Template:First Trimester Timeline

From Embryology
First Trimester Timeline 
Links: human timeline | first trimester timeline | second trimester timeline | third trimester timeline
Gestational Day GA
Menstrual Phase
Menstrual cycle.png

menstrual cycle changes: uterus endometrium (loss), ovary (follicle development)

  Human- menstrual uterine endometrium.jpg
Proliferative Phase Smear- early proliferative.jpgOva41he.jpg menstrual cycle changes: uterus endometrium (proliferation), ovary (Follicle Development)
Proliferative Phase
  Smear- mid-proliferative.jpg Human- mid-proliferative uterine endometrium.jpg Ovary10x.jpg Ova20he.jpg menstrual cycle - Mid proliferative
  Smear- late-proliferative.jpg Human- late proliferative uterine endometrium.jpg Menstrual cycle.png menstrual cycle - Late Proliferative


Human ovulation 06.jpg Human oocyte.jpg Follicle 001 icon.jpg
Fertilization Day
Secretory Phase

Stage 1

Early zygote.jpg Stage1 size with ruler.jpgSmear- secretory.jpg Human- secretory uterine endometrium.jpg fertilization, zygote, Secretory Phase
Stage 2 Stage2.jpg Week1 001 icon.jpg morula, Blastula
  Human-blastocyst-day-3-6-icon.jpg blastocyst
Stage 3 CSt3.jpg blastocyst Hatching (zona pellucida lost)
  Smear- late secretory.jpg Human- late secretory uterine endometrium.jpg Late Secretory, blastocyst (free floating)
Stage 4 Adplantation
Stage 5 Week2 001 icon.jpgStage5 bf11L.jpg
Week2 001 icon.jpg implantation
Stage 6 Stage6 bf03.jpgChorion 001 icon.jpg Chorionic Cavity
Stage 7 Stage7-bf1.jpg Stage7-sem2.jpg Stage7.jpg gastrulation, ectoderm, mesoderm, endoderm
Stage 8 Stage8 human.jpg Neuralplate 001 icon.jpg neural neurogenesis, neural groove and folds are first seen
Stage8 SEM1.jpg
Stage 9 Stage9 bf2c.jpg Stage9 sem1b.jpg 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[1]

  heart cardiogenesis, week 3 begins as paired heart tubes.
Stage 10 Stage10 bf2c.jpg Stage10 sem10c.jpg Neuraltube 001 icon.jpg

Neural Crest differentiation at spinal cord level from day 22 until day 26

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[1]

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.

  heart begins to beat in Humans by day 22-23, first functioning embryonic organ formed.
Stage 11 Stage11 bf2c.jpg

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 Optic ventricle appears and the neural groove/tube space is initially filled with amniotic fluid.[2]

Stage 12 Stage12 bf2b.jpg Stage12 sem1.jpg

pituitary Week 4 hypophysial pouch, Rathke's pouch, diverticulum from roof

liver septum transversum forming liver stroma and hepatic diverticulum forming hepatic trabeculae[3]

neural caudal neuropore takes a day to close (closure is approximately at future somitic pair 31/sacral vertebra 2)

neural secondary neurulation begins

ventricular onset of the ventricular system and separates the ependymal from the amniotic fluid.[2]

neural crest cardiac crest, neural crest from rhombomeres 6 and 7 that migrates to pharyngeal arch 3 and from there the truncus arteriosus[1]

neural crest vagal neural crest enter the foregut (20-25 somite stage)

Stage 13 Stage13 bf1c.jpg Stage13 sem1c.jpg 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.[4]

telencephalon cavity appears

Neural - Vascular Development - hindbrain is supplied by two parallel neural arteries (or channels) that obtain their blood supply from carotid-vertebrobasilar anastomoses given by the pharyngeal arch arteries; trigeminal artery, the otic artery, hypoglossal artery, and the proatlantal artery.[5]

liver epithelial cord proliferation enmeshing stromal capillaries[3]

smell Crest comes from the nasal plates[6]

integumentary 4 weeks - simple ectoderm epithelium over mesenchyme

integumentary 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.[7]

Diaphragm - pleuroperitoneal fold (PPF) first discernible in human embryos (CRL 6mm).[8]

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)

Stage 14 Stage14 bf2l.jpg Stage14 sem1c.jpg Placodes sensory placodes, lens pit, otocyst, nasal placode, primary/secondary vesicles, fourth ventricle of brain

somite 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.

Abdominal Wall mesoderm of the primary body wall coalesced in the ventral midline to create the abdominal cavity.[9]

neural first appearance of the future cerebral hemispheres. Cerebellar plate differentiated to an intermediate layer, and future rhombic lip identifiable[10]

Neural - Vascular Development - basilar artery forms from the consolidation of the neural arteries.[5]

ventricular Subarachnoid space initially as irregular spaces on the ventral surface of the spinal cord.[11]

liver hepatic gland and its vascular channels enlarge, hematopoietic function appears[3]

vision lens placode is indented by the lens pit.[7]

Stage 15 Stage15 bf1c.jpg

neural cranial nerves (except olfactory and optic) are identifiable in more advanced embryos[12]

Neural - Vascular Development - vertebral arteries form from transverse anastomoses between cervical intersegmental arteries, beginning with the proatlantal artery and proceeding downward to the 6th intersegmental artery,[5]

vision lens pit is closed. The lens vesicle and optic cup lie close to the surface ectoderm and appear to press against the surface.[7]

  vision 35 to 37 days retinal pigment present
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

respiratory Week 6 - descent of heart and lungs into thorax. Pleuroperitoneal foramen closes

tongue Week 6 - gustatory papilla, caudal midline near the foramen caecum (week 6 to 7 - nerve fibers approach the lingual epithelium)

Stage 16 Stage16 bf1c.jpg Neural first parasympathetic ganglia, submandibular and ciliary, are identifiable[13]

Neural - Vascular Development - development of the middle cerebral artery is first identified as small buds originating proximal to the anterior cerebral artery on the anterior division of the primitive internal carotid artery.[5]

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.

Abdominal Wall muscle cell migration about 25% of the hemicircumference of the abdominal cavity, the lateral plate mesoderm has become more condensed and thicker in the area around the myoblasts.[9]

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.[7]

Stage 17 Stage17 bf1c.jpg
  • neural
    • telencephalon areas of the future archicortex, paleocortex, and neocortex, visible. Beginning of future choroid plexus[14]
    • ventricular primordium of the epidural space appears first on the ventral part of the vertebral canal and develops rostro-caudally[15]
  • smell olfactory nerve fibres enter the brain[6]
  • Eyelid sulcus (groove) above and below eye deepen and eyelid folds develop (below first and then above)[7]
  • Diaphragm - pleuroperitoneal fold (PPF) no longer separated from the diaphragm (CRL 14mm)[8]
  • Abdominal Wall muscle cells now migrated approximately 50% of the distance to the ventral midline, inner and outer layers were not discernible yet.[9]
  heart separation of common cardiac outflow (aortic arch and pulmonary aorta)
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

Stage 18 Stage18 bf1c.jpg

limb bone forms by endochondrial ossification and throughout embryo replacement of cartilage with bone (week 5-12).

neural smell vomeronasal fibres and nervus terminalis[6]

liverobturation due to epithelial proliferation, bile ducts became reorganized, continuity between liver cells and gut[3]

ventricular duramater appears and spaces surround the circumference of the spinal cord, which coalesce and contain many blood vessels.[15]

Female uterus opening of the paramesonephric (Müllerian) duct to the coelomic cavity formed as an invagination of the coelomic epithelium[16]

Abdominal Wall separation of the myoblasts into distinct inner and outer layers, with unidirectional orientation. Abdominal wall thicker in the region where secondary structures were forming compared with the primary body wall region, dorsally outermost layer of connective tissue approximately half of this thickness.[9]


liver (stage 18 to 23) biliary ductules developed in periportal connective tissue

produces ductal plates that receive biliary capillaries[3]

Stage 19 Stage19 bf1c.jpg
  • vision - (stage 19 -22) eyelid folds develop into the eyelids and cover more of the eye as the palpebral fissure takes shape. The upper and the lower eyelids meet at the outer canthus in Stage 19.[7]
  • cardiovascular
  • Respiratory - first generation of subsegmental bronchi now complete, see bronchial tree reconstruction[20] (plates 3 and 4).
  • gastrointestinal tract - anal membrane defined.
  • renal - Cloacal membrane ruptures from urinary pressure at stage 18 or stage 19,
  • genital
    • testis - Rete testis develops from the seminiferous cords at stages 19–23, and tunica albuginea forms.[21].
    • ovary - Rete ovarii cords are developing.[22]
    • uterus Müllerian duct grows independently from the invagination of the coelomic epithelium during stages 19-23[16]
  • musculoskeletal
    • Sternum right and left sternal bars are present.[23] (figs. 7-17 and 7-22)
    • Abdominal Wall segregation of the myoblasts into four distinct muscle groups with unidirectional orientation of myoblasts. Myoblast migrated over half of the distance to the ventral midline, abdominal wall thickest where the muscles migrated and the outermost layer of connective tissue comprises approximately half of the total thickness of the abdominal wall. Rectus muscle completely separated after migrating over half the distance to the midline.[9]
  • neural
    • rhombencephalon migration for olivary and arcuate nuclei begins.
    • choroid plexus of the fourth ventricle present.
    • stria medullaris thalami reaches the habenular nuclei.
    • habenular commissure begins to develop.
    • accessory olivary nucleus appears[24]
    • Neural - Vascular Development - middle cerebral artery becomes more prominent, the plexi fuse into a single artery and further branches pierce the cerebral hemisphere.[5]
Stage 20 Stage20 bf1c.jpg

Head scalp vascular plexus visible

limb upper limbs begin to rotate ventrally

neural amygdaloid body has at least four individual nuclei[24]

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.[7]

  gastrointestinal tract anal membrane perforates
Stage 21 Stage21 bf1c.jpg

neural cortical plate appears in the area of future insula[25]

Neural - Vascular Development - formation of the anterior communicating artery.[5]

limb upper and lower limbs rotate

Intraembryonic Coelom pericardioperitoneal canals close

Abdominal Wall Myoblasts have reached the ventral midline and myotubes were present and oriented uniformly within all muscle groups. The rectus abdominis formed distinct bundles of muscle. Connective tissue layers comprised the majority of the thickness of the abdominal wall, outermost layer of connective tissue accounted for the majority of this thickness.[9]

Stage 22 Stage22 bf1c.jpg neural neocortical fibres project to epithalamus, to dorsal thalamus, and to mesencephalon[25]

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[6]

Uterus Vagina fused duct (uterovaginal canal) bifurcated at the caudal portion at Carnegie stages 22 and 23[16]

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)[26]

Stage 23 Stage23 bf1c.jpg 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[27]

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)[28]

Abdominal Wall Rectus muscle forms 2 or 3 distinct layers with myotube orientation uniform in all muscles. The external oblique and internal oblique started to expand in thickness, transversus a thin layer of muscle.[9]

Week 8 Stomach Week 8 - Gastrin containing cells in stomach antrum. Somatostatin cells in both the antrum and the fundus.

Genital - Female Development paired paramesonephric (Müllerian) ducts contact each other and are fused into a single tube that separates again and returns to the mesonephric (Wolffian) ducts. The paramesonephric ducts have not yet reached the urogenital sinus.[16]

Week 9 Beginning of Fetal Development

Historic Embryology
Embryology History George Streeter In 1949 the embryologist George Streeter[29] used the replacement of cartilage within the humerus by bone marrow as an arbitrary definition of the embryo to fetus transition.
"If the onset can be recognized in a given specimen, that specimen is straightway classed as a fetus."

CRL 43 mm, femur length 6 mm

9 weeks CRL 50 mm - genital genitalia in both sexes look identical[30]

uterus - paramesonephric ducts come into apposition with the urorectal septum and begin to fuse

Size comparison embryo-fetus actual.jpg

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

outer ear 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.

inner ear Week 10 - neural-crest-derived melanocytes migrate into the cochlea. They penetrate the basement membrane of the lateral wall epithelium and develop into the intermediate cells of the stria vascularis.[31]

Week 10 - CRL 55 mm, femur length 9 mm, biparietal diameter 17 mm

neural - Cerebrum appearance of the first sulcus (week 11-15, GA 13-17 weeks)[32]

Size comparison embryo-fetus actual.jpg

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.

Week 11 - CRL 68 mm, femur length 12 mm, biparietal diameter 20 mm
Systems: bone timeline | eye neural crest timeline | heart abnormality timeline | hearing EAM timeline | muscle timeline | ovary timeline | placental villi timeline | shoulder timeline | smell timeline | spleen timeline | ventricular timeline
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