Carnegie Staging Guide

Carnegie Stage
Somite stage
Greatest length (mm) [mean]
Morphological land marks
9
1-3*
1.4-2.5 [1.4]
Head fold present. Neural folds may be apposed but are not yet fused at any point. No visible pharyngeal arch. *May be more than 3 somites and may not be paired.
10
4-12
1.5-3.6 [2.1]
Neural tube closing ie closed/fused at least at closure point 1. First and, possibly, second pharyngeal arches visible. Looped heart tube visible.
11
13-20
1.4-5.0 [3.2]
Rostral neuropore closing. Otic placodes present. Optic vesicles formed. No limb buds.
21-29
3.0-5.4 [3.9]
Rostral neuropore closed. Caudal neuropore closing. Upper limb buds appear. Optic vesicles clearly visible. Three pairs of pharyngeal arches visible. Otic pits present. May have thin tail.
30 or more
3.9-6.0 [4.9]
Caudal neuropore closed. Upper limb buds clearly visible and flipper like. Lower limb buds appear. Lens placodes distinct. Four pairs of pharyngeal arches visible. Thin tail present. Otic vesicles present.
More than 30
4.9-8.2 [6.5]
Four clearly visible limb buds- upper limbs paddle-shaped and lower limb buds flipper like. Nasal pits visible. Optic cups present and optic fissure may be visible.
 
6-11 [7.8]
Hand plates formed. Lower limbs paddle-shaped. Nasal pits prominent. Future cerebral hemispheres distinct.
 
7-12.2 [9.6]
Foot plates formed. No/unclear digital rays in handplate. Pigment visible in retina.
 
10-14.5 [12.2]
Digital rays clearly visible in hand plates. Trunk beginning to straighten. Cerebral vesicles prominent.
 
11.7-17.2 [14.9]
Notches between the digital rays in the hand. Elbow region visible. Toe rays begin to appear in foot plates.
 
15.5-21.5 [18.2]
Five distinct toe rays but interdigital notches have not yet appeared. Limbs extend ventrally/forward. Trunk elongating and straightening. Midgut herniation prominent.
 
18-23 [20.7]
Fingers distinct but webbed. Notches between the digital rays in the feet. Upper limbs longer and bent at elbows. Scalp vascular plexus appears.
 
20-26.4 [22.9]
Fingers free and longer. Toes distinct but webbed. Hands and feet approach each other. Stubby tail present. Scalp vascular plexus forms a line at somewhat more than half the distance from eye-ear level to the vertex.
 
23.4-27.5 [25.5]
Toes free and longer. Fingers longer and overlap, scalp vascular plexus at ¾ up head.  Eyelids and auricles (external ears) more developed.
 
23-32.2 [28.8]
Forehead more prominent and head shows human characteristics. Scalp vascular plexus at vertex. Limbs longer and more developed. Distinct bulge still present in umbilical cord caused by herniation of intestines.

The updated HDBR staging guide is largely based on a) O'Rahilly R, Müller F (2001) Human Embryology and Teratology 3rd Edition (ISBN-10: 0471382256) b) O'Rahilly R, Müller F (2010) Developmental stages in human embryos: revised and new measurements. Cells Tissues Organs. 192:73-84. PubMed PMID: 20185898.  In addition, the experience of HDBR staff in viewing and staging embryos refined the staging criteria to only include features that are visible externally in unfixed material.