Results

Calculated 12:04:46 PM

Gestational age

20+0

First-trimester CRL

Worst long-bone Z

—

No long bones

Lethality markers

0/3

0/3 evaluable

Top pattern

No flag

Thanatophoric dysplasia

FL/AC

—

Need femur + AC

TC/AC

—

Need TC + AC

Long bones < -2 SD

0 below -4 SD of 0 measured

Femur standard

Chitty

Chitty & Altman 2002

Dashboard

Differential

ISDS Guidelines

Survey checklist

Missing views

Text outputs

References & assumptions

Lethality / pulmonary-hypoplasia screen

Lower concern from entered data. No validated lethality marker met from entered data. This does not exclude skeletal dysplasia or pulmonary compromise, especially if thoracic and proportionality measurements are missing. Note: only 0/3 lethality markers were evaluable from entered data.

Validated marker	Evaluable	Result	Reported performance
FL/AC < 0.16	—	not evaluable	sens ~84% / spec ~57%
TC/AC < 0.60	—	not evaluable	sens ~10% / spec ~100%
TC < 5th centile	—	not evaluable	sens ~59% / spec ~81%

Concordance PPV (Crane 2024): 1 marker ~50%, 2 markers ~78%, 3 markers ~100% for a life-limiting outcome.

Biometry and skeletal measurements

No measurements entered yet.

Pattern analysis

Pattern labels

no specific skeletal dysplasia pattern identified from entered data

Segment Z-score averages
Proximal (femur, humerus): — Middle (radius/ulna, tibia/fibula): — Distal (foot): — Mean long bones: —

Segment classification flags a dominant segment when its mean Z is ≥1.5 SD more negative than the comparator.

Ratio	Value	Comment
FL/AC	—	Need femur + AC
TC/AC	—	Need TC + AC
Femur/foot	—	Need femur + foot
Femur/humerus	—	Need femur + humerus

Ranked differential is not a diagnosis. Scores are rule-based feature matches against the 2023 Nosology backbone. Prenatal 2D ultrasound accuracy for the specific entity is ~40–60%; definitive diagnosis requires genetic testing and expert phenotype correlation. Suggested ICD-10 codes are documentation aids only.

Rank	Condition	ISDS Nosology	Candidate genes	ICD-10*	Lethality	Why it appears
1 score 1	Thanatophoric dysplasia	Group 1: FGFR3 chondrodysplasias	FGFR3	Q77.1	Usually perinatal lethal	severe rhizomelia/micromelia; narrow thorax; short ribs
2 score 0	Achondroplasia / hypochondroplasia	Group 1: FGFR3 chondrodysplasias	FGFR3	Q77.4	Non-lethal (heterozygous)	rhizomelia; macrocephaly; frontal bossing
3 score 0	Homozygous achondroplasia	Group 1: FGFR3 chondrodysplasias	FGFR3	Q77.4	Usually lethal	both parents affected; severe limb shortening; small thorax
4 score 0	Osteogenesis imperfecta type II (lethal)	Group 8: Osteogenesis imperfecta and decreased bone density	COL1A1, COL1A2, CRTAP, P3H1, IFITM5	Q78.0	Type II perinatal lethal	multiple fractures; beaded/fractured ribs; poor mineralization
5 score 0	Achondrogenesis (1A/1B/2)	Group 2 & 4: Type 2 collagen & Sulfation	TRIP11, SLC26A2, COL2A1	Q77.0	Usually lethal	extreme micromelia; very poor spine/skull ossification; short trunk
6 score 0	Hypophosphatasia (perinatal severe)	Group 8: Osteogenesis imperfecta and decreased bone density	ALPL	E83.39	Perinatal form often lethal	markedly reduced mineralization; thin/bowed bones; fractures

*Suggested ICD-10 for the suspected entity; the billable maternal encounter is generally an O35.- "maternal care for known or suspected fetal abnormality" code. Reconcile with your coding standard.

Genetic strategy: If the phenotype is suspicious for skeletal dysplasia, do not rely on cfDNA aneuploidy screening as the diagnostic test. Use diagnostic fetal sampling with chromosomal microarray plus rapid trio exome (pooled incremental yield ~69% after normal karyotype/CMA; Tse 2023), or a broad skeletal-dysplasia panel with CNV calling. For an isolated achondroplasia/thanatophoric phenotype, targeted cfDNA FGFR3 testing can non-invasively distinguish lethal thanatophoric from non-lethal achondroplasia. Store fetal DNA for reanalysis.

1/17 survey items documented (Krakow/ACMG 2009 standardized fetal skeletal survey).

#	Item	Status
1	Gestational age confirmed (first-trimester US / reliable dating)	documented
2	All long bones measured (femur, humerus, radius, ulna, tibia, fibula)	pending
3	Clavicle measured	pending
4	Long-bone shape (straight/curved, uni- vs bilateral) documented	pending
5	Metaphyseal/epiphyseal ends documented	pending
6	Echodensity / mineralization documented	pending
7	Foot length / shape documented	pending
8	Hands (digit number, shape, mineralization) documented	pending
9	Circumferences measured (HC, AC, chest/TC)	pending
10	Lateral view of chest / thoracic shape documented	pending
11	Ribs assessed (length, fractures, beading)	pending
12	Cranium mineralization & shape documented	pending
13	Vertebral mineralization & shape documented	pending
14	Mandible / facial profile (bossing, nasal bone, micrognathia)	pending
15	Limb posturing / contractures assessed	pending
16	Amniotic fluid volume documented	pending
17	Other anomalies surveyed (cardiac, renal, CNS, hydrops, genitalia)	pending

ISDS Nosology & Literature

The differential diagnosis in this tool is mapped to the Nosology of Genetic Skeletal Disorders: 2023 revision (American Journal of Medical Genetics A, 2023).

2023 Nosology Guidelines
Group 1: FGFR3 chondrodysplasias (Achondroplasia, Thanatophoric Dysplasia)
Group 2 & 11: Type 2 / 11 collagen disorders (Achondrogenesis, Stickler)
Group 4: Sulfation disorders (Diastrophic dysplasia)
Group 8: Osteogenesis imperfecta and decreased bone density
Group 10: Ciliopathies with major skeletal involvement (Short-rib thoracic dysplasias)
Group 20: Chondrodysplasia punctata (CDP) group

20 missing or incomplete items. Complete these before final counseling if technically possible.

#	Action
1	AC is missing: required for FL/AC and proportionality.
2	Thoracic circumference is missing: required for TC/AC and TC-centile (two of three lethality markers).
3	Femur: measure both left and right if technically possible.
4	Humerus: measure both left and right if technically possible.
5	Radius: measure both left and right if technically possible.
6	Ulna: measure both left and right if technically possible.
7	Tibia: measure both left and right if technically possible.
8	Fibula: measure both left and right if technically possible.
9	Foot: measure both left and right if technically possible.
10	Thoracic shape not documented (normal/narrow/bell-shaped/very small).
11	Rib length/shape not documented (short, beaded, fractured, normal).
12	Mineralization not documented (skull compressibility, vertebral ossification, long-bone echogenicity). Abnormal skull yields the highest exome diagnostic rate.
13	Fractures not documented (long bones, ribs, skull contour).
14	Long-bone shape not documented (straight, bowed, telephone-receiver, angulated).
15	Hands not documented (polydactyly, trident, hitchhiker thumb, syndactyly, contractures).
16	Feet not documented (clubfeet, rocker-bottom, ray anomalies).
17	Spine/pelvis not documented (platyspondyly, segmentation anomaly, iliac wings, acetabulum).
18	Skull not documented (macrocephaly, cloverleaf, craniosynostosis, compressibility).
19	Face not documented (bossing, midface hypoplasia, micrognathia, cleft, nasal hypoplasia).
20	Associated anomalies not selected (cardiac, renal, CNS, hydrops, genital ambiguity, FGR).

Expert EMR note

Targeted skeletal survey performed at 20+0 weeks. Dating basis: First-trimester CRL. Reference standard: Chitty & Altman 2002 long bones; thoracic circumference Chitkara 1987. Biometry/skeletal measurements: No measured skeletal parameter below -2 SD from entered data. FL/AC not calculated (normal ~0.20-0.24; lethal marker < 0.16); TC/AC not calculated (normal ~0.89; lethal marker < 0.60); femur/foot not calculated (normal ~1.0). Worst long-bone Z-score: —. Pattern assessment: no specific skeletal dysplasia pattern identified from entered data. Thorax: Unknown / not documented. Ribs: Unknown / not documented. Mineralization: Unknown / not documented. Fractures: Unknown / not documented. Long-bone shape: Unknown / not documented. Hands: Unknown / not documented. Feet: Unknown / not documented. Spine/pelvis: Unknown / not documented. Associated anomalies: none selected / not documented. Lethality / pulmonary-hypoplasia screen (validated triad FL/AC<0.16, TC/AC<0.60, TC<5th centile): Lower concern from entered data. Validated markers present: 0/3 (none); 0/3 evaluable. Differential diagnosis suggested by the pattern (not definitive; prenatal US accuracy ~40-60%): 1. Thanatophoric dysplasia (FGFR3; ICD-10 Q77.1; Usually perinatal lethal) 2. Achondroplasia / hypochondroplasia (FGFR3; ICD-10 Q77.4; Non-lethal (heterozygous)) 3. Homozygous achondroplasia (FGFR3; ICD-10 Q77.4; Usually lethal) 4. Osteogenesis imperfecta type II (lethal) (COL1A1, COL1A2, CRTAP, P3H1, IFITM5; ICD-10 Q78.0; Type II perinatal lethal) 5. Achondrogenesis (1A/1B/2) (TRIP11, SLC26A2, COL2A1; ICD-10 Q77.0; Usually lethal) Recommendation: Decision-support only; not diagnostic by ultrasound alone. Recommend completion of an expert skeletal survey, genetic counseling, diagnostic fetal testing if not already performed, and multidisciplinary planning per severity and parental wishes. If skeletal dysplasia remains suspected: chromosomal microarray plus rapid trio exome sequencing (or a broad skeletal-dysplasia panel with CNV calling), retain fetal DNA for reanalysis, and confirm postnatally with radiographs/autopsy.

Patient explanation

At 20+0 weeks, the ultrasound measurements and bone pattern do not show a strong bone-shortening pattern from the information entered, but the assessment depends on completing all key views. The most important question is whether the baby's chest is small enough to affect lung development. The entered findings do not currently meet the warning thresholds for a small chest, but missing chest, rib, or mineralization information can change this. Ultrasound can suggest a pattern, but often cannot name the exact condition. Genetic testing from amniocentesis or CVS is usually the best way to identify the cause, estimate prognosis, and understand the chance of recurrence in a future pregnancy. The current possibilities being considered include: 1. Thanatophoric dysplasia 2. Achondroplasia / hypochondroplasia 3. Homozygous achondroplasia 4. Osteogenesis imperfecta type II (lethal) 5. Achondrogenesis (1A/1B/2) This should be discussed with maternal-fetal medicine, clinical genetics, neonatology, and pediatric specialists if the pregnancy continues.

Genetic testing request

Indication: suspected / possible fetal skeletal dysplasia on fetal skeletal survey at 20+0 weeks. Key ultrasound phenotype: - Pattern: no specific skeletal dysplasia pattern identified from entered data - Shortened measurements: No measured skeletal parameter below -2 SD from entered data. - Ratios: FL/AC not calculated (normal ~0.20-0.24; lethal marker < 0.16); TC/AC not calculated (normal ~0.89; lethal marker < 0.60); femur/foot not calculated (normal ~1.0). - Lethality markers: 0/3 (none) - Thorax/ribs: Unknown / not documented; Unknown / not documented - Mineralization/fractures: Unknown / not documented; Unknown / not documented - Bowing/metaphyses: Unknown / not documented; Unknown / not documented - Skull/face: Unknown / not documented; Unknown / not documented - Hands/feet/spine-pelvis: Unknown / not documented; Unknown / not documented; Unknown / not documented - Associated anomalies: none selected / not documented - Family history / exposure: None known; None known Requested testing (subject to local lab pathway): 1. Diagnostic fetal DNA (amniocentesis/CVS), not cfDNA screening, where a diagnosis is required. 2. QF-PCR / karyotype if locally required for rapid aneuploidy or culture workflow. 3. Chromosomal microarray (captures CNVs, e.g. 17q24/SOX9 in campomelic dysplasia). 4. Rapid trio exome sequencing preferred when the phenotype is broad or severe (pooled incremental yield ~69% after normal karyotype/CMA; Tse 2023); alternatively a comprehensive skeletal-dysplasia panel with CNV calling if faster locally. 5. For an isolated achondroplasia/thanatophoric phenotype, targeted cfDNA FGFR3 testing can non-invasively distinguish lethal thanatophoric from non-lethal achondroplasia. 6. Store fetal DNA and parental samples for reanalysis/segregation if first-line testing is negative. Top ultrasound-based differential to guide variant interpretation: 1. Thanatophoric dysplasia (FGFR3; ICD-10 Q77.1; Usually perinatal lethal) 2. Achondroplasia / hypochondroplasia (FGFR3; ICD-10 Q77.4; Non-lethal (heterozygous)) 3. Homozygous achondroplasia (FGFR3; ICD-10 Q77.4; Usually lethal) 4. Osteogenesis imperfecta type II (lethal) (COL1A1, COL1A2, CRTAP, P3H1, IFITM5; ICD-10 Q78.0; Type II perinatal lethal) 5. Achondrogenesis (1A/1B/2) (TRIP11, SLC26A2, COL2A1; ICD-10 Q77.0; Usually lethal)

Delivery / perinatal plan

Perinatal planning at 20+0 weeks: Current lethality screen: Lower concern from entered data (0/3 validated markers). If the pregnancy continues and skeletal dysplasia remains suspected: - Refer / manage in a tertiary fetal-medicine center. - Arrange genetic counseling and diagnostic testing if not already performed. - Arrange fetal echocardiography and review for renal/CNS/other anomalies. - Involve neonatology before viability/delivery, especially with thoracic restriction. - If high concern for lethality, discuss realistic neonatal survival, comfort-care options, and the local legal/ethical framework. - Delivery location should have NICU, pediatric radiology, genetics, pediatric orthopedics, airway/ENT/anesthesia support, and palliative care where appropriate. - Avoid traumatic instrumental delivery (forceps/vacuum) if significant dysplasia, poor mineralization, skull abnormality, or cervical/spine risk is suspected. - Mode and timing of delivery should be obstetric-led unless macrocephaly, abnormal skull/spine, severe mineralization disorder, or a change in fetal/neonatal plan alters the balance. Missing items before final counseling: 1. AC is missing: required for FL/AC and proportionality. 2. Thoracic circumference is missing: required for TC/AC and TC-centile (two of three lethality markers). 3. Femur: measure both left and right if technically possible. 4. Humerus: measure both left and right if technically possible. 5. Radius: measure both left and right if technically possible. 6. Ulna: measure both left and right if technically possible. 7. Tibia: measure both left and right if technically possible. 8. Fibula: measure both left and right if technically possible. 9. Foot: measure both left and right if technically possible. 10. Thoracic shape not documented (normal/narrow/bell-shaped/very small). 11. Rib length/shape not documented (short, beaded, fractured, normal). 12. Mineralization not documented (skull compressibility, vertebral ossification, long-bone echogenicity). Abnormal skull yields the highest exome diagnostic rate.

Reference-curve implementation. Long bones (femur, humerus, radius, ulna, tibia, fibula, foot) are Z-scored with the Chitty & Altman (2002) fractional-polynomial mean and SD equations, valid 12–42 weeks; values were verified against the published median table before embedding. Thoracic circumference uses the Chitkara (1987) percentile nomogram (16–40 weeks), interpolated, with SD derived from the published 5th–95th band. An optional INTERGROWTH-21st femur median is provided as a switchable standard; its SD is approximated from the Chitty femur SD and is flagged as such. BPD/HC/AC use embedded approximate curves for contextual Z only and are not the basis of any flag. Clavicle is recorded but not Z-scored (no embedded validated chart).

Lethality thresholds. The validated triad is FL/AC < 0.16 (sens ~84%, spec ~57%), TC/AC < 0.60 (sens ~10%, spec ~100%), and TC < 5th centile (sens ~59%, spec ~81%) (Crane 2024; Rahemtullah 1997; Chitkara 1987). Positive predictive value for a life-limiting outcome rises with concordance: ~50% with one marker, ~78% with two, ~100% with three. Normal TC/AC is ~0.89, so a ratio of 0.60–0.79 is below normal but is not the validated lethal cutoff. Single markers have modest specificity; rely on concordance, never on one marker.

Chitty LS, Altman DG. Charts of fetal size: limb bones. BJOG. 2002;109:919–929. Per-bone mean and SD fractional-polynomial equations for all seven long bones; the core Z-score engine here.
Chitkara U, Rosenberg J, Chervenak FA, et al. Prenatal sonographic assessment of the fetal thorax: normal values. Am J Obstet Gynecol. 1987;156:1069–1074. Thoracic circumference nomogram; normal TC/AC ~0.89; TC < 5th centile screens for pulmonary hypoplasia.
Rahemtullah A, McGillivray B, Wilson RD. Suspected skeletal dysplasias: FL/AC ratio for predicting outcome. Am J Obstet Gynecol. 1997;177:864–869. FL/AC < 0.16 lethality threshold.
Crane J, et al. Sonographic predictors of lethality in suspected skeletal dysplasia (TC/AC, FL/AC, TC centile; combined-marker PPV ladder). Prenat Diagn. 2024. Per-marker sensitivity/specificity and concordance PPV used in the lethality engine.
Papageorghiou AT, et al. International standards for fetal growth (INTERGROWTH-21st). Lancet. 2014;384:869–879. Optional femur median standard.
Krakow D, Lachman RS, Rimoin DL. Guidelines for the prenatal diagnosis of fetal skeletal dysplasias. Genet Med. 2009;11:127–133. Structured survey checklist and referral/lethality logic.
Salomon LJ, et al. ISUOG Practice Guidelines (updated): mid-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol. 2022;59:840–856. Limb survey requirements.
Unger S, Ferreira CR, Mortier GR, et al. Nosology of genetic skeletal disorders: 2023 revision. Am J Med Genet A. 2023;191:1164–1209. 771 entries / 552 genes; differential backbone.
Tse KY, et al. Diagnostic yield of exome sequencing in fetuses with sonographic skeletal dysplasia. Genes. 2023;14:1203. Pooled incremental exome yield ~69% after normal karyotype/CMA; informs the genetics pathway.
Suggested ICD-10 codes are documentation aids only and must be reconciled with your coding standard. For the maternal encounter, an O35.- "maternal care for known or suspected fetal abnormality" code is generally the billable code; the Q-code documents the suspected entity. Gene/phenotype mapping is simplified and must be maintained by a genetics lead. Do not use the ranked differential as a final diagnosis.

Fetal Skeletal Dysplasia Workbench

Results

Lethality / pulmonary-hypoplasia screen

Biometry and skeletal measurements

Pattern analysis

ISDS Nosology & Literature

Expert EMR note

Patient explanation

Genetic testing request

Delivery / perinatal plan

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