Calculator Form
Example Data Table
| Gestational Age | Weight | Sex | Approximate Result | Use Case |
|---|---|---|---|---|
| 30 weeks 0 days | 1200 g | Unknown | Below median, close review needed | Preterm growth check |
| 36 weeks 3 days | 2450 g | Female | Near lower expected range | Late pregnancy estimate |
| 39 weeks 0 days | 2550 g | Male | Likely small for gestational age | Birth weight review |
| 40 weeks 2 days | 3400 g | Unknown | Appropriate for gestational age | Routine newborn check |
Formula Used
The calculator uses an educational gestational weight reference table from 24 to 42 weeks. Values are linearly interpolated for extra days.
Gestational age: weeks + days ÷ 7
Sex adjustment: male values use a small upward adjustment, female values use a small downward adjustment, and unknown uses no adjustment.
Log standard deviation: sigma = ln(P90 ÷ P10) ÷ 2.563103
Z score: z = ln(weight ÷ P50) ÷ sigma
Percentile: percentile = normal cumulative probability of z × 100
Classification uses common screening limits. Less than the 3rd percentile is severe small size. From the 3rd to below the 10th percentile is small for gestational age. From the 10th to 90th percentile is appropriate for gestational age. Above the 90th percentile is large for gestational age.
How to Use This Calculator
- Enter completed gestational weeks.
- Enter extra days from zero to six.
- Enter fetal or newborn weight.
- Select the correct weight unit.
- Select baby sex, if known.
- Choose the measurement type.
- Select pregnancy type.
- Press calculate and review the result above the form.
- Download the result as CSV or PDF when needed.
Small for Gestational Age Guide
Understanding Small Size
Small for gestational age describes a baby whose estimated or birth weight is lower than expected for the completed pregnancy age. Many clinicians use the tenth percentile as the common screening line. A result below that line does not always mean disease. Some babies are constitutionally small and healthy. Others may need closer review because growth restriction, placental problems, maternal conditions, or dating errors can influence weight.
Reading Calculator Results
This calculator gives a structured estimate from gestational age, fetal or newborn weight, and sex. It compares the entered weight with reference values inside the tool. The result shows an estimated percentile, a z score, the tenth percentile weight, and the distance from the median. These details help users understand whether the measurement sits far below, near, or above the expected range.
Input Accuracy Matters
Gestational age should be entered carefully. Use completed weeks plus extra days. A seven day mistake can move the percentile noticeably, especially in preterm pregnancy. Weight quality also matters. Ultrasound estimated fetal weight has normal error. Newborn scale readings are usually more direct, but timing, fluid balance, and recording mistakes can still affect interpretation.
Clinical Meaning
The tool marks values below the third percentile as severe small size. Values from the third to below the tenth percentile are marked small for gestational age. Values from the tenth to ninetieth percentile are marked appropriate for gestational age. Values above the ninetieth percentile are marked large for gestational age. These labels are screening labels. They are not a final diagnosis.
Safe Use
Use the output with clinical context. Doppler studies, amniotic fluid, maternal blood pressure, diabetes status, smoking exposure, fetal anatomy, serial growth, and family size patterns may change the meaning of a percentile. A single low value can prompt repeat measurements or specialist review. A falling percentile trend can matter even when the latest value remains above the tenth percentile.
Final Note
This calculator is useful for education, chart review, and quick checks. It should not replace professional medical advice. Pregnant patients should contact a qualified clinician when growth is low, movements reduce, blood pressure rises, bleeding occurs, or symptoms feel concerning. Safe care depends on complete assessment, not one number every time.
FAQs
What does small for gestational age mean?
It usually means the baby weighs below the tenth percentile for the pregnancy age. It is a screening label, not a complete diagnosis.
Is every small baby unhealthy?
No. Some babies are naturally small because of family patterns. Others may need closer review for growth restriction or placental concerns.
What percentile suggests small size?
The tenth percentile is commonly used. Values below the third percentile often suggest more severe small size and need careful clinical review.
Can ultrasound weight be exact?
No. Ultrasound estimated fetal weight has normal error. A clinician may compare scans, Doppler findings, fluid levels, and overall pregnancy risk.
Does sex affect fetal weight?
Yes. Male babies are often slightly heavier on average. This calculator uses a small educational sex adjustment when sex is selected.
Can this tool diagnose growth restriction?
No. Diagnosis requires clinical assessment. Growth trends, Doppler results, maternal health, dating accuracy, and fetal wellbeing must be considered.
Why enter days with weeks?
Fetal weight changes quickly near many pregnancy stages. Extra days make the percentile estimate more precise than using weeks alone.
Should twins use this result?
Twins and higher order multiples may need special growth references. Use this result only as an educational screen and ask a clinician.