Baby Weight Calculator - UK Fetal Weight Estimator
What is a Baby Weight Calculator?
A baby weight calculator estimates fetal weight during pregnancy based on gestational age (weeks pregnant) and optional fundal height measurements. In the UK, these calculators use medically-validated formulas aligned with NHS guidelines and Royal College of Obstetricians and Gynaecologists (RCOG) standards.
This free UK baby weight calculator provides:
- Estimated fetal weight by week - Based on UK medical research and NHS percentile charts
- Weight in grams, kilograms, pounds and ounces - All UK-standard units
- Size comparison - Helps visualize baby's size (e.g., "size of a pineapple")
- Expected length - Crown to heel measurement estimates
- Weight range - 10th to 90th percentile healthy variation
- NHS Red Book percentile reference - Compare to UK child health programme standards
How Baby Weight is Measured in UK Hospitals
NHS hospitals use ultrasound scans to estimate fetal weight during pregnancy:
Early Pregnancy (8-20 weeks)
- Dating scan (10-13 weeks) - Measures crown-rump length (CRL) to confirm due date
- Anomaly scan (18-21 weeks) - Checks baby's anatomy and measures head, abdomen, femur
- Weight estimated from length measurements using validated formulas
Late Pregnancy (20+ weeks)
- Growth scans - Offered if concerns about baby's size
- Four key measurements:
- Biparietal diameter (BPD) - Head width
- Head circumference (HC)
- Abdominal circumference (AC) - Most important for weight
- Femur length (FL) - Thigh bone
- Hadlock formula - Used to calculate estimated fetal weight (EFW)
- Accuracy - ±15% margin of error (±450-500g for 3kg baby)
NHS Red Book and UK Growth Charts
Every baby in the UK receives the NHS Personal Child Health Record (Red Book) at birth. This contains:
- UK-WHO growth charts - Percentile charts for weight, length, and head circumference
- Birth to 4-5 years tracking - Monitor healthy growth trajectory
- Percentile lines - 0.4th, 2nd, 9th, 25th, 50th (median), 75th, 91st, 98th, 99.6th
- Separate charts for boys and girls - Account for gender differences
- Preterm charts - For babies born before 37 weeks
Health visitors plot baby's weight at regular checks. Concerns arise if:
- Baby drops more than 2 centile spaces (e.g., 75th to 25th)
- Weight below 0.4th centile (very small)
- Not regaining birth weight by day 14
- Crossing upward rapidly (may indicate overfeeding or medical issue)
How the Baby Weight Calculator Works
Step 1: Enter Gestational Age
Input your current pregnancy week (8-42 weeks). This calculator uses UK-standard gestational age counting from the first day of your last menstrual period (LMP). Your dating scan (10-13 weeks) confirms accurate dating.
Step 2: Optional Fundal Height
From 24 weeks onwards, UK midwives measure fundal height at antenatal appointments. Fundal height (in cm) approximately equals gestational age (in weeks) from 20-36 weeks. For example:
- 28 weeks pregnant = approximately 28cm fundal height
- 32 weeks pregnant = approximately 32cm fundal height
- More than 2cm difference may trigger growth scan
Step 3: Calculator Estimates Weight
The calculator uses medically-validated weight-by-week data from:
- NHS fetal medicine research
- Royal College of Obstetricians and Gynaecologists (RCOG) guidelines
- UK Intergrowth-21st standards for fetal growth
- Hadlock formula adjustments based on fundal height (if provided)
Step 4: View Results
Results include:
- Estimated weight - Grams, kilograms, pounds, ounces
- Expected length - Crown to heel measurement
- Size comparison - Fruit/vegetable analogy for visualization
- Percentile range - 10th to 90th percentile healthy variation
- Trimester information - Where you are in pregnancy
- NHS guidance - What to expect and when to seek advice
Formula Used
This calculator uses UK-validated average fetal weights by gestational week. If fundal height is provided, the estimate adjusts using:
Adjustment = (Fundal Height - Gestational Age) × 50g If difference >3cm, weight estimate adjusts accordingly
Important: This is an estimation tool. Actual ultrasound measurements by trained NHS sonographers are more accurate, especially for identifying growth concerns.
UK Baby Weight Guidelines and NHS Standards
Average Baby Weight by Trimester (UK Standards)
First Trimester (Weeks 1-13)
Minimal weight gain, focus on organ development:
- Week 8: 1g (size of raspberry)
- Week 9: 2g (size of cherry)
- Week 10: 4g (size of strawberry)
- Week 11: 7g (size of fig)
- Week 12: 14g (size of plum)
- Week 13: 23g (size of lemon)
Second Trimester (Weeks 14-27)
Moderate growth, skeleton developing:
- Week 14: 43g (size of peach)
- Week 16: 100g (size of avocado)
- Week 18: 190g (size of bell pepper)
- Week 20: 300g (size of banana) - Halfway milestone
- Week 22: 430g (size of papaya)
- Week 24: 600g (viability threshold in UK)
- Week 26: 760g (size of lettuce head)
- Week 27: 875g (size of cauliflower)
Third Trimester (Weeks 28-40)
Rapid growth, fat accumulation for warmth and energy:
- Week 28: 1000g (1kg) - Gaining ~200-250g/week from now
- Week 30: 1319g
- Week 32: 1702g (1.7kg) - Common growth scan week
- Week 34: 2146g (2.1kg)
- Week 36: 2622g (2.6kg) - Common growth scan week
- Week 37: 2859g (2.9kg) - Full term
- Week 38: 3083g (3.1kg)
- Week 39: 3288g (3.3kg)
- Week 40: 3462g (3.5kg) - Average UK birth weight
UK Birth Weight Classifications
- Low birth weight (LBW): <2.5kg - Requires special care, frequent feeding, temperature monitoring
- Very low birth weight (VLBW): <1.5kg - Neonatal intensive care unit (NICU) admission
- Extremely low birth weight (ELBW): <1kg - Extended NICU stay, specialist care
- Normal birth weight: 2.5-4kg - Standard postnatal care
- Macrosomia: >4kg - Monitor for birth injuries, hypoglycemia
- Severe macrosomia: >4.5kg - Higher risk of shoulder dystocia, C-section offered
When NHS Offers Growth Scans
UK hospitals offer additional growth scans (beyond routine 12 and 20 week scans) if:
- Fundal height measuring small (>2cm below gestational age)
- Fundal height measuring large (>2cm above gestational age)
- Previous baby with growth restriction (IUGR)
- Previous large baby (>4.5kg)
- Gestational diabetes (diagnosed via glucose tolerance test)
- Maternal hypertension or pre-eclampsia
- Multiple pregnancy (twins, triplets)
- Maternal BMI >35
- Reduced fetal movements
- Maternal age >40
- Smoking or substance use
Scans typically scheduled at 28, 32, and 36 weeks if concerns identified.
UK Healthy Start Programme
Low-income families qualify for Healthy Start vouchers (£4.25/week) for:
- Milk, fresh fruit, vegetables
- Infant formula milk
- Free vitamins (Healthy Start vitamins containing folic acid, vitamin C, vitamin D)
Supports healthy pregnancy and baby growth. Apply at healthystart.nhs.uk
Tips for Healthy Baby Weight
Nutrition Tips (UK Guidelines)
- Folic acid - 400mcg daily until 12 weeks (prevents neural tube defects). Free Healthy Start vitamins available.
- Vitamin D - 10mcg daily throughout pregnancy (NHS recommends year-round in UK due to limited sunlight)
- Protein - 51g daily (71g in third trimester). Sources: lean meat, fish, eggs, beans, lentils
- Iron - 27mg daily (prevents anemia). Sources: red meat, spinach, fortified cereals. Pair with vitamin C for absorption.
- Omega-3 - 200mg DHA daily for brain development. Sources: salmon, mackerel (limit to 2 portions/week), walnuts, flaxseed
- Calcium - 1000mg daily for bone development. Sources: milk, cheese, yogurt, fortified plant milk
- Hydration - 8-10 glasses water daily (prevents constipation, supports amniotic fluid)
Foods to Avoid (UK FSA Guidelines)
- Unpasteurized dairy - Risk of listeria (can cause miscarriage/stillbirth)
- Soft mould-ripened cheese - Brie, camembert, Danish blue (listeria risk)
- Pâté - All types including vegetable (listeria risk)
- Raw/undercooked eggs - Unless British Lion stamped (salmonella risk)
- Raw/undercooked meat - Ensure cooked through (toxoplasmosis risk)
- Liver/liver products - High vitamin A (can harm baby)
- High-mercury fish - Shark, swordfish, marlin (limit tuna to 2 steaks/week or 4 cans/week)
- Alcohol - NHS recommends complete avoidance (safest option)
- Caffeine - Limit to 200mg/day (2 mugs instant coffee or 2 cups tea)
Weight Gain Tips (Maternal Weight)
- Healthy weight gain - 10-12.5kg total for normal BMI (18.5-24.9)
- First trimester - Minimal gain (1-2kg), focus on quality nutrition
- Second/third trimester - 0.5kg/week average
- Underweight (BMI <18.5) - Gain 12.5-18kg, refer to dietitian
- Overweight (BMI 25-29.9) - Gain 7-11.5kg, monitor for gestational diabetes
- Obese (BMI ≥30) - Gain 5-9kg, extra monitoring, glucose tolerance test
Monitoring Tips
- Attend all antenatal appointments - Fundal height measured from 24 weeks
- Track fetal movements - From 16-24 weeks onwards. Contact immediately if reduced.
- Use NHS Kicks Count app - Free app to monitor baby's movement patterns
- Report concerns early - Don't wait - call midwife or triage if worried
- Keep Red Book updated - Track all weights and measurements after birth
Common Mistakes to Avoid
Calculator Interpretation Mistakes
- Treating estimates as exact - Online calculators provide averages. ±15% variation is normal. Only ultrasound provides medical-grade estimates.
- Comparing to wrong percentile - UK uses WHO growth charts. US/other countries may differ. Use NHS Red Book percentiles only.
- Worrying about single measurement - Growth velocity (trend over time) matters more than one data point. Consistent growth on same percentile is healthy.
- Ignoring ethnic variations - South Asian babies typically 200-300g lighter, Afro-Caribbean babies similar to white British, Polynesian babies heavier. All can be healthy.
- Using LMP dates if irregular periods - Dating scan (10-13 weeks) is gold standard. If cycles irregular, LMP-based dates may be inaccurate.
Medical Advice Mistakes
- Delaying care if concerned - If you feel something's wrong, call immediately. Maternity triage available 24/7 in all UK NHS hospitals. Never "wait and see" with reduced movements.
- Declining growth scans - If midwife/consultant recommends scan, there's clinical reason. Accept monitoring - early detection prevents complications.
- Self-diagnosing IUGR or macrosomia - Only fetal medicine specialists diagnose growth restriction. Don't panic based on online calculator.
- Ignoring fundal height concerns - If midwife refers for scan due to fundal height, attend. 30-40% of growth-restricted babies detected this way.
- Assuming "small = problem" or "large = problem" - Many small/large babies are healthy. Investigation needed to determine cause.
Lifestyle Mistakes
- Smoking "just a few" - No safe level. Every cigarette reduces placental blood flow. Babies 150-250g lighter on average. Free NHS Stop Smoking Service available.
- "Eating for two" - Only need extra 200 calories/day in third trimester (1 slice toast with peanut butter). Overeating increases gestational diabetes risk.
- Restricting food to keep baby small - Never diet during pregnancy. Malnutrition causes growth restriction. Consult dietitian if concerns about baby's size.
- Excessive caffeine - >200mg/day linked to low birth weight. 1 mug instant coffee = 100mg, 1 mug tea = 75mg, 1 can cola = 40mg.
- Herbal supplements without GP approval - Many herbals unsafe in pregnancy (black cohosh, dong quai, saw palmetto). Always check with GP/midwife first.
Postnatal Mistakes
- Missing weight checks - Babies should regain birth weight by day 14. Missing checks can miss feeding problems or medical issues.
- Forcing percentile change - Don't overfeed to "move up percentiles". Babies have genetic growth pattern. Overfeeding increases obesity risk.
- Ignoring weight loss >10% - Normal to lose 5-7% birth weight in first week. >10% needs investigation (dehydration, breastfeeding problems).
- Comparing to other babies - "My friend's baby is bigger" is irrelevant. Growth charts account for individual variation. Focus on your baby's trend.
- Not using Red Book - Bring to every appointment. Continuity of records vital if you change GP or move house.
Frequently Asked Questions
What are NHS Red Book percentile charts?
NHS Red Book percentile charts track your baby's growth from birth to 4-5 years. The charts show weight, length, and head circumference percentiles (9th, 25th, 50th, 75th, 91st, 98th). These UK-specific growth charts account for variations in ethnicity and are used by health visitors and GPs to monitor healthy development. Babies typically follow same percentile line - crossing 2+ lines requires investigation.
How does UK child health programme monitor baby weight?
The UK Healthy Child Programme includes regular weight checks: birth (in hospital), day 5 with midwife, 6-8 week check with GP, 9-12 months, 1-2 years, and 2-2.5 years with health visitor. Weights are plotted in the NHS Red Book. Health visitors flag if baby drops more than 2 centile lines or is below 0.4th centile. Additional weighing available at children's centres and GP surgeries.
What is the average UK newborn weight?
The average UK newborn weight is 3.5kg (7lb 8oz) for boys and 3.4kg (7lb 4oz) for girls at 40 weeks. Normal birth weight ranges from 2.5kg to 4.5kg. Babies below 2.5kg are considered low birth weight and may need special care. Babies over 4kg are large for gestational age. UK charts account for ethnic variations - South Asian babies average 200-300g lighter, which is normal.
How accurate are ultrasound weight estimates in UK hospitals?
NHS ultrasound fetal weight estimates have a ±15% margin of error (±450-500g for a 3kg baby). Accuracy decreases after 36 weeks. UK hospitals use Hadlock formula measuring biparietal diameter, head circumference, abdominal circumference, and femur length. Growth velocity over multiple scans is more reliable than single estimates. 50% of "big baby" predictions are overestimates - many mothers successfully deliver vaginally despite 4kg+ predictions.
When do UK hospitals offer growth scans?
NHS offers growth scans if: fundal height measures small (>2cm below gestational age), previous growth-restricted baby, gestational diabetes, maternal hypertension, reduced fetal movements, or multiple pregnancy. Scans typically at 28, 32, and 36 weeks if concerns arise. Not routinely offered for reassurance - private scans available but remember ±15% error margin can cause unnecessary anxiety.
What is considered small for gestational age (SGA) in UK?
UK defines SGA as birth weight below 10th percentile for gestational age. Severely SGA is below 3rd percentile. Babies measuring small need Doppler blood flow scans to check placental function. If constitutionally small (genetic) with normal Doppler, monitoring continues. If placental insufficiency detected, early delivery may be recommended at 34-37 weeks. Many SGA babies are healthy - just petite like parents.
What support does NHS offer for low birth weight babies?
NHS provides: admission to neonatal unit if under 2.5kg, temperature regulation support (incubators), frequent feeding assistance, blood sugar monitoring (hypoglycemia risk), jaundice phototherapy if needed, and extended postnatal stay. Health visitors provide additional home visits. Babies qualify for Healthy Start vouchers for vitamins and food. Exclusive breastfeeding supported with lactation consultant input.
How does gestational diabetes affect baby weight in UK?
Gestational diabetes (diagnosed via 75g glucose tolerance test at 24-28 weeks in UK) can cause macrosomia (baby >4kg). NHS offers 2-4 weekly growth scans, glucose monitoring 4 times daily, dietitian support, and metformin/insulin if diet control insufficient. Induction offered at 37-38 weeks if on medication. C-section discussed if estimated weight >4.5kg. Baby's blood sugar checked after birth to prevent hypoglycemia.
Pro Tips for Accurate Results
- Double-check your input values before calculating
- Use the correct unit format (metric or imperial)
- For complex calculations, break them into smaller steps
- Bookmark this page for quick future access
Understanding Your Results
Our Baby Weight Calculator provides:
- Instant calculations - Results appear immediately
- Accurate formulas - Based on official UK standards
- Clear explanations - Understand how results are derived
- 2025/26 updated - Using current rates and regulations
Common Questions
Is this calculator free?
Yes, all our calculators are 100% free to use with no registration required.
Are the results accurate?
Our calculators use verified formulas and are regularly updated for accuracy.
Can I use this on mobile?
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