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
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 2Early Events Zygote divides into two blastomeres (24 – 30 hours from start of fertilization)
Day 4Early Events Free floating blastocyst
  Early Events Inner cell mass
  Early Events See where the back and chest will be
Day 6Early Events Embryo attaches to wall of uterus
1 weekEarly Events Placenta begins to form
Unit 2:   1 to 2 Weeks 
1 week, 1 dayEarly Events Positive pregnancy test
1 week, 2 daysEarly Events Cells in womb engorged with nutrients
1 week, 5 daysEarly Events Implantation complete
  Early Events Yolk sac
Unit 3:   2 to 3 Weeks 
2 weeks, 1 dayEarly Events Rostral-caudal orientation
2 weeks, 2 daysBlood & Immune Three types of blood-forming cells in yolk sac
2 weeks, 4 daysGastrointestinal Foregut, midgut, and hindgut
  Special Senses Brain is first organ to appear
2 weeks, 6 daysCardiovascular Beginnings of the heart can be seen
  Special Senses Forebrain, midbrain, and hindbrain
  Special Senses Three main divisions of brain
3 weeksCardiovascular Blood and blood vessels
Unit 4:   3 to 4 Weeks 
3 weeks, 1 dayRespiratory Respiratory outgrowth
  Cardiovascular Circulatory system function begins
  Cardiovascular Heart begins beating
  Cardiovascular Tubular heart begins folding
  Cardiovascular Umbilical arteries
  Cardiovascular Umbilical veins (right and left)
  Early Events Body cavities
3 weeks, 3 daysGastrointestinal Liver
  Gastrointestinal Membrane between future mouth and throat may begin to rupture
3 weeks, 5 daysGastrointestinal First part of pancreas
  Respiratory Lung bud
  Cardiovascular Descending aorta
  Special Senses Lowermost spinal cord formation begins
  Special Senses Neural tube closes (lower back)
  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 Pancreas: Ventral pancreas
  Gastrointestinal Pharynx
  Gastrointestinal Small & large intestines
  Respiratory Lungs begin filling chest cavity
  Respiratory Trachea
  Cardiovascular Circulatory system "well established"
  Cardiovascular Functioning two-chamber heart
  Cardiovascular Heart rate (about) 113 beats/min
  Special Senses Fourth ventricle
  Early Events Cervical flexure
  Early Events Limb buds - the first sign of arms and legs
  Early Events Umbilical cord emerging
  Early Events Upper and lower limb buds
Unit 5:   4 to 5 Weeks 
4 weeks, 4 daysEarly Events Nose: Nasal pits
  Special Senses Brain: Cerebral hemispheres appear and begin rapid growth
4 weeks, 5 daysCardiovascular Blood vessels penetrate diencephalon
  Special Senses Brain with five main sections
  Special Senses First nerve fibers
5 weeksRespiratory Lobar pattern mimics adult pattern
  Cardiovascular Pacemaker cells
  General Head is one third of entire embryo
Unit 6:   5 to 6 Weeks 
5 weeks, 2 daysSpecial Senses All cranial nerves identifiable
  Skeletal Initial tooth formation
  Movement Subtle movement begins
5 weeks, 6 daysGastrointestinal Primordial vermiform appendix
6 weeksMovement Face withdraws from light touch around mouth
  Blood & Immune Blood forming in liver
  Skeletal Tooth buds (primary teeth)
  Gastrointestinal Intestines fill base of umbilical cord
  Gastrointestinal External ears
Unit 7:   6 to 7 Weeks 
6 weeks, 2 daysSkeletal Elbow regions sometimes identifiable
  Skeletal Humerus, radius, and ulna
  Gastrointestinal Submandibular gland primordia
  Special Senses Brainwave activity has begun
  Movement The hands begin to move
  Skeletal Bones first form in the collar bones and lower jaw
6 weeks, 5 daysSkeletal Arms point forward
  Skeletal Beginnings of occipital and sphenoid bones
6 weeks, 6 daysGastrointestinal Cloacal membrane ruptures
7 weeksMovement Head rotates
  Movement Leg movements
  Reproductive Ovaries
  Muscular Hiccups
  Cardiovascular The heart has four chambers and is nearly complete.
  Cardiovascular The heart rate peaks at 165 to 170 beats per minute.
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 Eyelids growing rapidly
  Skin Fingertips thicken
  Cardiovascular EKG pattern similar to adult
7 weeks, 4 daysSkeletal The fingers are free
7 weeks, 5 daysSkeletal Bone-forming cells emerge
  Skeletal Hands can reach one another and fingers can overlap
8 weeksMovement Complex response to touch
  Movement More frequent hand-to-face contact
  Renal/Urinary Urine production and release
  Respiratory Occasional breathing motions begin
  Cardiovascular Blood supply to the brain closely resembles adult pattern
  Nervous Ear drum
  Special Senses "The hindbrain "presents striking resemblance to that of the newborn."
  Special Senses Brain represents 43% of embryo
  Special Senses Right- and left-handedness emerges
  General Embryonic Period Ends
Unit 9:   8 to 9 Weeks 
  General Eyelids completely fused
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
  Endocrine Thyroid gland weighs 2 grams
  Special Senses Face, hands, and feet sense light touch
Unit 10:   9 to 10 Weeks 
9 weeks - 10 weeksGeneral My weight will rise more than 75% this week
9 weeks, 4 daysSpecial Senses Yawns
10 weeksBlood & 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)
Unit 11:   10 to 11 Weeks 
  Skin Volar and plantar pads regress
11 weeksSkin 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
Unit 12:   11 to 12 Weeks 
12 weeksEndocrine Many different hormones are present in pituitary gland
  Skeletal Upper limbs reach final proportion
  Gastrointestinal Bowel movements
  Gastrointestinal Liver: Bile production begins
  Nervous There are taste buds all over the mouth
  General Head circumference 10 cm
Unit 13:   3 to 4 Months 
13 weeksSkeletal Teeth are growing
  Special Senses Most of body sensitive to touch
14 weeksMovement Girls move their jaws more than the boys do
  Movement Light touch to mouth evokes turn toward stimulus
  Special Senses Cerebellum resembles adult structure
  General Fat deposits in cheeks
15 weeksSkin Body fat emerges throughout the body
16 weeksMovement Quickening
  Skin Fat deposits upper & lower limbs
Unit 14:   4 to 5 Months 
18 weeksSkin Cream-like substance protects skin
  Skin Sweat glands
19 weeksSkin Melanin production
  Special Senses Daily cycles in biological rhythms
20 weeksSkin All skin layers and structures
  Nervous Hearing and responding to sound begins
  Nervous Hearing and responding to sound begins
  General Head circumference 20 cm
Unit 15:   5 to 6 Months 
20 weeks - 24 weeksGeneral Eyelids separate, eyes open and close
21 weeks - 22 weeksGeneral If born prematurely from this point on, survival is possible
23 weeksGeneral Brain weight 100 grams
Unit 16:   6 to 7 Months 
25 weeksGastrointestinal Intestinal lining contains all adult cell types
  Nervous The ability to taste
26 weeksSkin Additional fat deposits decrease wrinkles
  Nervous The ability to smell has arrived
26 weeks - 38 weeksSpecial Senses Brain weight increases 400% to 500%
27 weeksNervous Pupils react to light
Unit 17:   7 to 8 Months 
30 weeksRespiratory Breathing motions are common even though there is no air in the womb
  General Head circumference 30 cm
32 weeksMuscular Esophagus: Lower esophagus muscles functional
Unit 18:   8 to 9 Months 
32 weeks - 36 weeksNervous Prenatal food affects newborn taste preferences
35 weeksMuscular Firm grip
36 weeksGeneral Brain weight 300 grams
Unit 19:   9 Months to Birth 
37 weeksGastrointestinal Fetus drinks an estimated 15 oz (or 450cc) of amniotic fluid/day
38 weeksCardiovascular Heart beats 54 million times before birth
  General Brain weight 350 grams
  General Fetus initiates labor
  General Head circumference 35 cm
  General Time to be born!