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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	Unit 1: The First Week
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(Day 0): Embryonic Period Begins Fertilization Zygote formation (Day 1): Activation of the genome (Day 1 - Day 2): First cell division - 24 to 30 hours Early pregnancy factor (EPF) (Day 3): Morula (meaning mulberry) (Day 4): Free-floating blastocyst Blastocyst Dorsal & ventral surfaces Embryonic disc (Day 5): Hatching Inner cell mass (Day 6): Implantation begins with attachment of blastocyst to uterine wall (1 week): Amniotic cavity formation begins Hypoblast & epiblast Placenta formation begins Less Events...

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	Unit 3: 2 to 3 Weeks
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(2 weeks, 1 day): 3 germ layers Bilateral symmetry (2 weeks, 2 days): Amnion with two cell layers Secondary villi Erythroblasts in yolk sac Three types of blood-forming cells in yolk sac (2 weeks, 3 days): Portion of the thyroid (2 weeks, 4 days): Connecting stalk Neural plate Foregut, midgut, and hindgut Brain is first organ to appear (2 weeks, 6 days): Somites 1 - 3 pairs of somites Forebrain, midbrain, and hindbrain Neural crest Neural groove deepens substantially Otic disc Stomodeum forming Blood vessels appear in yolk sac, body, and amnion Dorsal aortae (paired) Heart Paired tubular heart Foregut Numerous blood-forming locations in yolk sac (3 weeks): Blood cells 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 (4½ weeks): Spleen (4 weeks, 4 days): Cerebral hemispheres Eyes located on sides of head Lens pit Sinu-atrial (SA) node Right and left main stem bronchi (4 weeks, 5 days): First nerve fibers Five secondary brain subdivisions Hypothalamus Lateral ventricle Most cranial nerves seen Synapses among motor neurons in spinal cord Third ventricle Lens vesicles beneath surface Nasal pits Optic chiasma Blood vessels penetrate diencephalon Coronary arteries (terminal end) Cecum (5 weeks): Pigmented retina Pacemaker cells in sinus venosus Bronchial tree branching accelerates Lungs have 3 right lobes & 2 left lobes Circular muscles in esophagus Permanent kidneys Hand plates ACTH [adrenocorticotropin hormone] Growth hormone Pituitary gland Head is 1/3 of entire embryo Less Events...

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	Unit 7: 6 to 7 Weeks
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(6 weeks, 2 days): Brainwaves Cerebrospinal fluid production begins Nasolacrimal duct Optic fibers Origin of left coronary artery Submandibular glands Hands polygon-shaped (6½ weeks): Hand movement Ossification in clavicle & jaws Volar pads on palms (6 weeks, 5 days): Occipital pole of cerebral hemispheres Cartilaginous styloid process Circle of Willis complete Origin of right coronary artery Beginnings of occipital and sphenoid bones Three semicircular ducts (6 weeks, 6 days): Membrane blocking rectum ruptures Feet polygon-shaped (7 weeks): Head rotates Leg movements 4-chambered heart nearly complete Heart rate peaks (165 to 170 beats/minute) Hiccups Ovaries Testes begin to differentiate Foot plates notched Tendons attach muscle to bone Insulin in pancreas B lymphocytes in liver 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): Neurons synapse in cerebral cortex Eyelids completely fused Plantar pads toes (9 weeks): Face, hands, and feet sense light touch 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 Small intestine peristalsis Oogonia replicating in fetal ovary Uterus Thyroid gland weighs 2 grams (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): Thickened pads begin regressing on hands and feet (11 weeks): Auditory cells: inner & outer hair cells The face now makes complex expressions Nose & lips completely formed Intestines absorb water & glucose External genitalia clearly male or female Intermediate layer Thyroid gland weighs 12 grams Immunological competence 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): Most of body sensitive to touch Tooth development Cilia lining airways Crown-heel length 15 cm (14 weeks): 4-lobed cerebral cortex Cerebellum resembles adult structure Light touch to mouth evokes turn toward stimulus Fat deposits in cheeks Jaw motion more frequent in females Crown-heel length 17 cm (15 weeks): Digestive enzymes Fat deposits in abdomen, back, kidneys & shoulders Glucagon in fetal bloodstream Stem cells arrive in bone marrow Crown-heel length 19.5 cm (16 weeks): Hormonal stress response to invasive procedures Quickening Tooth enamel Bronchial tree nearly complete Fat deposits upper & lower limbs Crown-heel length 21 cm Less Events...

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	Unit 15: 5 to 6 Months
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(21 weeks): Sratum corneum (21 weeks - 22 weeks): Age of viability (22 weeks): Behavioral states Cornea structure (23 weeks): Brain weight 100 grams (24 weeks): Blink-startle response; females before males Crown-heel length 34.5 cm

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	Unit 17: 7 to 8 Months
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(30 weeks): 6-layered cerebral cortex Breathing motions 30% to 40% of the time Head circumference 30 cm (32 weeks): Memory - music preferences Alveoli Lower esophagus muscles functional Glomeruli formation complete Crown-heel length 45 cm (32 weeks - 36 weeks): Prenatal food affects newborn taste preferences

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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): Birth Fetus initiates labor via estrogen release Spinal cord ends at third lumbar vertebrae Major circulatory changes Transition to air breathing at birth Brain weight 350 grams Crown-heel length 50 cm Head circumference 35 cm Heart beats 54 million times before birth This timeline has more than 3.1 million configurations 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

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	Unit 2: 1 to 2 Weeks
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(1 week, 1 day): HCG (human chorionic gonadotropin) (1 week, 2 days): Cells in womb engorged with nutrients Chorion Primary yolk sac (1 week, 4 days): Longitudinal axis (1 week, 5 days): Implantation complete (1 week, 6 days): Amnion with single cell layer Chorionic villi Primitive streak Secondary yolk sac Less Events...

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	Unit 4: 3 to 4 Weeks
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(3 weeks, 1 day): Body cavities Folding of the embryo begins Neural tube Hyoid arch Atria (right and left) far apart Circulatory system function begins Endocardial tubes fuse forming tubular heart Heart begins beating Pericardium Sinus venosus Tubular heart begins folding Umbilical arteries Umbilical veins (right and left) Respiratory primordium Midgut emerging (3 weeks, 3 days): Neuropore (near brain) closes Notochord Membrane between future mouth and throat may begin to rupture Cystic primordium Liver Thyroid complete (3 weeks, 5 days): 21 - 29 pairs of somites Progressively C-shaped embryo Brain involves 40% of neural tube Lowermost spinal cord formation begins Neuropore (lower back) closes Pharyngeal arches 1, 2, and 3 Descending aorta Internal carotid arteries Unidirectional circulation Lung bud First part of pancreas Earliest hint of upper limbs (4 weeks): Amnion surrounds embryo Umbilical cord Cerebellum Fourth ventricle Most cranial nerve ganglia 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 Bronchial buds Lungs begin filling chest cavity Right and left lung buds Trachea or windpipe Esophagus and stomach Intestines growing in length Pharynx Small & large intestines Ventral pancreas Upper & lower limb buds Upper and lower limb buds Periderm Skin transparent Less Events...

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	Unit 6: 5 to 6 Weeks
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(5 weeks, 1 day): Wrists (5 weeks, 2 days): All cranial nerves identifiable Cochlear nerve present Atrioventricular (AV) node Circle of Willis almost complete Thyroid detaches from pharynx (5½ weeks): Initial tooth formation Cartilage 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): 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 Appendix Cartilage in occipital sclerotomes (1-4) (6 weeks): Cerebral cortex Synapses form in spinal cord External ear Face withdraws from light touch around mouth Tooth buds (primary teeth) Inferior vena cava Internal and external jugular veins Diaphragm largely formed Intestines fill base of umbilical cord Handplates develop subtle flattening Joints Nipples along side of trunk Adrenal glands Glucagon in pancreas Parathyroid glands Blood formation & lymphocytes in liver 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): Lowermost spinal cord formation complete (7 weeks, 1 day - 8 weeks): Folds in stomach wall (7 weeks, 2 days): Coronary arteries and veins complete (7 weeks, 3 days): Cerebral hemispheres cover more than half of diencephalon Eyelids grow rapidly Cartilaginous nasal septum (7½ weeks): Hands begin to touch face EKG pattern similar to adult Hands & feet touch in midline Knee joints Plantar pads toes Volar pads on fingertips (7 weeks, 4 days): Fingers completely separate (7 weeks, 5 days): Cerebral hemispheres cover 75% of diencephalon Cortical plate expanding rapidly Internal capsule (7 weeks, 6 days): Toes free (8 weeks): Embryonic Period Ends Brain highly complex Brain represents 43% of embryo Cranial nerve distribution resembles adult pattern Handedness Pons Retina with 4 of the10 layers found in adult Complex response to touch More frequent hand-to-face contact Squinting The embryo floats and rolls over in the womb Mouth opens & closes Blood supply to the brain closely resembles adult pattern Superior vena cava Breathing motion Diaphragm complete Longitudinal muscles in esophagus Peristalsis in large intestine Urine production and release Joints resemble adult joints Hair in eyebrows & around mouth Skin multi-layered, loses transparency Hypothalamus Testes release testosterone Crown-heel length 4.3 cm Embryo contains approximately 1 billion (10^9) cells Over 4,000 body structures 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): Corpus callosum begins Eyes roll downward reflexively Tonsils Tooth buds (permanent teeth) Physiologic herniation ends Glomeruli formation begins Ossification underway in most bones Fingernails & toenails begin Skin with three layers Three-layered epidermis Unique fingerprints 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): Corpus callosum Taste buds throughout mouth Hands touch the mouth up to 50 times per hour Palate fuses Bile production begins in liver Bowel movements Bladder resembles smooth muscle Upper limbs reach final proportion Thyroid gland produces hormone T lymphocytes leave thymus Crown-heel length 12 cm Head circumference 10 cm

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	Unit 14: 4 to 5 Months
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(17 weeks): Cerebral cortex activity Retina has discrete layers (18 weeks): Corpus callosum complete Speaking motion of larynx sweat glands Vernix caseosa covers fetus Insulin secretion (19 weeks): Circadian rhythms Number of oogonia peak within fetal ovaries Melanin production (20 weeks): Cochlea reaches adult size Organ of hearing reaches adult size Surfactant production (low levels) All skin layers and structures 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): Responds to taste Rods & cones Intestinal lining contains all adult cell types (26 weeks): Sense of smell Tear production Additional fat deposits decrease wrinkles (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): Amniotic fluid volume peaks Firm grip (36 weeks): Surfactant production accelerates Brain weight 300 grams Crown-heel length 48.5 cm

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Nervous	Cardiovascular	Musculoskeletal	Early Events
Special Senses	Respiratory	Skin	Growth Parameters
Movement	Gastrointestinal	Endocrine	General
Facial	Genitourinary	Blood & Immune	"Gee Whiz" Facts