7 1/2 week embryo, 7.5 week embryo, seven and a half week embryo

Interactive Prenatal Development Timeline

[All ages refer to time since fertilization] [Features] [Help] [Tutorial] [Print preview] [PDF]

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0-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 10-11 11-12 12-16 16-20 20-24 24-28 28-32 32-36 36-38

	Unit 1: The First Week
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(Day 0 - Day 1): Fertilization - development begins with a single-cell embryo!!! Fertilization resulting in zygote formation Embryonic Period Begins (Day 1): Activation of the genome (Day 1 - Day 2): Early pregnancy factor (EPF) (Day 2): Zygote divides into two blastomeres (perhaps 30 hours or so after fertilization) Zygote divides into two blastomeres (perhaps 30 hours or so after fertilization) (Day 3): Embryo is spherically shaped with 12 to 16 cells (Day 4): Embryonic disc Free floating blastocyst See where the back and chest will be (Day 5): Hatching Inner cell mass (Day 6): The embryo implants into wall of womb (1 week): Amnioblasts present; amnion and amniotic cavity formation begins Chorion Hypoblast & epiblast Placenta begins to form Less Events...

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	Unit 3: 2 to 3 Weeks
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(2 weeks, 1 day): 3 germ layers Rostral-caudal orientation (2 weeks, 2 days): Erythroblasts in yolk sac Three types of blood-forming cells in yolk sac Amnion with two cell layers Secondary villi (2 weeks, 4 days): Foregut, midgut, and hindgut Brain is first organ to appear Neural plate Connecting stalk (2 weeks, 6 days): Numerous blood islands in umbilical vesicle Foregut Stomodeum forming Beginnings of the heart can be seen Blood vessels emerge simultaneously in umbilical vesicle, embryo proper, amnion, and connecting stalk Dorsal aortae (paired) Paired tubular heart Otic disc Forebrain, midbrain, and hindbrain Neural crest Neural groove deepens substantially Three main divisions of brain Somites: 1 - 3 pairs (3 weeks): Blood and blood vessels Less Events...

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	Unit 5: 4 to 5 Weeks
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(4 weeks, 3 days): Early eyes (4 weeks, 3 days - 5 weeks): Germ cells migrate to gonads (4 weeks, 4 days): Lungs: Right and left primary (or main stem) bronchi Sinu-atrial (SA) node Eyes located on sides of head Brain enlarges 50% since Carnegie Stage 13 Brain: Cerebral hemispheres appear and begin rapid growth Brain: Lateral ventricles (4 weeks, 5 days): Cecum Blood vessels penetrate diencephalon Coronary arteries (terminal end) Lens vesicles beneath surface Optic chiasm Brain with five main sections First nerve fibers Hypothalamus Most cranial nerves seen Synapses among motor neurons in spinal cord Third ventricle (5 weeks): ACTH [adrenocorticotropin hormone] Growth hormone Pituitary gland Limb buds form hand plates Permanent kidneys Bronchial tree branching accelerates Lobar pattern mimics adult pattern Pacemaker cells Head is one third of entire embryo Less Events...

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	Unit 7: 6 to 7 Weeks
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(6 weeks, 2 days): Elbow regions sometimes identifiable Hands polygon-shaped Toe rays sometimes present Submandibular gland primordia Inferior vena cava Origin of left coronary artery Eyelid folds sometimes present Optic fibers Brainwave activity has begun Cerebrospinal fluid production begins (6½ weeks): The hands begin to move Volar pads on palms Bones first form in the collar bones and lower jaw (6 weeks, 5 days): Beginnings of occipital and sphenoid bones Cartilaginous styloid process Humerus, radius, and ulna Deltoid muscle Anal membrane Lung, left: Oblique fissure defines upper and lower lobes Circulus arteriosus (Circle of Willis) complete Origin of right coronary artery Tricuspid and mitral valves Eyelids: Upper and lower lids present and growing Primitive nasal cavity Three semicircular ducts or canals Occipital pole of cerebral hemispheres (6 weeks, 6 days): Feet polygon-shaped Cloacal membrane ruptures (7 weeks): Head rotates Leg movements B lymphocytes in liver Ovaries Testes begin to differentiate Insulin in pancreas Foot plates notched Hiccups Tendons attach muscle to bone The heart has four chambers and is nearly complete. The heart rate peaks at 165 to 170 beats per minute. Crown-heel length 2.2 cm Less Events...

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	Unit 9: 8 to 9 Weeks
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(8 weeks, 1 day): Fetal Period Begins (8½ weeks): Eyelids completely fused Neurons synapse in cerebral cortex (marginal zone) (9 weeks): Bends hip & knee if sole of foot touched Drinking fluid is becoming routine Sucking the thumb The young fetus now sighs, stretches, moves the head, opens the mouth, and moves the tongue Tongue movement Female fetuses have early reproductive cells in their ovaries Thyroid gland weighs 2 grams Small intestine peristalsis Face, hands, and feet sense light touch (9 weeks - 10 weeks): Early vocal cords My weight will rise more than 75% this week Less Events...

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	Unit 11: 10 to 11 Weeks
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(10½ weeks): Volar and plantar pads regress (11 weeks): The face now makes complex expressions Immunological competence Intermediate layer Nose & lips completely formed Now you can tell if it's a girl or boy! Thyroid gland weighs 12 grams Intestines absorb water & glucose Auditory cells: inner & outer hair cells Crown-heel length (11 weeks - 12 weeks): Weight increases by 60% this week Less Events...

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	Unit 13: 3 to 4 Months
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(13 weeks): Teeth are growing Cilia lining airways Most of body sensitive to touch Crown-heel length 15 cm (14 weeks): Girls move their jaws more than the boys do Light touch to mouth evokes turn toward stimulus 4-lobed cerebral cortex Cerebellum resembles adult structure Crown-heel length 17 cm Fat deposits in cheeks (15 weeks): Stem cells arrive in bone marrow Body fat emerges throughout the body Glucagon in fetal bloodstream Digestive enzymes Crown-heel length 19.5 cm (16 weeks): Quickening Fat deposits upper & lower limbs Tooth enamel Bronchial tree nearly complete Hormonal stress response to invasive procedures Crown-heel length 21 cm Less Events...

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	Unit 15: 5 to 6 Months
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(21 weeks): Stratum corneum (21 weeks - 22 weeks): If born prematurely from this point on, survival is possible (22 weeks): Cornea structure Behavioral states (23 weeks): Brain weight 100 grams (24 weeks): Blink-startle response; females before males Crown-heel length 34.5 cm Less Events...

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	Unit 17: 7 to 8 Months
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(30 weeks): Breathing motions are common even though there is no air in the womb 6-layered cerebral cortex Head circumference 30 cm (32 weeks): Glomeruli formation complete Lower esophagus muscles functional Alveoli Memory - music preferences Crown-heel length 45 cm (32 weeks - 36 weeks): Prenatal food affects newborn taste preferences Less Events...

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	Unit 19: 9 Months to Birth
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(37 weeks): Fetus drinks an estimated 15 oz (or 450cc) of amniotic fluid/day (38 weeks): Air breathing begins Heart beats 54 million times before birth Major circulatory changes Spinal cord ends at third lumbar vertebrae Brain weight 350 grams Crown-heel length 50 cm Fetus initiates labor Head circumference 35 cm Time to be born! Less Events...

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Fertilization

Week 1 ends

Play Movie - 1 to 2 weeks, implantation and HCG

Week 2 ends

Play Movie - 2 to 3 week embryo, amnion, germ layer

Week 3 ends

Play Movie - 3 to 4 week embryo, folding of the embryo

Week 4 ends

Play Movie - 4 to 5 week embryo, The Heart in Action

Week 5 ends

Play Movie - 5 to 6 week embryo, hand plates develop

Week 6 ends

Play Movie - 6 to 7 week embryo, hand movement

Week 7 ends

Play Movie - 7 to 8 week embryo, fingers, toes

Week 8 ends

Play Movie - 8 to 9 week fetus, reflexive movement

Week 9 ends

Play Movie - 9 to 10 week fetus, sucking, swallowing

Week 10 ends

Play Movie - 10 to 11 week fetus, eyes roll, yawns

Week 11 ends

Play Movie - 11 to 12 week fetus, nails, fingerprints

Month 3 ends

Play Movie - 3 to 4 month fetus, the rooting reflex

Month 4 ends

Play Movie - 4 to 5 month fetus, bronchial tree

Month 5 ends

Play Movie - 5 to 6 months fetus, responds to sound

Month 6 ends

Play Movie - 6 to 7 month fetus, response to light

Month 7 ends

Play Movie - 7 to 8 month fetus, behavioral states

Month 8 ends

Play Movie - 8 to 9 month fetus, firm hand grasp

Month 9 ends

Birth

393 events on this timeline

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	Unit 2: 1 to 2 Weeks
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(1 week, 1 day): Positive pregnancy test (1 week, 2 days): Cells in womb engorged with nutrients Yolk sac (1 week, 4 days): Longitudinal axis (1 week, 5 days): Implantation complete Primary yolk sac (1 week, 6 days): Primordial blood vessels Amnion with single cell layer Chorionic villi (2 weeks): Yolk sac Yolk sac Less Events...

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	Unit 4: 3 to 4 Weeks
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(3 weeks, 1 day): Midgut emerging Respiratory outgrowth Atria (right and left) far apart Circulatory system function begins Endocardial tubes fuse forming tubular heart Heart begins beating Pericardium Primary head vein Sinus venosus Tubular heart begins folding Umbilical arteries Umbilical veins (right and left) Neural tube Body cavities Hyoid arch (3 weeks, 3 days): Thyroid complete Cystic primordium Liver Membrane between future mouth and throat may begin to rupture Neuropore (near brain) closes Notochord (3 weeks, 5 days): First part of pancreas Pharyngeal arches 1, 2, and 3 Lung bud Descending aorta Internal carotid arteries Unidirectional circulation Brain involves 40% of neural tube Lowermost spinal cord formation begins Neural tube closes (lower back) Somites: 21 - 29 pairs Upper limb primordium at level of somites 8 to 10 Progressively C-shaped embryo (4 weeks): Periderm Skin is so thin, you can see through it! Esophagus primordia Intestines growing in length Pancreas: Ventral pancreas Pharynx Small & large intestines Bronchial buds Lungs begin filling chest cavity Right and left lung buds Trachea Circulatory system "well established" Functioning two-chamber heart Heart chambers bulging with fluid Heart now functions as two parallel pumps Heart rate (about) 113 beats/min Cerebellum Fourth ventricle Most cranial nerve ganglia Amnion surrounds embryo Limb buds - the first sign of arms and legs Lower limb buds Umbilical cord emerging Upper and lower limb buds Less Events...

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	Unit 6: 5 to 6 Weeks
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(5 weeks, 1 day): Wrist joints are forming (5 weeks, 2 days): Thyroid detaches from pharynx Atrioventricular (AV) node Circle of Willis almost complete Musculocutaneous, radial, ulna, and median nerves enter upper limb bud All cranial nerves identifiable Cochlear nerve present (5½ weeks): Cartilage formation Initial tooth formation (5½ weeks - 6 weeks): Subtle movement begins (5 weeks, 5 days): Nerve cells differentiating (5 weeks, 5 days - 7 weeks, 1 day): Melanocytes in epidermis (5 weeks, 6 days): Cartilage in occipital sclerotomes (1-4) Appendix All 39 somite pairs present All spinal nerves present Dura begins forming in basal area Frontal and tempororal poles of cerebral hemispheres Synapses in spinal cord between interneurons and primary afferent neurons (6 weeks): Face withdraws from light touch around mouth Blood forming in liver Nipples along side of trunk Adrenal glands Glucagon in pancreas Parathyroid glands Handplates develop subtle flattening Joints Tooth buds (primary teeth) Diaphragm is largely formed Intestines fill base of umbilical cord External ear Cerebral cortex Synapses form in spinal cord Crown-heel length 1.6 cm Less Events...

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	Unit 8: 7 to 8 Weeks
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(7 weeks, 1 day): Upper limbs with slightly flexed elbows Sacrocaudal spinal cord formation (secondary neurulation) complete (7 weeks, 1 day - 8 weeks): Folds in stomach wall (7 weeks, 2 days): Arteries and veins of heart complete (7 weeks, 3 days): Cartilaginous nasal septum Wrists slightly flexed Eyelids grow rapidly Cerebral hemispheres cover more than half of diencephalon (7½ weeks): Hands begin to touch face The hands touch each other as do the feet! Fingertips thicken Plantar pads toes The knee joints have arrived EKG pattern similar to adult (7 weeks, 4 days): The fingers are free (7 weeks, 5 days): Bone-forming cells called osteoblasts emerge Hands can reach one another and fingers can overlap Cerebral hemispheres cover 75% of diencephalon Cortical plate expanding rapidly Internal capsule with connections to epithalamus, dorsal thalamus, and mesencephalon (7 weeks, 6 days): The toes are free (8 weeks): Complex response to touch More frequent hand-to-face contact Mouth opens & closes Squinting The embryo floats and rolls over in the womb Hair in eyebrows & around mouth Skin multi-layered, loses transparency Male embryos are making testosterone already! The embryo's joints are similar to adult joints Diaphragm complete Urethra Urine production and release Longitudinal muscles in esophagus Peristalsis in large intestine Occasional breathing motions begin Blood supply to the brain closely resembles adult pattern Superior vena cava Cranial nerve distribution mimics adult pattern Retina: Four of the ten adult layers present Tympanic membrane "The hindbrain "presents striking resemblance to that of the newborn." Brain represents 43% of embryo Grey and white matter Right- and left-handedness emerges Crown-heel length 4.3 cm Embryo contains approximately 1 billion (10^9) cells Embryonic Period Ends The embryo has more than 4,000 parts (adults have about 4,500) Less Events...

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	Unit 10: 9 to 10 Weeks
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(9½ weeks): I yawn when I want (9 weeks, 4 days): Yawns (10 weeks): Eyes roll downward reflexively Palatine tonsils Fingernails and toenails begin to grow! Three-layered epidermis Tiny unique fingerprints have arrived! Now, all the bones are getting harder Tooth buds (secondary teeth) Glomeruli formation begins Physiologic herniation ends Corpus callosum begins Crown-heel length 7.5 cm (10 weeks - 12 weeks): Langerhans cells enter epidermis Less Events...

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	Unit 12: 11 to 12 Weeks
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(12 weeks): Hands touch the mouth up to 50 times per hour T lymphocytes leave thymus Many different hormones are present in pituitary gland Thyroid gland produces hormone Palate fuses Upper limbs reach final proportion Bladder resembles smooth muscle Bile production begins in liver Bowel movements There are taste buds all over the mouth Corpus callosum Crown-heel length 12 cm Head circumference 10 cm Less Events...

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	Unit 14: 4 to 5 Months
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(17 weeks): Retina has discrete layers Cerebral cortex activity (18 weeks): Cream-like substance protects skin Sweat glands Insulin secretion Speaking motion of larynx Corpus callosum complete (19 weeks): Melanin production Number of oogonia peak within fetal ovaries Daily cycles in biological rhythms (20 weeks): All skin layers and structures Surfactant production (low levels) Hearing and responding to sound begins Hearing and responding to sound begins Crown-heel length 28 cm Head circumference 20 cm (20 weeks - 24 weeks): Eyelids separate, eyes open and close Less Events...

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	Unit 16: 6 to 7 Months
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(25 weeks): Intestinal lining contains all adult cell types Rods & cones The ability to taste (26 weeks): Additional fat deposits decrease wrinkles Tear production The ability to smell has arrived (26 weeks - 38 weeks): Brain weight increases 400% to 500% (27 weeks): Pupils react to light (28 weeks): Distinguishes sounds of different frequencies Crown-heel length 39.5 cm Less Events...

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	Unit 18: 8 to 9 Months
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(34 weeks): Rapid weight gain (35 weeks): Firm grip Amniotic fluid volume peaks (36 weeks): Surfactant production accelerates Brain weight 300 grams Crown-heel length 48.5 cm

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