.
  • Nutrition: Protein, iron, folic acid, and overall nutrition.
  • Placental function: Nutrient and oxygen delivery.
  • Maternal health: Diabetes (increases size), hypertension (decreases size).
  • Smoking and alcohol: Reduce fetal growth.
  • Multiple pregnancy: Twins and triplets weigh less.
  • Ethnicity: Some ethnic groups have lower average birth weights.
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