Perinatology.com Calculator

Fetal Biometry 5.0

Important: This tool is for clinical decision support and organized reporting. Management outputs are guidance prompts and should always be integrated with the full clinical picture, interval growth, antenatal testing, maternal status, and local practice standards.

Instructions
  1. Start by entering the gestational age in weeks and days.
  2. Select the working panel or group of measurements: Core Biometry/EFW, Doppler/FGR, Head, Thorax, GI/Liver, Renal/Urinary, or Skeletal.
  3. Enter the available measurements. The calculator updates automatically as values are entered or changed; use Update now only as a manual backup.
  4. For core biometry, select the EFW entry method and Hadlock EFW formula before entering BPD, HC, AC, FL, or a direct EFW.
  5. Optional proportionality ratios may be selected with core biometry when HC/AC, FL/AC, AC minus HC, BPD/AC, AC/FL, EFW/AC, or EFW/FL context is desired.
  6. Integrated Results will present entered measurements with z-scores, percentiles, estimated median-equivalent gestational age where supported, and clinical interpretation prompts.
Clinical use: This tool is for clinical decision support and organized reporting. Management outputs are guidance prompts and should always be integrated with the full clinical picture, interval growth, antenatal testing, maternal status, and local practice standards.
Direct panel anchors: EFW/Core Head Jeanty HC Doppler/FGR MCA PI CPR Renal Skeletal

Study Setup

Auto-update on
Results refresh as values are entered or changed. Use Update now only as a manual backup. Use Return to entry to go back to data entry without clearing values.

 

Ultrasound Biometry Threshold Values

Working Panel

Choose whether to calculate EFW from biometry or enter a known EFW directly for FGR synthesis.
Ratios are always calculated when source measurements are present, but are kept in a separate optional section after the primary z-score table.
BPD percentile
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Cephalic index
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HC percentile
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AC percentile
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FL percentile
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Hadlock EFW
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Optional proportionality ratios
HC/AC ratio
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FL/AC ratio
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AC − HC
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BPD/AC ×100
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AC/FL ×100
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EFW/AC
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EFW/FL
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Disproportion screen
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Optional ratio outputs appear after the primary biometry z-score results. They are contextual proportionality flags only.
Short femur / short humerus soft-marker ratios
Expected FL from BPD
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FL / expected FL
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Expected HL from BPD
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HL / expected HL
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Uses BPD-based expected long-bone length ratios for the second-trimester soft-marker screen. Femur uses the Core FL entry. Humerus uses the Skeletal-panel humerus entry. SMFM describes shortened femur as observed/expected FL <0.92 and shortened humerus as observed/expected HL <0.90.
How to measure biometry
  • BPD/OFD/HC: axial head plane at thalami and cavum septi pellucidi.
  • Cephalic index is calculated only when both BPD and OFD are entered: CI = BPD / OFD × 100. It is used as a head-shape quality and interpretation flag; it is not used to diagnose FGR.
  • Derived proportional ratios are calculated when source measurements are available: HC/AC, FL/AC, AC − HC, BPD/AC, AC/FL, EFW/AC, and EFW/FL. These are interpretive flags only and should not be used as standalone delivery recommendations.
  • Short femur/short humerus soft-marker ratios compare observed long-bone length with BPD-based expected length and should be interpreted only after confirming that the finding is isolated by a detailed anatomic survey.
  • AC: true transverse abdominal plane with stomach and portal sinus visible, ribs symmetric.
  • FL: full ossified diaphysis, exclude epiphyses.
  • Direct EFW entry: use when a formal EFW is already available from the scan report.
Interpretation focus: Emphasizes UA PI and end-diastolic flow status for management prompts, while MCA PI and CPR are shown as supportive redistribution markers. Expected mean values are shown in the integrated results table for PI, RI, S/D, MCA PI, and CPR. UA S/D now uses the exact INTERGROWTH-21st LMS-style equation rather than a simple normal approximation.
How to measure Dopplers
  • UA Doppler: sample from a free-floating loop of cord, fetus quiet, avoid venous signal, angle of insonation <30°, use 3 similar waveforms, and average them.
  • MCA Doppler: sample the proximal to mid straight segment of the MCA in an axial head view with minimal angle and fetal rest.
  • Record PI, RI, and S/D from the same high-quality tracing when possible.
Entering HC here displays INTERGROWTH-21st, Hadlock, and, from 20+0 to 42+0 weeks, Jeanty / Chervenak legacy table-anchored z-score results.

Head circumference z-score comparators

HC in the Head panel displays INTERGROWTH-21st fetal head circumference, Hadlock HC, and the Jeanty / Chervenak legacy HC z-score comparator by exact gestational age. This field is separate from the Core-panel HC used for Hadlock 4 EFW, which keeps head assessment and EFW workflow clearly separated while preserving the older Biometry 3.1 HC_Jeanty pathway.

INTERGROWTH HC z-score
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Hadlock HC z-score
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Hadlock HC percentile
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Jeanty HC z-score
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Jeanty HC percentile
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Jeanty expected HC
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Hadlock is shown as a familiar legacy growth comparator; Jeanty / Chervenak is retained for continuity with Biometry 3.1 and older microcephaly workflows. The z-score is anchored to the Chervenak Table III SD bands from 20+0 to 42+0 weeks; use with clinical context and modern HC standards.
Enter Head-panel HC to display INTERGROWTH-21st, Hadlock, and Jeanty / Chervenak legacy HC results.
How to measure head and face
  • HC/BPD/OFD: standard axial plane at the thalami and cavum septi pellucidi.
  • TCD: posterior fossa plane with cerebellar hemispheres and cisterna magna visualized; place calipers outer-to-outer.
  • Orbital Diameter (OD) maximum internal diameter of each orbit inner orbital wall to inner orbital wall. Inner Orbital Distance (IOD), inner edge of one orbit to inner edge of the opposite orbit.Outer Orbital Distance (OOD , )binocular distance (BOD) outer edge of one orbit to outer edge of the opposite orbit. IMAGE. In Axial orbital plane with symmetric lenses.
  • Mandible: true sagittal/oblique profile showing the full bony mandible; Otto reference values use mandible length in centimeters.
How to measure thorax
  • Thoracic circumference: transverse chest plane at the four-chamber heart view.IMAGE.
  • Thoracic length: sagittal chest view from thoracic inlet/superior sternum to diaphragm.
  • Clavicle/scapula: obtain the full ossified bone in a plane as close as possible to perpendicular to the beam.
Stomach module: dimensional percentiles use the Goldstein nomogram; AP-to-AC proportionality is cross-checked against the Kepkep constant (~1/3); gastric area and gastric area-to-abdominal area ratio provide the higher-value obstruction screen described by Sase.
How to measure GI and liver
  • Stomach: measure the largest visible stomach including the pyloric region; record maximal longitudinal dimension and the AP/transverse diameters at the center of the gastric corpus.
  • Interpret stomach size with caution because filling and emptying cycles can change the apparent size over short intervals; repeat assessment is helpful when clinical concern persists.
  • Small bowel/colon: measure the largest lumen short-axis inner-to-inner in a coronal abdominal plane.IMAGE.
  • Liver: right lobe length from superior dome to inferior edge in coronal image of abdomen.
How to measure renal and urinary tract
  • Kidney length: true longitudinal kidney view, avoid including adrenal gland.
  • Kidney AP and transverse: use the transverse kidney section where the renal pelvis is best visualized; measure the kidney outer-to-outer.
  • Bladder longitudinal diameter: measure in a true midsagittal plane from the inner bladder dome to the inner bladder neck (largest longitudinal diameter).
Optional skeletal-panel femur entry. If left blank, the core FL entry is used.
Also used for the short-humerus observed/expected ratio.
How to measure skeletal parameters
  • Measure each long bone in a plane where the full ossified diaphysis is visualized and as close as possible to a right angle to the ultrasound beam.
  • Do not include epiphyses or adjacent shadows.
  • Foot length is heel to tip of the longest toe in plantar view.

Integrated Results

Enter values; results update automatically.
Optional z-score graphs
Enter measurements to render the optional graph.
Graph scale −4 to +4; extreme values are capped visually only.

FGR / Delivery Planning auto-updating

Enter values; FGR and delivery-planning prompts update automatically.

Clinical synthesis

An integrated clinical synthesis will appear here as values are entered.

Color flags

Color-coded flags will appear here as values are entered.

Suggested report text

A concise integrated report will appear here as values are entered.

EMR-ready one-line impression

A compact one-line impression will appear here as values are entered.

Equations and References

Equations used for calculator outputs

Notation: GA is gestational age in completed weeks plus days/7. Linear measurements may be entered in millimeters or centimeters using the unit toggle near the top of the calculator. The calculator converts entered values internally to the reference unit required by each equation. Hadlock EFW equations use BPD, HC, AC, and FL in centimeters. Percentiles are derived from z-scores using the standard normal distribution.

General statistical calculations

OutputEquation usedNotes
Normal z-scorez = (observed - mean) / SDUsed when the reference provides a mean and SD or when SD is derived from a published prediction interval.
Percentilepercentile = 100 × Phi(z)Phi is the standard normal cumulative distribution function.
LMS z-scorez = ((observed / M)^L - 1) / (L × S)
if L = 0: z = ln(observed / M) / S		Used for LMS-style references when L, M, and S are available.
Table interpolationinterpolated value = y0 + ((GA - x0) × (y1 - y0)) / (x1 - x0)Used between gestational-age rows for tabular references.
SD from 95% prediction intervalSD = (upper - lower) / 3.92Used when the published table gives approximately mean ± 1.96 SD.

Core Hadlock biometry equations

The Hadlock parameter equations below generate the expected mean in centimeters. The calculator converts the mean and SD to millimeters for comparison with entered measurements.

ParameterMean equationSD used
BPDmean cm = -3.08 + 0.41 × GA - 0.000061 × GA^30.30 cm
HCmean cm = -11.48 + 1.56 × GA - 0.0002548 × GA^31.00 cm
ACmean cm = -13.3 + 1.61 × GA - 0.00998 × GA^21.34 cm
FLmean cm = -3.91 + 0.427 × GA - 0.0034 × GA^20.30 cm

Hadlock estimated fetal weight

EFW is returned in grams as 10^(log10 EFW). BPD, HC, AC, and FL are in centimeters in these equations.

Selected formulaEquation
Hadlock 4: BPD + HC + AC + FLlog10 EFW = 1.3596 - 0.00386 × AC × FL + 0.0064 × HC + 0.00061 × BPD × AC + 0.0424 × AC + 0.174 × FL
HC + AC + FLlog10 EFW = 1.326 - 0.00326 × AC × FL + 0.0107 × HC + 0.0438 × AC + 0.158 × FL
BPD + AC + FLlog10 EFW = 1.335 - 0.0034 × AC × FL + 0.0316 × BPD + 0.0457 × AC + 0.1623 × FL
EFW percentilez = (ln(EFW) - ln(interpolated Hadlock p50 weight)) / 0.127

Derived head-shape and proportionality ratios

OutputEquationInterpretive threshold
Cephalic indexCI = BPD / OFD × 100Low if <70; high if >86.
HC/ACHC / ACAC-dominant contextual flag if ≤0.952.
FL/ACFL / ACLow contextual flag if <0.18; marked skeletal-disproportion flag if <0.16 in the appropriate clinical setting.
AC - HCAC - HCContextual flag if AC exceeds HC by ≥35 mm; stronger caution if ≥50 mm.
BPD/AC × 100BPD / AC × 100Borderline contextual flag if ≤27.5; stronger flag if ≤26.8.
AC/FL × 100AC / FL × 100Borderline contextual flag if ≥466; stronger flag if ≥476.
EFW/ACEFW / ACSupportive mass-to-abdomen ratio only.
EFW/FLEFW / FLSupportive mass-to-femur ratio only.

Short femur and short humerus soft-marker ratios

OutputEquationThreshold used
Expected FL from BPDexpected FL = -9.3105 + 0.9028 × BPDObserved/expected FL <0.92 is flagged as shortened femur.
Expected HL from BPDexpected HL = -7.9404 + 0.8492 × BPDObserved/expected HL <0.90 is flagged as shortened humerus.

INTERGROWTH-21st head circumference, Jeanty / Chervenak HC comparator, and umbilical artery Doppler

OutputEquation usedNotes
Head-panel HC medianmedian = -28.2849 + 1.69267 × GA^2 - 0.397485 × GA^2 × ln(GA)Used for Head-panel HC only, not for Hadlock EFW.
Head-panel HC SDSD = 1.98735 + 0.0136772 × GA^3 - 0.00726264 × GA^3 × ln(GA) + 0.000976253 × GA^3 × ln(GA)^2z = (HC - median) / SD.
Jeanty / Chervenak legacy HC z-scoreTable III anchor interpolation
z is interpolated between the Chervenak/Jeanty head-perimeter curves at −5, −4, −3, −2, −1, mean, +1, and +2 SD
GA range: 20+0 to 42+0 weeks; HC in millimeters		Retained for continuity with the older Biometry 3.1 HC_Jeanty pathway, but corrected to match the uploaded Chervenak Table III SD bands. This avoids overcalling negative z-scores from an incorrect small SD formula. Use as a legacy comparator rather than the sole modern HC standard.
UA PIL = -0.0768617
M = 1.02944 + 77.7456 × GA^-2 - 0.000004455 × GA^3
S = -0.00645693 + 254.885 × ln(GA) × GA^-2 - 715.949 × GA^-2
z = (1 / L) × (exp((observed - M) × L / S) - 1)		INTERGROWTH-21st LMS-style equation implemented for UA PI.
UA S/DL = -0.2752483
M = 2.60358 + 445.991 × GA^-2 - 0.0000108754 × GA^3
S = -0.503202 + 1268.37 × ln(GA) × GA^-2 - 3417.37 × GA^-2
z = (1 / L) × (exp((observed - M) × L / S) - 1)		INTERGROWTH-21st LMS-style equation implemented for UA S/D.
UA RImean = interpolated p50
SD = ((p90 - p10) / 2) / 1.2815515655		Uses INTERGROWTH-style tabular RI centiles with interpolation.
MCA PI and CPRz = (observed - interpolated mean) / interpolated SDUses Baschat and Gembruch gestational-age tables. CPR is calculated as MCA PI / UA PI unless an override is entered.

Organ-specific and skeletal-panel calculations

PanelCalculation approach
TCDPinar table interpolation. SD = (upper - lower) / 4, because the table is treated as approximately mean ± 2 SD.
Orbital measurements, mandible, thorax, liver, bowel, bladder, scapulaGestational-age table interpolation. When percentile anchors are provided, the calculator derives an SD from the published interval and then calculates z-score and percentile.
Stomach dimensionsGoldstein table interpolation with SD = half-range / 2, because the table is treated as mean ± 2 SD.
Renal lengthmean ln = 5.4141 × ln(GA/10) - 5.0562 × sqrt(GA/10) + 6.3939; SD ln = 0.11650; z = (ln(observed) - mean ln) / SD ln.
Renal AP diametermean ln = 5.0861 × ln(GA/10) - 4.8576 × sqrt(GA/10) + 5.9114; SD ln = -0.0018793 × GA + 0.21053.
Renal transverse diametermean ln = 4.5320 × ln(GA/10) - 4.0868 × sqrt(GA/10) + 5.1504; SD ln = -0.0019741 × GA + 0.21256.
Skeletal-panel FLmean = 3.4162 × GA - 0.0004791 × GA^3 - 32.425; SD = 0.058328 × GA + 1.0605.
Humerusmean = 11.459 × GA - 2.2362 × GA × ln(GA) - 63.704; SD = 0.040292 × GA + 1.3464.
Radiusmean = 7983 / GA^2 - 1698.6 / GA + 91.634; SD = 0.046386 × GA + 1.1933.
Ulnamean = 11120 / GA^2 - 2146.3 / GA + 108.94; SD = 0.049218 × GA + 1.2021.
Tibiamean = 14451 / GA^2 - 2553.2 / GA + 120.05; SD = 0.049978 × GA + 1.1102.
Fibulamean = 13697 / GA^2 - 2458.0 / GA + 116.51; SD = 0.053841 × GA + 1.0451.
Foot lengthmean = 0.36909 × GA^2 - 0.084175 × GA^2 × ln(GA) - 14.158; SD = 0.10865 × GA + 0.27971.

Important: These equations document the implementation used by this calculator. For organ-specific measurements based primarily on tabular references, the displayed equations describe the interpolation and z-score method rather than replacing the original published nomograms.

Detailed references, PMIDs, and source mapping

Current source mapping used

Calculator outputCurrent source familyHow it is used
BPD percentile / z-score / median-equivalent GAHadlock 1984Uses the published parameter equation and SD for gestational-age z-score support. Median-equivalent GA is displayed as a descriptive 50th-percentile equivalent and should not be used for redating.
Cephalic indexDerived head-shape ratioCalculated as BPD/OFD × 100 when both entries are available; interpreted using the commonly used fetal range approximately 70–86%, with low values flagged as dolichocephaly pattern and high values as brachycephaly pattern.
HC/AC, FL/AC, AC − HC, BPD/AC, AC/FL, EFW/AC, and EFW/FLDerived proportionality ratiosCalculated from entered core biometry and/or EFW. HC/AC and AC − HC describe head-abdomen proportionality; FL/AC describes femur-abdomen proportionality; BPD/AC and AC/FL are shown as shoulder-dystocia contextual ratios; EFW/AC and EFW/FL describe mass-to-abdomen and mass-to-femur proportionality. These are contextual flags and are not used as standalone diagnoses or delivery triggers.
HC percentile / z-score / median-equivalent GAHadlock 1984Uses the published parameter equation and SD for gestational-age z-score support. Median-equivalent GA is displayed as a descriptive 50th-percentile equivalent and should not be used for redating.
AC percentile / z-score / median-equivalent GAHadlock 1984Uses the published parameter equation and SD for gestational-age z-score support. Median-equivalent GA is displayed as a descriptive 50th-percentile equivalent and should not be used for redating.
FL percentile / z-score / median-equivalent GAHadlock 1984Uses the published parameter equation and SD for gestational-age z-score support. Median-equivalent GA is displayed as a descriptive 50th-percentile equivalent and should not be used for redating.
Hadlock EFW percentile / median-equivalent GAHadlock 1985 / 1991Calculated from biometry using the selected Hadlock formula and compared with the published in utero weight standard. Median-equivalent GA is displayed as a descriptive weight-age equivalent, not as a redating recommendation.
Umbilical artery PIINTERGROWTH-21st / Drukker 2020Used for gestational-age centile and z-score support and integrated FGR prompts. UA S/D uses the exact published INTERGROWTH-21st equation for skewness, mean, and coefficient of variation.
MCA PI / CPRBaschat & Gembruch 2003Used as supportive redistribution markers rather than primary delivery triggers.
Head panel HC percentile / z-scoreINTERGROWTH-21st fetal standardUses the exact published official equations for HC median and SD by exact gestational age; kept separate from the Hadlock core panel.
Jeanty / Chervenak legacy HC z-scoreChervenak 1984 Table III / Jeanty fetal head biometryUses table-anchored interpolation of head perimeter SD bands from 20 to 42 weeks to preserve continuity with older protocols while matching the uploaded source table. Displayed as a legacy comparator, not as the sole modern HC standard.
Head / thorax / GI / renal / skeletal panelsMixed organ-specific referencesQuantitative z-score and percentile outputs are interpolation-supported organ-specific estimates for panel use; thoracic circumference and thoracic length now use Chitkara table-matched percentile interpolation with an estimated z-score anchored to the table centiles; bladder longitudinal diameter now uses the Fontanella 2021 largest-LBD nomogram (15-35 weeks) with table-interpolated 5th/50th/95th centiles and contextual LUTO interpretation. TCD now uses the Pinar 2002 gestational-age mean and ±2 SD table. Kidney AP, transverse, and length dimensions are reported side-specifically using the Chitty 2003 framework.

Core biometry and weight references

Hadlock FP, Deter RL, Harrist RB, Park SK. Estimating fetal age: computer-assisted analysis of multiple fetal growth parameters. Radiology. 1984;152:497-501. PubMed: PMID: 6739822. This is the source for the BPD, HC, AC, and FL gestational-age equations and SD values used for the current individual Hadlock parameter z-scores.

Hadlock FP, Harrist RB, Sharman RS, Deter RL, Park SK. Estimation of fetal weight with the use of head, body, and femur measurements-a prospective study. Am J Obstet Gynecol. 1985;151:333-337. PubMed: PMID 3881966. This is the classic Hadlock EFW formula paper underlying the HC+AC+FL and BPD+AC+FL pathways.

Hadlock FP, Harrist RB, Martinez-Poyer J. In utero analysis of fetal growth: a sonographic weight standard. Radiology. 1991;181:129-133. PubMed: PMID 1887021. This is the gestational-age weight standard used to contextualize EFW percentiles.

Comparator standards and alternate frameworks

INTERGROWTH-21st fetal growth standard. The earlier builds referenced INTERGROWTH-21st style head support. In the current repaired version, the active core BPD/HC/AC/FL percentiles are Hadlock-based because you specifically asked for Hadlock 4 alignment. If you want, I can add a visible framework toggle so users can switch between Hadlock core parameter percentiles and INTERGROWTH-21st comparator support.

Papageorghiou AT, Ohuma EO, Altman DG, et al. International standards for fetal growth based on serial ultrasound measurements: the Fetal Growth Longitudinal Study of the INTERGROWTH-21st Project. Lancet. 2014;384:869-879. PubMed: PMID 25209488.

Jeanty P, Cousaert E, Hobbins JC, Tack B, Bracken M, Cantraine F. A longitudinal study of fetal head biometry. Am J Perinatol. 1984 Jan;1(2):118-128. This is the underlying longitudinal fetal head biometry reference.

Chervenak FA, Jeanty P, Cantraine F, et al. The diagnosis of fetal microcephaly. Am J Obstet Gynecol. 1984;149:512-517. The legacy Jeanty / Chervenak HC z-score anchored to the published Table III head-perimeter SD bands from 20 to 42 weeks.

Doppler references

Drukker L, Staines-Urias E, Villar J, et al. International gestational age-specific centiles for umbilical artery Doppler indices: a longitudinal prospective cohort study of the INTERGROWTH-21st Project. Am J Obstet Gynecol. 2020;222:602.e1-15. PubMed: PMID: 31954701. This is the current backbone for UA PI gestational-age centiles and z-scores.

Acharya G, Wilsgaard T, Berntsen GK, Maltau JM, Kiserud T. Reference ranges for serial measurements of umbilical artery Doppler indices in the second half of pregnancy. Am J Obstet Gynecol. 2005;192:937-944. PubMed: PMID: 15746695.

Baschat AA, Gembruch U. The cerebroplacental Doppler ratio revisited. Ultrasound Obstet Gynecol. 2003;21:124-127. PubMed: PMID: 12601831.

Mari G, Deter RL, Carpenter RL, et al. Noninvasive diagnosis by Doppler ultrasonography of fetal anemia due to maternal red-cell alloimmunization. N Engl J Med. 2000;342:9-14. PubMed: PMID: 10620643.

FGR definitions and management references

Society for Maternal-Fetal Medicine (SMFM) Consult Series #52. Diagnosis and management of fetal growth restriction. Am J Obstet Gynecol. 2020. PubMed: PMID 32407785.

ACOG Practice Bulletin No. 227. Fetal Growth Restriction. 2021. PubMed: PMID: 33481528.

Head and facial biometry references

Chitty LS, Altman DG. Charts of fetal size: head measurements. BJOG. 1994. PubMed: PMID 8297866.

Merz E, Welter C, Sillek U, Pittmann P, Bahlmann F, Weber G. Orbital diameter, inner orbital distance and outer orbital distance: a growth model for fetal orbital parameters. Ultraschall in Med. 1995;16:12-17. This calculator uses the full Merz table-based 5th/50th/95th centiles with interpolation for inner and outer orbital distances.

Pinar H, Burke SH, Huang CW, Singer DB, Sung CJ. Reference values for transverse cerebellar diameter throughout gestation. Pediatr Dev Pathol. 2002 Sep-Oct;5(5):489-94. . PMID: 12192430..

Otto C, Platt LD. The fetal mandible measurement: an objective determination of fetal jaw size. Ultrasound Obstet Gynecol. 1991. PubMed: PMID 12797096. In this calculator, Otto mandible assessment is based on the gestational-age table of mean and 95% prediction limits, with interpolation between gestational weeks.

Thoracic references

Chitkara U, Rosenberg J, Chervenak FA, et al. Prenatal sonographic assessment of the fetal thorax: normal values. Am J Obstet Gynecol. 1987. PubMed: PMID 3555087.

Yarkoni S, Schmidt W, Jeanty P, et al. Clavicular measurement: a new biometric parameter for fetal evaluation. J Ultrasound Med. 1985. PubMed: PMID 3903198.

Sherer DM, Plessinger MA, Allen TA. Fetal scapular length in the ultrasonographic assessment of gestational age. J Ultrasound Med. 1994. PubMed: PMID 7933014.

GI, liver, renal, and skeletal references

Goldstein I, Reece EA, Yarkoni S, Wan M, Green JLJ, Hobbins JC. Growth of the fetal stomach in normal pregnancies. Obstet Gynecol. 1987;70:641-644.PMID: 3306513.

Kepkep K, Tuncay YA, Goynumer G, Yetim G. Nomogram of the fetal gastric size development in normal pregnancy. J Perinat Med. 2005;33:336-339.PMID: 16207120.

Lap CCM, Voskuilen CS, Pistorius LR, et al. Reference curves for the normal fetal small bowel and colon diameters; their usefulness in fetuses with suspected dilated bowel. J Matern Fetal Neonatal Med. 2020. PubMed: PMID 29985072.

Fontanella F, Groen H, Duin LK, et al. Z-scores of fetal bladder size for antenatal differential diagnosis between posterior urethral valves and urethral atresia. Ultrasound Obstet Gynecol. 2021;58:875-881.PMID: 33864313.

Chitty LS, Altman DG. Charts of fetal size: kidney and renal pelvis measurements. Prenat Diagn. 2003. PubMed: PMID 14634973.

Brennan S, Kandasamy Y, Rudd D, Schneider M, Watson D. Fetal kidney charts of a novel measurement of the renal parenchymal thickness to evaluate fetal kidney growth and potential function. Prenat Diagn. 2020. PubMed: PMID: 32277493.

van Vuuren SH, Damen-Elias HAM, Stigter RH, et al. Size and volume charts of fetal kidney, renal pelvis and adrenal gland. Ultrasound Obstet Gynecol. 2012. PubMed: PMID 22581671.

Chitty LS, Altman DG. Charts of fetal size: limb bones. BJOG. 2002. PubMed: PMID 12197373.

Short femur / short humerus soft-marker references

Nyberg DA, Resta RG, Luthy DA, Hickok DE, Williams MA. Humerus and femur length shortening in the detection of Down's syndrome. Am J Obstet Gynecol. 1993;168:534-538. PubMed: PMID 8438923. This calculator uses BPD-based expected long-bone lengths to generate observed/expected ratios for short femur and short humerus soft-marker screening.

Prabhu M, Kuller JA, Biggio JR; Society for Maternal-Fetal Medicine. SMFM Consult Series #57: Evaluation and management of isolated soft ultrasound markers for aneuploidy in the second trimester. Am J Obstet Gynecol. 2021;225:B2-B15. PubMed: PMID 34171388. SMFM describes shortened humerus as observed/expected humeral length <0.90 and shortened femur as observed/expected femoral length <0.92, and recommends third-trimester ultrasound reassessment and growth evaluation when isolated.

Derived proportionality ratio references

Ulusoy CO, Kurt A, Yıldız AG, et al. Machine Learning-Based Prediction of Shoulder Dystocia in Pregnancies Without Suspected Macrosomia Using Fetal Biometric Ratios. J Clin Med. 2025;14:5240. This paper supports display of fetal biometric ratios as contextual shoulder-dystocia risk features, especially BPD/AC and AC/FL, while emphasizing that prediction remains imperfect. It also reports EFW/AC and EFW/FL as significant sonographic ratio predictors, but these should remain supportive context rather than deterministic delivery recommendations.

Gerber et al. reported that an AC/HC ratio ≥1.05 had limited prediction performance for shoulder dystocia; this calculator therefore treats an equivalent HC/AC ≤0.952 as a contextual AC-dominant flag rather than a deterministic diagnosis.

Ramus RM, Martin LB, Twickler DM. Ultrasonographic prediction of fetal outcome in suspected skeletal dysplasias with use of the femur length/abdominal circumference ratio. Am J Obstet Gynecol. 1998;179:1348-1352. This is the basis for the marked FL/AC <0.16 skeletal-disproportion flag in the appropriate clinical setting.

AC − HC difference: AC exceeding HC by ≥50 mm is treated as an AC-dominant shoulder-dystocia context flag. The tool displays this as a cautionary prompt only because positive predictive value is limited and management must consider diabetes, EFW, prior shoulder dystocia, pelvic/labor factors, and local practice.

Implementation notes

Percentile and z-score: where a paper provides a clean mean/SD or LMS structure, the calculator can use direct z-score calculation; where a paper is better suited as a normative chart or organ-specific contextual anchor, the calculator uses interpolation-supported contextual interpretation rather than forcing an unsupported regression.