Fetal Weight Estimator
Estimate fetal weight from ultrasound biometrics using Hadlock 4-parameter formula
🧮 Fetal Weight Estimator — Formula
🩺 Based on ACOG/RCOG guidelines. Confirm with ultrasound measurement when indicated.
📌Hadlock FP et al. Radiology 1985 / ISUOG Practice Guidelines
📊 Quick Reference
| Input / Parameter | Description | Example Value |
|---|---|---|
| LMP (first day) | First day of last menstrual period | 15 March 2025 |
| Cycle length | Average menstrual cycle length in days | 28 days |
| Gestational age | Calculated weeks + days from LMP | 28+3 weeks |
| EDD output | Estimated due date (Naegele's Rule: LMP + 280 days) | 20 December 2025 |
| Fetal biometrics | BPD, HC, AC, FL for EFW by Hadlock formula | EFW 1,240g (50th centile) |
| Bishop Score | Cervical score 0–13 for induction readiness | Bishop 6 — borderline favourable |
ℹ️ About This Calculator
The Fetal Weight Estimator is a standard obstetric calculation tool used to derive pregnancy dating parameters, assess fetal growth, evaluate labour readiness, or estimate obstetric outcomes using validated formulas and reference data. Accurate obstetric calculations are fundamental to safe antenatal care — errors in gestational age, estimated fetal weight, or induction readiness assessment directly affect clinical decisions about screening test timing, corticosteroid administration, delivery planning, and neonatal management.
Gestational age is the most fundamental parameter in obstetric care — it determines the schedule of every antenatal assessment, screening test, and intervention from first trimester viability to delivery planning. The Fetal Weight Estimator applies the appropriate validated formula or reference standard — whether Naegele's Rule for due date calculation, Hadlock equations for fetal weight estimation, or Bishop scoring for cervical assessment — to provide a clinically reliable result.
The exact formula and variable definitions used by this calculator are shown in the Formula section below. For obstetric calculations, formula transparency matters because different equations and charts have different populations of origin, and clinicians should know which reference they are applying. For example, Hadlock equations for estimated fetal weight were derived from predominantly white American fetuses — they may produce different accuracy in other ethnic populations.
Important limitations: LMP-based dating (Naegele's Rule) assumes a regular 28-day cycle with ovulation on day 14. For patients with irregular cycles, unknown LMP, or cycles substantially longer or shorter than 28 days, first-trimester ultrasound dating by CRL measurement is significantly more accurate and should be used per ACOG guidelines. Fetal weight estimations have an inherent error of approximately ±15–20% regardless of the formula used. The Fetal Weight Estimator provides estimates — clinical findings, ultrasound measurements, and professional judgment define the management plan.
All calculations run in your browser only. No obstetric data, LMP dates, fetal measurements, or clinical information is transmitted to any server or stored anywhere. This ensures full compliance with healthcare data privacy requirements when using the tool during clinical consultations.
These calculators support evidence-based obstetric care — they do not replace it. All pregnant women should receive regular antenatal care from a qualified obstetrician or midwife. Contact your maternity care provider promptly for: vaginal bleeding at any gestation, decreased fetal movement after 24 weeks, symptoms of pre-eclampsia (severe headache, visual disturbance, epigastric pain, oedema), signs of preterm labour, or any other concerns about pregnancy. If in doubt, always seek prompt obstetric assessment.
📌Clinical Reference: Hadlock FP et al. Radiology 1985 / ISUOG Practice Guidelines
📋 How to Use This Calculator
- 1
Confirm the last menstrual period date
Enter the first day of the last normal menstrual period (LMP). Ensure the date is reliable — a patient with irregular cycles or uncertainty about the LMP date will require ultrasound dating.
- 2
Enter clinical measurements as required
For fetal weight estimation, input ultrasound biometric measurements (BPD, HC, AC, FL). For Bishop Score, enter the cervical examination findings at the time of assessment.
- 3
Calculate the obstetric parameter
The tool applies the validated formula and returns the gestational age, EDD, fetal weight estimate, or clinical score. The result is displayed with the applicable reference range.
- 4
Compare with ultrasound findings
For pregnancy dating, compare the LMP-derived EDD with first-trimester ultrasound dating. If the discordance is >5–7 days in the first trimester, the ultrasound date takes precedence per ACOG guidelines.
- 5
Document and plan antenatal care
Record the calculated GA and EDD in the patient's obstetric record. Gestational age determines the schedule for antenatal screening tests, growth scans, and delivery planning.
🎯 When to Use This Calculator
Pregnancy dating and EDD confirmation
Calculate EDD from LMP at the first antenatal visit. Compare with first-trimester ultrasound dating — if discordance >5–7 days, use ultrasound date per ACOG guidelines.
Gestational age for clinical decisions
Calculate exact gestational age in weeks and days when making decisions about antenatal screening timing, viability thresholds, corticosteroid administration, and delivery planning.
Labour induction readiness assessment
Calculate Bishop Score before cervical ripening or induction to predict likelihood of successful induction. A score ≥8 indicates a favourable cervix with high success probability.
Antenatal weight gain monitoring
Track gestational weight gain against IOM guidelines (based on pre-pregnancy BMI) at each visit. Excessive gain increases GDM, hypertension, and macrosomia risk; insufficient gain affects fetal growth.
IVF success rate estimation
Use the IVF success rate calculator before initiating fertility treatment to set realistic expectations and guide clinical counselling about the number of treatment cycles likely needed.
💡 Clinical Pro Tips
Naegele's Rule (EDD = LMP + 280 days) assumes a regular 28-day cycle with ovulation on day 14. For patients with cycles longer than 28 days, the EDD should be adjusted: add the difference between the patient's cycle length and 28 days. For example, a patient with 35-day cycles should have EDD adjusted forward by 7 days.
First-trimester ultrasound CRL dating is more accurate than LMP dating and should take precedence if the discordance is >5 days at 9–11 weeks, or >7 days at 11–14 weeks. ACOG recommends using ultrasound dates once assigned, even if LMP later becomes available.
For the Bishop Score, effacement is often misscored. It reflects the percentage of cervical shortening relative to the normal non-pregnant length (~3.5 cm). A cervix that has thinned to 1.75 cm is 50% effaced (score 2). Tactile assessment of effacement improves with experience — request senior review for borderline cases.
Gestational weight gain targets (IOM 2009) are BMI-based: underweight (<18.5): 12.5–18 kg; normal (18.5–24.9): 11.5–16 kg; overweight (25–29.9): 7–11.5 kg; obese (≥30): 5–9 kg. Women gaining outside these ranges have higher risks of GDM, macrosomia, preterm birth, and caesarean section.
More Obstetrics & Gynaecology Tools
Due Date Calculator (EDD)
Calculate estimated due date from last menstrual period or ultrasound gestational age.
Gestational Age Calculator
Calculate gestational age in weeks and days from LMP or ultrasound scan date.
Pregnancy Weight Gain Guide
Calculate recommended pregnancy weight gain based on pre-pregnancy BMI and IOM guidelines.
IVF Success Rate Calculator
Estimate IVF success probability using SART prediction model by age and cycle history.
Ovulation & Fertile Window Tool
Calculate ovulation date and fertile window from menstrual cycle length.
Bishop Score Calculator
Calculate Bishop score for cervical ripening assessment and induction of labour decisions.
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