Fetal Foot Ossification Centers by Gestational Age
Primary Ossification Centers
The metatarsals are the first foot bones to ossify, appearing at 9 weeks gestation, followed by the phalanges starting at 11-12 weeks. 1
Metatarsals and Phalanges Timeline
- All five metatarsals: Ossification begins at 9 weeks gestation 1
- Distal phalanx of first digit (great toe): Appears at 11 weeks 1
- Proximal phalanges of all five digits: Visible at 12 weeks 1
- Distal phalanges of digits 2-5: Appear at 12 weeks 1
- Middle phalanx of second digit: Ossifies at 17 weeks 1
- Middle phalanges of third and fourth digits: Both appear at 18 weeks 1
- Middle phalanx of fifth digit: Latest to appear at 21 weeks 1
Tarsal Bones (Hindfoot/Midfoot)
The calcaneus and talus are the tarsal bones most relevant for prenatal ultrasound assessment, with the calcaneus visible at 24 weeks and the talus at 26 weeks. 2
Ultrasound-Detectable Tarsal Centers
- Calcaneus: First visible at 24 weeks gestation by ultrasound 2, 3
- Talus: Becomes visible at 26 weeks gestation 2, 3
- Cuboid: Timing not well-established prenatally, but ossification center is centrally located within its cartilaginous anlage 4
- Navicular: Ossification begins in the central or lateral third of the cartilaginous anlage, with the medial aspect ossifying last 4
Important Clinical Considerations
The calcaneal and talar ossification centers appear as egg-shaped echo-rich areas on ultrasound at the level of the tarsus osseus. 2 These centers show almost linear growth until term, though the slight weekly increase limits their utility for precise gestational age determination compared to other parameters. 3
Early ossification in the talus, calcaneus, and navicular does not begin in the center of their cartilaginous anlagen but rather eccentrically. 4 Specifically:
- Talar ossification centers on the neck of the talus, with the proximal aspect ossifying last 4
- Calcaneal ossification centers on the distal two-thirds, with the proximal aspect and subtalar joint area ossifying last 4
- Navicular ossification centers on the central or lateral third, with the medial aspect ossifying last 4
Clinical Context for Skeletal Dysplasia Detection
While individual foot ossification centers are not primary markers for skeletal dysplasia screening, abnormal mineralization patterns can be detected at routine 18-20 week ultrasound. 5, 6 Undermineralized skull and extremities detected at 14-16 weeks gestation raise concern for severe skeletal dysplasias including osteogenesis imperfecta type II, achondrogenesis, and hypophosphatasia. 5
Short limb length (femur or humerus <5th percentile or >2 SD below mean) and angular deformities are the key ultrasound findings for skeletal dysplasia, not individual foot ossification assessment. 5, 6