Embryology · Skeleton, muscles, limbs and skin

Limb defects: amelia, meromelia, polydactyly and syndactyly !

Malformation

A limb can be missing completely or in part, have extra digits or joined digits; nearly all of it is decided between weeks 4 and 8.

Updated 2026-11-01

See it in 3D: Humerus · Radius · Ulna · Femur · 1st metacarpal · 5th metacarpal · Proximal phalanx, little finger

What it is

Limb defects arise in weeks 4 to 8, while the buds grow and the digits form. A limb can be completely absent (amelia) or partly absent (meromelia, such as phocomelia), have extra digits (polydactyly) or fused digits (syndactyly). Many are isolated, but a radial or thumb defect calls for a search for heart, kidney or blood anomalies.

Embryological cause

Amelia and meromelia: failure of the apical ectodermal ridge or of bud mesoderm in weeks 4 to 6; thalidomide is the classic example.
Phocomelia: the long segments are missing and the hand or foot sits close to the trunk.
Polydactyly: excess anteroposterior signalling (SHH from the ZPA or its regulator); postaxial (little-finger side) is the most common.
Syndactyly: failure of interdigital apoptosis in weeks 6 to 8; it can join skin only or bone as well.
Cleft hand or foot: the central ray (third metacarpal and its phalanges) is missing.
Amniotic bands: constriction rings and amputations from amnion adhesions; not a genetic error.

Markers

SHH
TBX5
HOXD13
FGF8
Thalidomide

How it presents

Thalidomide causes phocomelia mainly with exposure between days 24 and 36 after fertilisation.
Isolated postaxial polydactyly: common and inherited, especially in people of African descent; also in trisomy 13.
Most frequent syndactyly: between the second and third toes; seen in craniosynostosis syndromes (Apert).
Radial or thumb defect: think of Holt–Oram (TBX5), Fanconi anaemia, TAR (thrombocytopenia–absent radius) and VACTERL.
Some reduction defects come with heart defects, omphalocele and gastroschisis.

Diagnosis

Second-trimester ultrasound: count the long bones and digits of all four limbs.
X-ray at birth to see which bones are missing or fused.
Echocardiogram, renal ultrasound and blood count if there is a radial defect.
Karyotype or genetic testing if there are other anomalies.

Test yourself

Think of the answer, then tap the question to see it.

Which mechanism fails in syndactyly?

Apoptosis of the mesenchyme between the digits, which normally separates them between weeks 6 and 8.

What is phocomelia, and which drug is it linked to?

Absence of the long segments, with hands or feet close to the trunk; it is linked to thalidomide in weeks 4 to 6.

Which tests does a newborn with an absent thumb or radius need?

Echocardiogram, renal ultrasound and blood count, looking for Holt–Oram, Fanconi anaemia, TAR syndrome or VACTERL.

Which type of polydactyly is the most common?

Postaxial, with an extra digit on the little-finger side; often isolated and inherited.

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See it in 3D

The structures of Durgical's 3D anatomy where it is found.

← All of embryology

Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.

Sources: Limb Reduction Defects | Birth Defects | CDC (Centers for Disease Control and Prevention, 2026); Polydactyly: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Webbing of the fingers or toes: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Langman's Medical Embryology, 13th ed., ch. 12: Limbs (Wolters Kluwer, 2015).

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Representative model, for educational purposes only.