Prenatal Development Timeline

Nervous Nervous Cardiovascular Cardiovascular Muscular Muscular Early Events Early Events
Special Senses Special Senses Respiratory Respiratory Skeletal Skeletal Growth Parameters Growth Parameters
Blood & Immune Blood & Immune Gastrointestinal Gastrointestinal Endocrine Endocrine General General
Skin/Integument Skin/Integument Renal/Urinary Renal/Urinary Reproductive Reproductive Movement Movement

Unit 1:   The First Week 
Day 0Early Events Embryonic period begins
  Early Events Fertilization resulting in zygote formation
Day 1Early Events Embryo is spherically shaped with 12 to 16 cells
Day 1 - Day 1Early Events Fertilization - development begins with a single-cell embryo!!!
Day 2Endocrine Early pregnancy factor (EPF)
  Early Events Activation of the genome
  Early Events Zygote divides into two blastomeres (24 – 30 hours from start of fertilization)
Day 4Early Events Embryonic disc
  Early Events Free floating blastocyst
  Early Events Hypoblast & epiblast
  Early Events Inner cell mass
  Early Events See where the back and chest will be
Day 5Early Events Hatching blastocyst
Day 6Early Events Embryo attaches to wall of uterus
1 weekEarly Events Chorion
  Early Events Placenta begins to form
Unit 2:   1 to 2 Weeks 
1 week, 1 dayEarly Events Amnioblasts present; amnion and amniotic cavity formation begins
  Early Events Positive pregnancy test
1 week, 2 daysEarly Events Cells in womb engorged with nutrients
1 week, 4 daysEarly Events Longitudinal axis
1 week, 5 daysEarly Events Implantation complete
  Early Events Yolk sac
1 week, 6 daysCardiovascular Primordial blood vessels
  Early Events Amnion with single cell layer
  Early Events Chorionic villi
2 weeksEarly Events Yolk sac
  Early Events Yolk sac
Unit 3:   2 to 3 Weeks 
2 weeks, 1 dayEarly Events 3 germ layers
  Early Events Rostral-caudal orientation
2 weeks, 2 daysBlood & Immune Erythroblasts in yolk sac
  Blood & Immune Three types of blood-forming cells in yolk sac
  Early Events Amnion with two cell layers
  Early Events Secondary villi
2 weeks, 4 daysGastrointestinal Foregut, midgut, and hindgut
  Special Senses Brain is first organ to appear
  Special Senses Neural plate induced by notochordal process
  Early Events Connecting stalk
2 weeks, 6 daysBlood & Immune Numerous blood islands in umbilical vesicle
  Gastrointestinal Foregut
  Gastrointestinal Stomodeum forming
  Cardiovascular Beginnings of the heart can be seen
  Cardiovascular Blood vessels emerge simultaneously in umbilical vesicle, embryo proper, amnion, and connecting stalk
  Cardiovascular Dorsal aortae (paired)
  Cardiovascular Paired tubular heart
  Special Senses Forebrain, midbrain, and hindbrain
  Special Senses Neural groove deepens substantially
  Special Senses Three main divisions of brain
  Early Events Neural crest: Rostral and facial
3 weeksCardiovascular Blood and blood vessels
Unit 4:   3 to 4 Weeks 
3 weeks, 1 dayGastrointestinal Midgut emerging
  Respiratory Respiratory outgrowth
  Cardiovascular Atria (right and left) far apart
  Cardiovascular Circulatory system function begins
  Cardiovascular Endocardial tubes fuse forming tubular heart
  Cardiovascular Heart begins beating
  Cardiovascular Pericardium
  Cardiovascular Primary head vein
  Cardiovascular Sinus venosus
  Cardiovascular Tubular heart begins folding
  Cardiovascular Umbilical arteries
  Cardiovascular Umbilical veins (right and left)
  Special Senses Neural tube
  Early Events Body cavities
  Early Events Hyoid arch
3 weeks, 3 daysEndocrine Thyroid complete
  Gastrointestinal Cystic primordium
  Gastrointestinal Liver
  Gastrointestinal Membrane between future mouth and throat may begin to rupture
  Cardiovascular Internal carotid arteries
  Special Senses Neuropore (near brain) closes
  Special Senses Notochord
3 weeks, 5 daysGastrointestinal First part of pancreas
  Gastrointestinal Pharyngeal arch 3
  Respiratory Lung bud
  Cardiovascular Descending aorta
  Cardiovascular Unidirectional circulation
  Special Senses Brain involves 40% of neural tube
  Special Senses Lowermost spinal cord formation begins
  Special Senses Neural tube closes (lower back)
  Early Events Somites: Pairs 21 through 29
  Early Events Upper limb primordium at level of somites 8 to 10
  General Progressively C-shaped embryo
4 weeksSkin Skin is so thin, you can see through it!
  Gastrointestinal Esophagus primordia
  Gastrointestinal Intestines growing in length
  Gastrointestinal Pancreas: Ventral pancreas
  Gastrointestinal Pharynx
  Gastrointestinal Small & large intestines
  Respiratory Bronchial buds
  Respiratory Lungs begin filling chest cavity
  Respiratory Trachea
  Cardiovascular Circulatory system "well established"
  Cardiovascular Functioning two-chamber heart
  Cardiovascular Heart chambers bulging with fluid
  Cardiovascular Heart now functions as two parallel pumps
  Cardiovascular Heart rate (about) 113 beats/min
  Nervous Most cranial nerve ganglia
  Special Senses Cerebellum
  Special Senses Fourth ventricle
  Early Events Amnion surrounds embryo
  Early Events Limb buds - the first sign of arms and legs
  Early Events Lower limb buds
  Early Events Umbilical cord emerging
  Early Events Upper and lower limb buds
Unit 5:   4 to 5 Weeks 
4 weeks, 3 daysEarly Events Early eyes
4 weeks, 3 days - 5 weeksReproductive Germ cells migrate to gonads
4 weeks, 4 daysRespiratory Lungs: Right and left primary (or main stem) bronchi
  Cardiovascular Sinu-atrial (SA) node
  Cardiovascular Eyes located on sides of head
  Cardiovascular Lens pits
  Cardiovascular Nose: Nasal pits
  Special Senses Brain enlarges 50% since Carnegie Stage 13
  Special Senses Brain: Cerebral hemispheres appear and begin rapid growth
  Special Senses Brain: Lateral ventricles
  Special Senses Hypothalamus
4 weeks, 5 daysGastrointestinal Caecum
  Cardiovascular Blood vessels penetrate diencephalon
  Cardiovascular Coronary arteries (terminal end)
  Cardiovascular Optic chiasm
  Special Senses Brain with five main sections
  Special Senses First nerve fibers
  Special Senses Most cranial nerves seen
  Special Senses Synapses among motor neurons in spinal cord
  Special Senses Third ventricle
5 weeksEndocrine ACTH [adrenocorticotropin hormone]
  Endocrine Growth hormone
  Endocrine Pituitary gland
  Skeletal Limb buds form hand plates
  Renal/Urinary Permanent kidneys
  Respiratory Bronchial tree branching accelerates
  Respiratory Lobar pattern mimics adult pattern
  Cardiovascular Pacemaker cells
  General Head is one third of entire embryo
Unit 6:   5 to 6 Weeks 
5 weeks, 1 daySkeletal Wrist joints are forming
5 weeks, 2 daysEndocrine Thyroid detaches from pharynx
  Cardiovascular Atrioventricular (AV) node
  Cardiovascular Circle of Willis almost complete
  Nervous Cochlear nerve present
  Nervous Musculocutaneous, radial, ulna, and median nerves enter upper limb bud
  Special Senses All cranial nerves identifiable
  Skeletal Initial tooth formation
  Movement Subtle movement begins
5 weeks, 4 daysSkeletal Cartilage formation
5 weeks, 5 daysSpecial Senses Nerve cells differentiating
5 weeks, 5 days - 7 weeks, 1 daySkin Melanocytes in epidermis
5 weeks, 6 daysSkeletal Cartilage in occipital sclerotomes (1-4)
  Gastrointestinal Primordial vermiform appendix
  Nervous All spinal nerves present
  Special Senses Dura begins forming in basal area
  Special Senses Frontal and temporal poles of cerebral hemispheres
  Special Senses Somites: Pairs 38 and 39
  Special Senses Synapses in spinal cord between interneurons and primary afferent neurons
6 weeksMovement Face withdraws from light touch around mouth
  Blood & Immune Blood forming in liver
  Skin Nipples along side of trunk
  Endocrine Adrenal glands
  Endocrine Glucagon in pancreas
  Skeletal Handplates develop subtle flattening
  Skeletal Joints
  Skeletal Tooth buds (primary teeth)
  Muscular Diaphragm is largely formed
  Gastrointestinal Intestines fill base of umbilical cord
  Gastrointestinal External ears
  Special Senses Synapses form in spinal cord
  General Crown-heel length 1.6 cm
Unit 7:   6 to 7 Weeks 
6 weeks, 2 daysSkeletal Elbow regions sometimes identifiable
  Skeletal Hands polygon-shaped
  Skeletal Humerus, radius, and ulna
  Skeletal Toe rays sometimes present
  Muscular Deltoid muscle
  Gastrointestinal Submandibular gland primordia
  Cardiovascular Inferior vena cava
  Cardiovascular Left coronary artery arises from aorta
  Nervous Optic fibers
  Nervous Eyelid folds sometimes present
  Special Senses Brainwave activity has begun
  Special Senses Cerebrospinal fluid production begins
  Movement The hands begin to move
  Skin Volar pads on palms
  Skeletal Bones first form in the collar bones and lower jaw
6 weeks, 5 daysSkeletal Beginnings of occipital and sphenoid bones
  Skeletal Cartilaginous styloid process
  Skeletal Limbs point forward (ventrally)
  Gastrointestinal Anal membrane
  Respiratory Lung, left: Oblique fissure defines upper and lower lobes
  Cardiovascular Circulus arteriosus (Circle of Willis) complete
  Cardiovascular Right coronary artery arises from aorta
  Cardiovascular Tricuspid and mitral valves
  Nervous Primitive nasal cavity
  Nervous Eyelids: Upper and lower lids present and growing
  Special Senses Occipital pole of cerebral hemispheres
6 weeks, 6 daysSkeletal Feet polygon-shaped
  Gastrointestinal Cloacal membrane ruptures
7 weeksMovement Head rotates
  Movement Leg movements
  Blood & Immune B lymphocytes in liver
  Reproductive Ovaries
  Reproductive Testes begin to differentiate
  Endocrine Insulin in pancreas
  Skeletal Foot plates notched
  Muscular Hiccups
  Muscular Tendons attach muscle to bone
  Cardiovascular The heart has four chambers and is nearly complete.
  Cardiovascular The heart rate peaks at 165 to 170 beats per minute.
  General Crown-heel length 2.2 cm
Unit 8:   7 to 8 Weeks 
7 weeks, 1 daySkeletal Upper limbs with slightly flexed elbows
  Special Senses Sacrocaudal spinal cord formation (secondary neurulation) complete
7 weeks, 1 day - 8 weeksGastrointestinal Stomach: Folds in stomach wall
7 weeks, 2 daysCardiovascular Arteries and veins of heart complete
7 weeks, 3 daysSkeletal The knee joints have arrived
  Skeletal Wrists slightly flexed
  Skeletal Eyelids growing rapidly
  Special Senses Cerebral hemispheres cover more than half of diencephalon
  Movement Hands begin to touch face
  Movement The hands touch each other as do the feet!
  Skin Fingertips thicken
  Skin Plantar pads toes
  Cardiovascular EKG pattern similar to adult
7 weeks, 4 daysSkeletal The fingers are free
7 weeks, 5 daysSkeletal Bone-forming cells called osteoblasts emerge
  Skeletal Hands can reach one another and fingers can overlap
  Special Senses Brain: Internal capsule with connections to epithalamus, dorsal thalamus, and mesencephalon
  Special Senses Cerebral hemispheres cover 75% of diencephalon
  Special Senses Cortical plate expanding rapidly
7 weeks, 6 daysSkeletal The toes are free
8 weeksMovement Complex response to touch
  Movement More frequent hand-to-face contact
  Movement Mouth opens & closes
  Movement Squinting
  Movement The embryo floats and rolls over in the womb
  Skin Hairs first appear in eyebrows & around mouth
  Skin Skin multi-layered, loses transparency
  Endocrine Male embryos are making testosterone already!
  Skeletal The embryo's joints are similar to adult joints
  Muscular Diaphragm complete
  Muscular Esophagus: Longitudinal muscles
  Renal/Urinary Urethra
  Renal/Urinary Urine production and release
  Gastrointestinal Peristalsis in large intestine
  Respiratory Occasional breathing motions begin
  Cardiovascular Blood supply to the brain closely resembles adult pattern
  Nervous Cranial nerve distribution mimics adult pattern
  Nervous Retina: Four of the ten adult layers present
  Nervous Tympanic membrane
  Special Senses "The hindbrain "presents striking resemblance to that of the newborn."
  Special Senses Brain represents 43% of embryo
  Special Senses Grey and white matter
  Special Senses Right- and left-handedness emerges
  General Crown-heel length 4.3 cm
  General Embryo contains approximately 1 billion (10^9) cells
  General Embryonic Period Ends
  General The 8-week embryo has formed more than 4,000 permanent body parts.
Unit 9:   8 to 9 Weeks 
  General Eyelids completely fused
  Special Senses Neurons synapse in cerebral cortex (marginal zone)
9 weeksMovement Bends hip & knee if sole of foot touched
  Movement Drinking fluid is becoming routine
  Movement Sucking the thumb
  Movement The young fetus now sighs, stretches, moves the head, opens the mouth, and moves the tongue
  Movement Tongue movement
  Reproductive Female fetuses have early reproductive cells in their ovaries
  Endocrine Thyroid gland weighs 2 grams
  Gastrointestinal Small intestine peristalsis
  Special Senses Face, hands, and feet sense light touch
Unit 10:   9 to 10 Weeks 
9 weeks - 10 weeksRespiratory Early vocal cords
  General My weight will rise more than 75% this week
  Movement I yawn when I want
9 weeks, 4 daysSpecial Senses Yawns
10 weeksMovement Eyes roll downward reflexively
  Blood & Immune Palatine tonsils
  Skin Fingernails and toenails begin to grow!
  Skin Three-layered epidermis
  Skin Tiny unique fingerprints have arrived!
  Skeletal Now, all the bones are getting harder
  Skeletal Tooth buds (secondary teeth)
  Renal/Urinary Glomeruli formation begins
  Gastrointestinal Physiologic herniation ends
  Special Senses Corpus callosum begins
  General Crown-heel length 7.5 cm
Unit 11:   10 to 11 Weeks 
10 weeks - 12 weeksBlood & Immune Langerhans cells enter epidermis
  Skin Volar and plantar pads regress
11 weeksMovement The face now makes complex expressions
  Blood & Immune Immunological competence
  Skin Intermediate layer
  Skin Nose & lips completely formed
  Reproductive Now you can tell if your baby is a girl or a boy!
  Endocrine Thyroid gland weighs 12 grams
  Gastrointestinal Intestines absorb water & glucose
  Nervous Auditory cells: inner & outer hair cells
  General Crown-heel length
Unit 12:   11 to 12 Weeks 
11 weeks - 12 weeksGeneral Weight increases by 60% this week
12 weeksMovement Hands touch the mouth up to 50 times per hour
  Blood & Immune T lymphocytes leave thymus
  Endocrine Many different hormones are present in pituitary gland
  Endocrine Thyroid gland produces hormone
  Skeletal Palate fuses
  Skeletal Upper limbs reach final proportion
  Renal/Urinary Bladder resembles smooth muscle
  Gastrointestinal Bowel movements
  Gastrointestinal Liver: Bile production begins
  Nervous There are taste buds all over the mouth
  Special Senses Corpus callosum
  General Crown-heel length 12 cm
  General Head circumference 10 cm
Unit 13:   3 to 4 Months 
13 weeksSkeletal Teeth are growing
  Respiratory Cilia lining airways
  Special Senses Most of body sensitive to touch
  General Crown-heel length 15 cm
14 weeksMovement Girls move their jaws more than the boys do
  Movement Light touch to mouth evokes turn toward stimulus
  Special Senses 4-lobed cerebral cortex
  Special Senses Cerebellum resembles adult structure
  General Crown-heel length 17 cm
  General Fat deposits in cheeks
15 weeksBlood & Immune Stem cells arrive in bone marrow
  Skin Body fat emerges throughout the body
  Endocrine Glucagon in fetal bloodstream
  Gastrointestinal Digestive enzymes
  General Crown-heel length 19.5 cm
16 weeksMovement Quickening
  Skin Fat deposits upper & lower limbs
  Skeletal Tooth enamel
  Respiratory Bronchial tree nearly complete
  Nervous Hormonal stress response to invasive procedures
  General Crown-heel length 21 cm
Unit 14:   4 to 5 Months 
17 weeksNervous Retina has discrete layers
18 weeksSkin Cream-like substance protects skin
  Skin Sweat glands
  Endocrine Insulin secretion
  Respiratory Speaking motion of larynx
  Special Senses Corpus callosum complete
19 weeksSkin Melanin production
  Reproductive Number of oogonia peak (at about 7 million) within fetal ovaries
  Special Senses Daily cycles in biological rhythms
20 weeksSkin All skin layers and structures
  Respiratory Surfactant production (low levels)
  Nervous Hearing and responding to sound begins
  Nervous Hearing and responding to sound begins
  General Crown-heel length 28 cm
  General Head circumference 20 cm
Unit 15:   5 to 6 Months 
20 weeks - 24 weeksGeneral Eyelids separate, eyes open and close
21 weeksSkin Stratum corneum
21 weeks - 22 weeksGeneral If born prematurely from this point on, survival is possible
22 weeksGeneral Cornea structure
  Special Senses Behavioral states
23 weeksGeneral Brain weight 100 grams
24 weeksNervous Blink-startle response; females before males
  General Crown-heel length 34.5 cm
Unit 16:   6 to 7 Months 
25 weeksGastrointestinal Intestinal lining contains all adult cell types
  Nervous Rods & cones
  Nervous The ability to taste
26 weeksSkin Additional fat deposits decrease wrinkles
  Skin Tear production
  Nervous The ability to smell has arrived
26 weeks - 38 weeksSpecial Senses Brain weight increases 400% to 500%
27 weeksNervous Pupils react to light
28 weeksNervous Distinguishes sounds of different frequencies
  General Crown-heel length 39.5 cm
Unit 17:   7 to 8 Months 
30 weeksRespiratory Breathing motions are common even though there is no air in the womb
  Special Senses 6-layered cerebral cortex
  General Head circumference 30 cm
32 weeksMuscular Esophagus: Lower esophagus muscles functional
  Renal/Urinary Glomeruli formation complete
  Respiratory Alveoli
  Special Senses Memory - music preferences
  General Crown-heel length 45 cm
Unit 18:   8 to 9 Months 
32 weeks - 36 weeksNervous Prenatal food affects newborn taste preferences
34 weeksGeneral Rapid weight gain
35 weeksMuscular Firm grip
  General Amniotic fluid volume peaks
36 weeksRespiratory Surfactant production accelerates
  General Brain weight 300 grams
  General Crown-heel length 48.5 cm
Unit 19:   9 Months to Birth 
37 weeksGastrointestinal Fetus drinks an estimated 15 oz (or 450cc) of amniotic fluid/day
38 weeksRespiratory Air breathing begins
  Cardiovascular By term, the typical umbilical cord measures 20 to 24 inches (50 to 60 cm)
  Cardiovascular Heart beats 54 million times before birth
  Cardiovascular Major circulatory changes
  Special Senses Spinal cord ends at third lumbar vertebrae
  General Brain weight 350 grams
  General Crown-heel length 50 cm
  General Fetus initiates labor
  General Head circumference 35 cm
  General Time to be born!