Timeline human development

Embryology - 27 Apr 2024        Expand to Translate
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Introduction

Embryonic Development

From a cell	to a newborn infant
	in 9 months.

This page is organised to show week by week human development features and approximate timing of key events with more detailed information about specific events in different systems. For a less detailed timeline see week by week.

Note:

• The "weeks" refer to embryonic development and differ from clinical weeks (shown in brackets, from last menstrual period, LMP or GA)
• The "stages" refer to Carnegie stages of development.
• The "timing" refers to days from fertilization or post conception age (PC), not the clinical or gestational age (GA) calculated from LMP (add 2 weeks).
• The dates and staging are also "ideal", and there is significant biological variability in the general timing of events.
• Week 1 to Week 8 are considered the embryonic period of development.
• Week 9 to week 37 or birth are considered the fetal period of development.
• The first month after birth is the neonatal period of development.

Each developmental feature is linked to online content with more detailed information and resources such as images and movies. The superscript numbers are the original source references.

There are similar "timelines" for other species shown below.

Week -2

(Clinical Week 1)

Day	Menstrual cycle	Event
1	Menstrual Phase	Menstrual Cycle changes: Uterine endometrium (loss), Ovary (Follicle Development)
2
3
4
5	Proliferative Phase	Menstrual Cycle changes: Uterine endometrium (proliferation), Ovary (Follicle Development)
6
7

Week -1

(Clinical Week 2)

Day	Menstrual cycle	Event
8	Proliferative Phase
9		Menstrual Cycle - Mid proliferative
10
11
12
13		Menstrual Cycle - Late Proliferative
14	Ovulation Capacitation

Week 1

Week 1 (Clinical Week 3)

Week 1 summary

Day	Stage	Event
1	Secretory Phase Stage 1	Fertilization, Secretory Phase
2	Stage 2	Morula, Blastula
3		Blastocyst Development
4	Stage 3	Blastocyst Hatching (zona pellucida lost)
5		Late Secretory, Blastocyst (free floating)
6	Stage 4	Adplantation
7	Stage 5

Week 2

Week 2 (Clinical Week 4)

Day	Stage	Event
8		Implantation
9
10
11
12
13	Stage 6	Chorionic Cavity
14

Week 3

Week 3 (Clinical Week 5)

Day	Stage	Event
15
16	Stage 7
17
18	Stage 8	Neural neurogenesis, neural groove and folds are first seen
19
20	Stage 9	Musculoskeletal somitogenesis, first somites form and continue to be added in sequence caudally (1 - 3 somite pairs). Neural the three main divisions of the brain, which are not cerebral vesicles, can be distinguished while the neural groove is still completely open Neural Crest mesencephalic neural crest is visible
21		Heart cardiogenesis, week 3 begins as paired heart tubes.

Week 4

Week 4 (Clinical Week 6)

Day	Stage	Event
22	Stage 10	Neural Crest differentiation at spinal cord level from day 22 until day 26 Neural neural folds begin to fuse near the junction between brain and spinal cord, when Neural Crest cells are arising mainly from the neural ectoderm Neural Crest trigeminal, facial, and postotic ganglia components visible Neural Crest migration of vagal level neural crest cells begins (7-10 somite stage) Neural rostral neural tube forms 3 primary brain vesicles (week 4) Respiratory Week 4 - laryngotracheal groove forms on floor foregut.
23		Heart begins to beat in Humans by day 22-23, first functioning embryonic organ formed.
24	Stage 11	Thyroid thyroid median endodermal thickening in the floor of pharynx Neural rostral (or cephalic) neuropore closes within a few hours; closure is bidirectional, it takes place from the dorsal and terminal lips and may occur in two areas simultaneously. The two lips, however, behave differently. Ventricular System Optic ventricle appears and the neural groove/tube space is initially filled with amniotic fluid.
25	Stage 12	Pituitary Week 4 hypophysial pouch, Rathke's pouch, diverticulum from roof Liver septum transversum forming liver stroma and hepatic diverticulum forming hepatic trabeculae Neural caudal neuropore takes a day to close (closure is approximately at future somitic pair 31/sacral vertebra 2) Neural secondary neurulation begins Ventricular System onset of the ventricular system and separates the ependymal from the amniotic fluid. Neural Crest cardiac crest, neural crest from rhombomeres 6 and 7 that migrates to pharyngeal arch 3 and from there the truncus arteriosus Neural Crest vagal neural crest enter the foregut (20-25 somite stage)
26
27
28	Stage 13	Neural the neural tube is normally completely closed, ventricular system now separated from amniotic fluid. Neural crest at spinal level is segregating, and spinal ganglia are in series with the somites. Spinal cord ventral roots beginning to develop. telencephalon cavity appears Liver epithelial cord proliferation enmeshing stromal capillaries Smell Crest comes from the nasal plates Skin 4 weeks - simple ectoderm epithelium over mesenchyme Skin 1-3 months ectoderm- germinative (basal) cell repeated division of generates stratified epithelium; mesoderm- differentiates into connective tissue and blood vessels Vision Optic vesicle lies close to the surface ectoderm. The surface ectoderm overlying the optic vesicle, in response to this contact, has thickened to form the lens placode. Diaphragm - pleuroperitoneal fold (PPF) first discernible in human embryos (CRL 6mm).

Week 5

Week 5 (Clinical Week 7)

Day	Stage	Event
29		Pituitary Week 5 elongation, contacts infundibulum, diverticulum of diencephalon Heart Week 5 septation starts, atrial and ventricular Respiratory Week 5 left and right lung buds push into the pericardioperitoneal canals (primordia of pleural cavity) Respiratory Week 5 to 17 lung histology - pseudoglandular Hearing Week 5 cochlear part of otic vesicle elongates (humans 2.5 turns)
30
31
32	Stage 14	Placodes sensory placodes, lens pit, otocyst, nasal placode, primary/secondary vesicles, fourth ventricle of brain Mesoderm continued segmentation of paraxial mesoderm (somite pairs), heart prominence Head 1st, 2nd and 3rd pharyngeal arch, forebrain, site of lens placode, site of otic placode, stomodeum Body - heart, liver, umbilical cord, mesonephric ridge visible externally as bulges. Limb upper and lower limb buds growing. Neural first appearance of the future cerebral hemispheres. Cerebellar plate differentiated to an intermediate layer, and future rhombic lip identifiable Ventricular System Subarachnoid space initially as irregular spaces on the ventral surface of the spinal cord. 16228957 Liver hepatic gland and its vascular channels enlarge, hematopoietic function appears Eye - Lens the lens placode is indented by the lens pit.
33	Stage 15	Neural cranial nerves (except olfactory and optic) are identifiable in more advanced embryos Eye - Lens the lens pit is closed. The lens vesicle and optic cup lie close to the surface ectoderm and appear to press against the surface.
34
35		Vision 35 to 37 days retinal pigment present

Week 6

Week 6 (Clinical Week 8)

Day	Stage	Event
36		Pituitary Week 6 - connecting stalk between pouch and oral cavity degenerates Parathyroid Week 6 - diverticulum elongate, hollow then solid, dorsal cell proliferation Thymus Week 6 - diverticulum elongate, hollow then solid, ventral cell proliferation Adrenal Week 6 - fetal cortex forms from mesothelium adjacent to dorsal mesentery, medulla neural crest cells from adjacent sympathetic ganglia Respire Week 6 - descent of heart and lungs into thorax. Pleuroperitoneal foramen closes Tongue Week 6 - descent of heart and lungs into thorax. Pleuroperitoneal foramen closes gustatory papilla, caudal midline near the foramen caecum (week 6 to 7 - nerve fibers approach the lingual epithelium)
37	Stage 16	Neural first parasympathetic ganglia, submandibular and ciliary, are identifiable Limb upper limb bud nerves median nerve, radial nerve and ulnar nerve entered into hand plate, myoblasts spindle shaped and oriented parallel to limb bud axis. Heart outflow tract elliptical configuration with four cushions, the two larger fusing at this stage. Semilunar valve leaflets form at the downstream end of the cushions Head lip and palate components of the upper lip, medial nasal prominence and maxillary process present, median palatine process appears. Eyelid prior to the development of the eyelids, one small sulcus or groove forms above the eye (eyelid groove) and another below it.
38
39
40
41	Stage 17	Neural telencephalon areas of the future archicortex, paleocortex, and neocortex, visible. Beginning of future choroid plexus Smell olfactory nerve fibres enter the brain Neural primordium of the epidural space appears first on the ventral part of the vertebral canal and develops rostro-caudally Eyelid sulcus (groove) above and below eye deepen and eyelid folds develop (below first and then above) Diaphragm - pleuroperitoneal fold (PPF) no longer separated from the diaphragm (CRL 14mm)
42		Heart separation of common cardiac outflow (aortic arch and pulmonary aorta)

Week 7

Week 7 (Clinical Week 9)

Day	Stage	Event
43		Pancreas Week 7 to 20 pancreatic hormones secretion increases, small amount maternal insulin Respiratory Week 7 - enlargement of liver stops descent of heart and lungs
44	Stage 18	Limb Bone forms by endochondrial ossification and throughout embryo replacement of cartilage with bone (week 5-12). Neural Smell vomeronasal fibres and nervus terminalis Liver obturation due to epithelial proliferation, bile ducts became reorganized, continuity between liver cells and gut Ventricular System duramater appears and spaces surround the circumference of the spinal cord, which coalesce and contain many blood vessels. Uterus Vagina opening of the Müllerian duct to the coelomic cavity formed as an invagination of the coelomic epithelium
45		Liver (stage 18 to 23) biliary ductules developed in periportal connective tissue produces ductal plates that receive biliary capillaries
46
47
48	Stage 19	Neural accessory olivary nucleus appears Uterus Müllerian duct grows independently from the invagination of the coelomic epithelium during stages 19-23 Eyelid the upper and the lower eyelids meet at the outer canthus
49

Week 8

Week 8 (Clinical Week 10)

Day	Stage	Event
50	Stage 20	Head scalp vascular plexus visible Limb upper limbs begin to rotate ventrally Neural amygdaloid body has at least four individual nuclei oculomotor nerve shows a dorsolateral and a ventromedial portion rhombic lip (rhombencephalon) formation of the cerebellum (intermediate layer) and of the cochlear nuclei cerebellum cell layer (future Purkinje cells) develops choroid plexuses of the fourth and lateral ventricles Eyelid the inner canthus is established.
51		Gastrointestinal Tract anal membrane perforates
52	Stage 21	Neural cortical plate appears in the area of future insula Limb upper and lower limbs rotate Intraembryonic Coelom pericardioperitoneal canals close
53
54	Stage 22	Neural neocortical fibres project to epithalamus, to dorsal thalamus, and to mesencephalon Limb fingers and toes lengthen Smell Stage 22 to early fetal period - migratory streams of neurons from the subventricular zone of the olfactory bulb towards the future claustrum Uterus Vagina fused duct (uterovaginal canal) bifurcated at the caudal portion at Carnegie stages 22 and 23
55		Genital 8 Weeks Testis - mesenchyme, interstitial cells (of Leydig) secrete testosterone, androstenedione Genital 8 to 12 Weeks - hCG stimulates testosterone production Tongue Week 8 - nerves penetrate epitheilai basal lamina and synapse with undifferentiated, elongated, epithelial cells (taste bud progenitor cell)
56	Stage 23	Stage 23 defines the end of the embryonic (organogenesis) period Mesoderm heart prominence, ossification continues Head nose, eye, external acoustic meatus, eyelids, external ears, rounded head Body - straightening of trunk, umbilical cord, intestines herniated at umbilicus Limb upper limbs longer and bent at elbow, hands and feet turned inward, foot with separated digits, wrist, hand with separated digits Extraembryonic Coelom chorionic cavity is now lost by fusion with the expanding amniotic cavity Neural rhombencephalon, pyramidal decussation present, nuclei and tracts similar to those present in the newborn cerebellum present as only a plate connected to midbrain and hindbrain through fibre bundles Axial Skeleton vertebral column 33 or 34 cartilaginous vertebrae (20-33 mm in total length), vertebral pedicles, articular and transverse processes identifiable (no spinous processes)
	Week 8	Stomach Week 8 - Gastrin containing cells in stomach antrum. Somatostatin cells in both the antrum and the fundus. Uterus Development 56 days - paramesonephric duct fusion (female)

Week 9

(Clinical Week 11)

Day	Stage	Event
57	Fetal Period	Hearing Week 9 - mesenchyme surrounding membranous labrynth (otic capsule) chondrifies Smell Embryonic/Fetal transition - localized incomplete lamination of the olfactory bulb
58
59
60
61
62
63		Week 9 - CRL 43 mm, femur length 6 mm 9 weeks CRL 50 mm - Genital genitalia in both sexes look identical uterus - paramesonephric ducts come into apposition with the urorectal septum and begin to fuse

Week 10

(Clinical Week 12)

Day	Stage	Event
64		Gastrointestinal Tract Week 10 intestines in abdomen Pituitary growth hormone and ACTH detectable Pancreas Week 10 glucagon (alpha) differentiate first, somatostatin (delta), insulin (beta) cells differentiate, insulin secretion begins Tongue Week 10 shallow grooves above the taste bud primordium Stomach Week 10 - Glucagon containing cells in stomach fundus. Nail Development fingernails appear Hearing - Outer Ear Development Week 10 - Meatal plug extends in a disc-like fashion, the meatus is boot-shaped with a narrow neck and the sole of the meatal plug spreading widely to form the future tympanic membrane medially. Proximal portion of the neck starts to be resorbed.
65
66
67
68
69
70		Week 10 - CRL 55 mm, femur length 9 mm, biparietal diameter 17 mm

Week 11

(Clinical Week 13)

Day	Stage	Event
71		Thyroid colloid appearance in thyroid follicles, iodine and thyroid hormone (TH) synthesis Stomach Week 11 - Serotonin containing cells in both the antrum and the fundus.
72
73
74
75
76
77		Week 11 - CRL 68 mm, femur length 12 mm, biparietal diameter 20 mm

Second Trimester

(Clinical Week 14) Second Trimester

Week	Stage	Event
12	Clinical second trimester	Week 12 - CRL 85 mm, femur length 15 mm, biparietal diameter 25 mm Hearing Week 12-16 - Capsule adjacent to membranous labrynth undegoes vacuolization to form a cavity (perilymphatic space) around membranous labrynth and fills with perilymph Genital male and female external genital differences observable Respiratory Month 3-6 - lungs appear glandular, end month 6 alveolar cells type 2 appear and begin to secrete surfactant Tongue Week 12 - first differentiated epithelial cells (Type II and III) Genital female genital canal (80 days) formed with absorption of the median septum
13		Tongue Week 12 to 13 - maximum synapses between cells and afferent nerve fibers Hearing - Outer Ear Development Week 13 - Meatal plug disc-like, innermost surface in contact with the primordial malleus, contributes to the formation of the tympanic membrane.
14		Tongue Week 14 to 15 - taste pores develop, mucous Ovary Development 100 days - primary follicles present Nail Development toenails appear Head Development facial skeleton remodelling begins
15		Pancreas glucagon detectable in fetal plasma. Spleen Week 15 -alpha-smooth muscle actin (alpha-SMA)-positive reticulum cells scattered around the arterioles. Fetal Timeline
16	14 cm	Hearing Week 16-24 - Centres of ossification appear in remaining cartilage of otic capsule form petrous portion of temporal bone. Continues to ossify to form mastoid process of temporal bone. Pituitary adenohypophysis fully differentiated Respiratory Week 16 to 25 lung histology - canalicular Hearing - Outer Ear Development Week 16.5 - External auditory meatus is fully patent throughout its length, lumen is still narrow and curved. Skin 4 months - basal cell- proliferation generates folds in basement membrane; neural crest cells- (melanocytes) migrate into epithelium; embryonic connective tissue- differentiates into dermis, a loose ct layer over a dense ct layer. Beneath the dense ct layer is another loose ct layer that will form the subcutaneous layer. Ectoderm contributes to nails, hair follictles and glands. Nails form as thickening of ectoderm epidermis at the tips of fingers and toes. These form germinative cells of nail field. Cords of these cells extend into mesoderm forming epithelial columns. These form hair follocles, sebaceous and sweat glands. primary follicles begin to form in the ovary and are characterized by an oocyte glandular urethra forms and skin folds present
17		Neural - Brain development histology week 17
18		Tongue Week 18 - substance P detected in dermal papillae, not in taste bud primordia Skin vernix caseosa covers skin Spleen Week 18 - alpha-SMA-positive reticulum cells increase in number and began to form a reticular framework. An accumulation of T and B lymphocytes occurred within the framework, and a primitive white pulp was observed around the arterioles. Hearing - Outer Ear Development week 18 - External auditory meatus is already fully expanded to its complete form.
19
20		Pituitary week 20 to 24 growth hormone levels peak, then decline Skin lanugo, skin hair Skin 5 months - Hair growth initiated at base of cord, lateral outgrowths form associated sebaceous glands; Other cords elongate and coil to form sweat glands; Cords in mammary region branch as they elongate to form mammary glands.
21
22		Neural brain cortical sulcation - sylvian fissure, interhemispheric fissure, callosal sulcus, parietooccipital fissure, and hippocampic fissures present Spleen - Week 22 - antigenic diversity of the reticular framework was observed, and T and B lymphocytes were segregated in the framework. T lymphocytes were sorted into the alpha-smooth muscle actin-positive reticular framework, and the periarteriolar lymphoid sheath (PALS) was formed around the arteriole. B lymphocytes aggregated in eccentric portions to the PALS and formed the lymph follicle (LF). The reticular framework of the LF was alpha-SMA-negative.
23
24		Respiratory Week 24 to 40 lung histology - terminal sac Spleen Week 24 - marginal zone appeared in the alpha-smooth muscle actin-positive reticular framework around the white pulp. Earliest potential survival expected if born ovarian follicles can consist of growing oocytes surrounded by several layers of granulosa cells
25		Respiratory end month 6 alveolar cells type 2 appear and begin to secrete surfactant

Third Trimester

(Clinical Week 28) Third Trimester

Week	Stage	Event
	Clinical third trimester	Hearing 3rd Trimester - vibration acoustically of maternal abdominal wall induces startle respone in fetus.
27
28		Respire Month 7 - respiratory bronchioles proliferate and end in alveolar ducts and sacs
29
30		Genital male gonad (testes) descending
31
32		Nail Development fingernails reach digit tip
33		Neural brain cortical sulcation - primary sulci present
34		Neural brain cortical sulcation - insular, cingular, and occipital secondary sulci present
35
36		Nail Development toenails reach digit tip Lens Development - lens growth and interocular distance plateaus after 36 weeks of gestation
37
38	Birth	Clinical Week 40 Heart pressure difference closes foramen ovale leaving a fossa ovalis Thyroid TSH levels increase, thyroxine (T3) and T4 levels increase to 24 h, then 5-7 days postnatal decline to normal levels Adrenal - zona glomerulosa, zona fasiculata present

Postnatal

Week	Stage	Event
+1
+2
+3
+4		Vision Development - eye globe growth plateaus after 42 weeks of gestation
+5
Month		Testis | Spermatozoa - about 2 months of age, primordial germ cells (gonocytes) are replaced by adult dark (Ad) and pale (Ap) spermatogonia forming the spermatogonial stem cell (SSC) population that at puberty will commence differentiation into spermatozoa.
Year
Year 1		Neural Hearing (6 months to 5 years) thalamocortical afferents to the deeper cortical layers mature and are the first source of input to the auditory cortex
Year 2
Year 3		Adrenal - Year 3 zona reticularis present
4
5		Neural Hearing - (5 to 12 years) commissural and association axons in the superficial cortical layers allows communication between subdivisions of the auditory cortex
7-13		Puberty - Female
9-15		Puberty - Male

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Cite this page: Hill, M.A. (2024, April 27) Embryology Timeline human development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Timeline_human_development

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G