Fetal Growth Calculator: Fetal Percentile & Ultrasound Guide
Introduction: Prenatal Development & Biometry
Monitoring fetal growth is a core component of prenatal care. From the early weeks of organogenesis to the final stages of hypertrophic weight gain, fetal development provides essential insights into placental health and neonatal viability. Because direct measurement in utero is impossible, clinicians utilize ultrasound biometry to estimate weight and evaluate growth trajectories.
Our Fetal Growth Calculator computes estimated fetal weight (EFW) and growth percentiles based on ultrasound measurements. By comparing biometrics against Hadlock reference standards, this tool helps expectant parents and sonography learners track gestational progress.
Use this clinical reference guide and calculator to:
Important Note: Ultrasound weight estimates carry a normal margin of error of ±10% to 15%. This tool is for educational purposes only and is not a substitute for diagnostic prenatal scanning or clinical guidance from your OB-GYN or midwife.
Ultrasound Biometry Calculator
Enter your gestational week and ultrasound measurements (BPD, HC, AC, FL) below to calculate estimated weight and milestones.
Gestational Progression
Avg. Length
25.6 cm
~10.1 in
Avg. Weight
300 g
~0 lb 10.6 oz
Fetal Developmental Milestones
Midway point of pregnancy. Movements (quickening) are easily felt.
Size Comparison analogy:
At week 20, the baby is about the size of a Banana.
Estimated weights correspond directly to Hadlock regression formulas.
The Three Phases of Fetal Development
Fetal growth is not linear. It progresses through three cellular phases as the fetus matures:
1. Hyperplastic Phase (Weeks 0 to 14)
Characterized by rapid cell division and organogenesis. Fetal weight accumulation is minimal, but structural complexity is high. Measured primarily by Crown-Rump Length (CRL).
2. Hyperplastic-Hypertrophic Phase (Weeks 15 to 28)
Cell division continues alongside cell enlargement. The fetal skeletal structure elongates rapidly. Anatomical structures are fully evaluated during the 18–22 week anatomy scan.
3. Hypertrophic Phase (Weeks 29 to 40)
Rapid cellular hypertrophy (enlargement) leads to exponential weight gain. The fetus stores glycogen in the liver and deposits subcutaneous fat, gaining approximately 150 to 200 grams per week in late pregnancy.
Ultrasound Biometry Markers
Four primary skeletal and soft-tissue landmarks are measured on prenatal scans to calculate growth:
Biparietal Diameter (BPD)
The maximum width of the fetal skull, measured from outer-to-inner bone edge at the level of the thalami and CSP.
Head Circumference (HC)
The outer perimeter of the fetal skull, adjusting for head shape variations.
Abdominal Circumference (AC)
The outer perimeter of the fetal abdomen at the level of the umbilical vein. Highly sensitive to glycogen storage and fetal nutrition.
Femur Length (FL)
The maximum length of the ossified thigh bone shaft. Serves as a key indicator of skeletal development.
Estimated Fetal Weight (EFW)
A calculation of overall mass in grams, combining the four biometrics using regression formulas.
Fetal Milestones by Gestational Week
Refer to this reference matrix to find average weights, lengths, and active developmental phases across gestation:
| Gestational Week | Average Weight (g) | Average Length (cm) | Developmental Phase |
|---|---|---|---|
| Week 12 | 14 g | 5.4 cm | Hyperplastic (Organogenesis) |
| Week 16 | 100 g | 11.6 cm | Hyperplastic-Hypertrophic |
| Week 20 | 300 g | 25.6 cm | Anatomy Scan Baseline |
| Week 24 | 600 g | 30.0 cm | Viability Threshold |
| Week 28 | 1,000 g | 37.6 cm | Third Trimester Surge |
| Week 32 | 1,700 g | 42.4 cm | Hypertrophic Weight Gain |
| Week 36 | 2,600 g | 47.4 cm | Lungs & Brain Maturation |
| Week 40 | 3,500 g | 51.2 cm | Term Birth weight Ready |
Fetal Growth Standards Compared
Clinical teams utilize different reference standards to evaluate fetal weight percentiles:
| Growth Standard | Primary Data Source | Clinical Strengths | Limitations |
|---|---|---|---|
| Hadlock Standards | Single-center U.S. cohorts (1980s) | The most widely studied, clinically tested, and integrated standard globally. | Based primarily on homogenous demographic data. |
| NICHD Standards | Multi-center U.S. diverse cohorts (2015) | Provides racially and ethnically specific growth percentiles to prevent diagnostic errors. | Requires maternal racial/ethnic inputs for calculations. |
| WHO Standards | Multinational healthy cohorts (2017) | Establishes a universal optimal growth target for maternal populations with access to clean nutrition. | May underdiagnose growth restriction in specific populations. |
Symmetric vs. Asymmetric FGR
Fetal Growth Restriction (FGR) occurs when the fetus fails to achieve its genetic growth potential, leaving weight below the 10th percentile:
Symmetric FGR (Early Onset)
All biometric measurements (BPD, HC, AC, FL) are equally reduced. Usually caused by an early gestational insult, such as chromosomal anomalies or congenital TORCH infections. The absolute body cell count is reduced.
Asymmetric FGR (Late Onset)
The head size (BPD, HC) is preserved while abdominal circumference (AC) is significantly reduced (the "head-sparing effect"). This brain-protective redistribution of blood flow is caused by placental insufficiency in late pregnancy.
EFW Biophysics & Variabilities
Since we cannot weigh the baby in utero, estimated fetal weight is calculated from bone and tissue markers. Because of this, several variables affect measurement accuracy:
Growth Percentile Risk Ranges
Growth percentiles indicate how a baby's weight compares to other healthy babies at the same gestational age:
| Percentile Range | Risk Classification | Clinical Implications |
|---|---|---|
| < 3rd Percentile | Severe SGA | High risk of pathologic growth restriction; requires Doppler evaluation. |
| 3rd to 9th Percentile | Moderate SGA | Borderline growth restriction; requires frequent biophysical profiles. |
| 10th to 90th Percentile | AGA (Appropriate) | Normal fetal growth. Continue standard prenatal schedule. |
| 91st to 97th Percentile | Moderate LGA | Large baby; monitor maternal blood glucose levels closely. |
| > 97th Percentile | Severe LGA | High risk of fetal macrosomia; evaluate delivery plan. |
Fetal Size Classifications
Clinical definitions and risks associated with fetal size variances:
| Classification | Definition | Common Causes | Clinical Risks |
|---|---|---|---|
| Fetal Growth Restriction (FGR) | Estimated fetal weight (EFW) < 10th percentile due to pathology. | Placental insufficiency, chronic maternal hypertension, preeclampsia. | Stillbirth, intrapartum fetal distress, neonatal hypoglycemia. |
| Small for Gestational Age (SGA) | Fetal weight < 10th percentile regardless of cause. | Constitutional (healthy small parents) or pathological (FGR). | Slightly increased neonatal morbidity; requires MFM tracking. |
| Appropriate for Gestational Age (AGA) | Fetal weight between the 10th and 90th percentiles. | Healthy placental blood flow and genetic progression. | Standard baseline delivery risk. |
| Large for Gestational Age (LGA) | Estimated fetal weight > 90th percentile. | Maternal gestational diabetes, maternal obesity, post-term pregnancy. | Shoulder dystocia, perineal lacerations, cesarean delivery. |
| Fetal Macrosomia | Actual birth weight exceeding 4,000 grams. | Hyperglycemia-induced hyperinsulinemia, maternal over-nutrition. | Birth trauma, neonatal hypoglycemia, respiratory distress. |
Pathological Factors Influencing Fetal Growth
Maternal, placental, and fetal factors can alter healthy growth trajectories:
| Factor | Physiological Mechanism | Impact on Fetal Weight |
|---|---|---|
| Maternal: Gestational Diabetes | Maternal hyperglycemia crosses the placenta, stimulating fetal insulin secretion (a potent growth factor). | Accelerates fetal fat deposition, leading to macrosomia/LGA. |
| Maternal: Chronic Hypertension | Vasoconstriction restricts uterine and placental blood flow, limiting nutrient delivery. | Slows fetal weight gain, leading to late-onset asymmetric FGR. |
| Placental: Insufficiency | Placental vascular thrombi or abnormal cord insertion (velamentous) restricts flow. | Reduces oxygenation, leading to fetal growth restriction. |
| Fetal: Congenital Infections | TORCH infections damage fetal organs and cellular division directly. | Results in early-onset symmetric FGR. |
Standard Prenatal Ultrasound Schedule
Recommended scans and markers measured throughout a healthy pregnancy:
| Ultrasound Scan Type | Recommended Timing | Primary Clinical Focus | Measurements Measured |
|---|---|---|---|
| Dating Ultrasound | 7 to 12 Weeks | Confirm viability, establish gestational age, and determine due date. | Crown-Rump Length (CRL) |
| Nuchal Translucency (NT) Scan | 11 to 13 Weeks | Screen for chromosomal anomalies (e.g. Down Syndrome). | Nuchal fold thickness |
| Detailed Anatomy Scan | 18 to 22 Weeks | Evaluate organ structure, cardiac chambers, and baseline size. | BPD, HC, AC, FL |
| Growth Scan (if indicated) | 28 to 36 Weeks | Monitor fetal growth velocity, amniotic fluid, and placental blood flow. | BPD, HC, AC, FL, EFW |
Ultrasound Dosing Scenarios
Three worked gestational biometrics examples showing estimated weight and percentiles.
Scenario 1: Normal 20-Week Scan
GA: 20w0d. Biometrics: BPD = 46mm, HC = 175mm, AC = 150mm, FL = 32mm. Reference: Hadlock.
Estimated Fetal Weight: ~300g (Approx. 50th Percentile).
Bedside Setup: Normal growth profile. Schedule standard checkup in 4 weeks.
Scenario 2: Late Growth Restriction
GA: 32w0d. Biometrics: HC = 295mm, AC = 240mm, FL = 62mm. Reference: Hadlock.
Estimated Fetal Weight: ~1,520g (Approx. 8th Percentile - Asymmetric SGA).
Bedside Setup: High risk of FGR. Initiate weekly umbilical artery Doppler flow scans.
Scenario 3: Large for Gestational Age
GA: 36w0d. Biometrics: BPD = 94mm, HC = 338mm, AC = 350mm, FL = 73mm. Reference: Hadlock.
Estimated Fetal Weight: ~3,120g (Approx. 95th Percentile - LGA).
Bedside Setup: Risk of macrosomia. Monitor maternal glucose levels and plan delivery route.
Frequently Asked Questions
Common prenatal questions regarding ultrasound biometry, percentiles, and growth standards.
1. What is a Fetal Growth Calculator?
2. How accurate is a Fetal Growth Calculator?
3. How do I calculate my fetal growth percentile?
4. How is fetal growth percentile calculated?
5. What is a fetal growth percentile?
6. What is considered normal fetal growth?
7. What is the Hadlock fetal growth calculator?
8. What is the NICHD fetal growth calculator?
9. What is the WHO growth calculator?
10. How often is fetal growth measured?
11. Can ultrasound accurately estimate weight?
12. Why do fetal growth percentiles matter?
13. What factors affect fetal growth?
14. Can a Fetal Growth Calculator diagnose growth restriction?
15. Should I worry if my baby's growth percentile changes?
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Medical Prenatal Disclaimer
Fetal growth calculations, estimated weight percentiles, and biometry targets are provided for educational and prenatal tracking purposes only. Digital calculators are not diagnostic medical tools. A true diagnosis of fetal growth restriction, oligohydramnios, or fetal macrosomia requires a professional diagnostic ultrasound evaluation, biophysical profiles, and Doppler blood flow indices analyzed by a licensed maternal-fetal medicine specialist or OB-GYN. Never make clinical decisions about induction, delivery, or pregnancy care based solely on estimated outputs.