| Unit 1: The First Week | |
| Day 0 | – | – |  Embryonic period begins |
| | | Fertilization resulting in zygote formation |
| Day 1 | – | – | Embryo is spherically shaped with 12 to 16 cells |
| Day 1 - Day 1 | – | – | Fertilization - development begins with a single-cell embryo!!! |
| Day 2 | – | – |  Early pregnancy factor (EPF) |
| | | Activation of the genome |
| | | Zygote divides into two blastomeres (24 – 30 hours from start of fertilization) |
| Day 4 | – | – | Embryonic disc |
| | | Free floating blastocyst |
| | | Hypoblast & epiblast |
| | | Inner cell mass |
| | | See where the back and chest will be |
| Day 5 | – | – | Hatching blastocyst |
| Day 6 | – | – | Embryo attaches to wall of uterus |
| 1 week | – | – | Chorion |
| | | Placenta begins to form |
| Unit 2: 1 to 2 Weeks | |
| 1 week, 1 day | – | – | Amnioblasts present; amnion and amniotic cavity formation begins |
| | | Positive pregnancy test |
| 1 week, 2 days | – | – | Cells in womb engorged with nutrients |
| 1 week, 4 days | – | – | Longitudinal axis |
| 1 week, 5 days | – | – | Implantation complete |
| | | Yolk sac |
| 1 week, 6 days | – | – |  Primordial blood vessels |
| | | Amnion with single cell layer |
| | | Chorionic villi |
| 2 weeks | – | – | Yolk sac |
| | | Yolk sac |
| Unit 3: 2 to 3 Weeks | |
| 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 induced by notochordal process |
| | | 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 |
| | | Forebrain, midbrain, and hindbrain |
| | | Neural groove deepens substantially |
| | | Three main divisions of brain |
| | | Neural crest: Rostral and facial |
| 3 weeks | – | – | Blood and blood vessels |
| | | |
| Unit 4: 3 to 4 Weeks | |
| 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 |
| | | Internal carotid arteries |
| | | Neuropore (near brain) closes |
| | | Notochord |
| 3 weeks, 5 days | – | – | First part of pancreas |
| | | Pharyngeal arch 3 |
| | | Lung bud |
| | | Descending aorta |
| | | Unidirectional circulation |
| | | Brain involves 40% of neural tube |
| | | Lowermost spinal cord formation begins |
| | | Neural tube closes (lower back) |
| | | Somites: Pairs 21 through 29 |
| | | Upper limb primordium at level of somites 8 to 10 |
| | |  Progressively C-shaped embryo |
| 4 weeks | – | – |  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 |
| | | 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 |
| | |  Most cranial nerve ganglia |
| | | Cerebellum |
| | | Fourth ventricle |
| | | Amnion surrounds embryo |
| | | Limb buds - the first sign of arms and legs |
| | | Lower limb buds |
| | | Umbilical cord emerging |
| | | Upper and lower limb buds |
| Unit 5: 4 to 5 Weeks | |
| 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 |
| | | Lens pits |
| | | Nose: Nasal pits |
| | | Brain enlarges 50% since Carnegie Stage 13 |
| | | Brain: Cerebral hemispheres appear and begin rapid growth |
| | | Brain: Lateral ventricles |
| | | Hypothalamus |
| 4 weeks, 5 days | – | – | Caecum |
| | | Blood vessels penetrate diencephalon |
| | | Coronary arteries (terminal end) |
| | | Optic chiasm |
| | | Brain with five main sections |
| | | First nerve fibers |
| | | 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 |
| Unit 6: 5 to 6 Weeks | |
| 5 weeks, 1 day | – | – | Wrist joints are forming |
| 5 weeks, 2 days | – | – | Thyroid detaches from pharynx |
| | | Atrioventricular (AV) node |
| | | Circle of Willis almost complete |
| | | Cochlear nerve present |
| | | Musculocutaneous, radial, ulna, and median nerves enter upper limb bud |
| | | All cranial nerves identifiable |
| 5� weeks | – | – | Initial tooth formation |
| 5� weeks - 6 weeks | – | – |  Subtle movement begins |
| 5 weeks, 4 days | – | – | Cartilage formation |
| 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) |
| | | Primordial vermiform appendix |
| | | All spinal nerves present |
| | | Dura begins forming in basal area |
| | | Frontal and temporal poles of cerebral hemispheres |
| | | Somites: Pairs 38 and 39 |
| | | 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 |
| | | Handplates develop subtle flattening |
| | | Joints |
| | | Tooth buds (primary teeth) |
| | |  Diaphragm is largely formed |
| | | Intestines fill base of umbilical cord |
| | | External ears |
| | | Synapses form in spinal cord |
| | | Crown-heel length 1.6 cm |
| | | |
| Unit 7: 6 to 7 Weeks | |
| 6 weeks, 2 days | – | – | Elbow regions sometimes identifiable |
| | | Hands polygon-shaped |
| | | Humerus, radius, and ulna |
| | | Toe rays sometimes present |
| | | Deltoid muscle |
| | | Submandibular gland primordia |
| | | Inferior vena cava |
| | | Left coronary artery arises from aorta |
| | | Optic fibers |
| | | Eyelid folds sometimes present |
| | | Brainwave activity has begun |
| | | Cerebrospinal fluid production begins |
| 6� 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 |
| | | Limbs point forward (ventrally) |
| | | Anal membrane |
| | | Lung, left: Oblique fissure defines upper and lower lobes |
| | | Circulus arteriosus (Circle of Willis) complete |
| | | Right coronary artery arises from aorta |
| | | Tricuspid and mitral valves |
| | | Primitive nasal cavity |
| | | Eyelids: Upper and lower lids present and growing |
| | | 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 |
| | | |
| Unit 8: 7 to 8 Weeks | |
| 7 weeks, 1 day | – | – | Upper limbs with slightly flexed elbows |
| | | Sacrocaudal spinal cord formation (secondary neurulation) complete |
| 7 weeks, 1 day - 8 weeks | – | – | Stomach: Folds in stomach wall |
| 7 weeks, 2 days | – | – | Arteries and veins of heart complete |
| 7 weeks, 3 days | – | – | The knee joints have arrived |
| | | Wrists slightly flexed |
| | | Eyelids growing rapidly |
| | | Cerebral hemispheres cover more than half of diencephalon |
| 7� weeks | – | – | Hands begin to touch face |
| | | The hands touch each other as do the feet! |
| | | Fingertips thicken |
| | | Plantar pads toes |
| | | 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 |
| | | Brain: Internal capsule with connections to epithalamus, dorsal thalamus, and mesencephalon |
| | | Cerebral hemispheres cover 75% of diencephalon |
| | | Cortical plate expanding rapidly |
| 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 |
| | | Hairs first appear 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 |
| | | Esophagus: Longitudinal muscles |
| | | Urethra |
| | | Urine production and release |
| | | Peristalsis in large intestine |
| | | Occasional breathing motions begin |
| | | Blood supply to the brain closely resembles adult pattern |
| | | 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 8-week embryo has formed more than 4,000 permanent body parts. |
| Unit 9: 8 to 9 Weeks | |
| 8� 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 |
| Unit 10: 9 to 10 Weeks | |
| 9 weeks - 10 weeks | – | – | Early vocal cords |
| | | My weight will rise more than 75% this week |
| 9� 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 |
| Unit 11: 10 to 11 Weeks | |
| 10 weeks - 12 weeks | – | – | Langerhans cells enter epidermis |
| 10� 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 your baby is a girl or a boy! |
| | | Thyroid gland weighs 12 grams |
| | | Intestines absorb water & glucose |
| | | Auditory cells: inner & outer hair cells |
| | | Crown-heel length |
| Unit 12: 11 to 12 Weeks | |
| 11 weeks - 12 weeks | – | – | Weight increases by 60% this week |
| 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 |
| | | Bowel movements |
| | | Liver: Bile production begins |
| | | There are taste buds all over the mouth |
| | | Corpus callosum |
| | | Crown-heel length 12 cm |
| | | Head circumference 10 cm |
| | | |
| Unit 13: 3 to 4 Months | |
| 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 |
| Unit 14: 4 to 5 Months | |
| 17 weeks | – | – | Retina has discrete layers |
| 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 (at about 7 million) 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 |
| Unit 15: 5 to 6 Months | |
| 20 weeks - 24 weeks | – | – | Eyelids separate, eyes open and close |
| 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 |
| | | |
| Unit 16: 6 to 7 Months | |
| 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 |
| | | |
| Unit 17: 7 to 8 Months | |
| 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 | – | – | Esophagus: Lower esophagus muscles functional |
| | | Glomeruli formation complete |
| | | Alveoli |
| | | Memory - music preferences |
| | | Crown-heel length 45 cm |
| | | |
| Unit 18: 8 to 9 Months | |
| 32 weeks - 36 weeks | – | – | Prenatal food affects newborn taste preferences |
| 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 |
| Unit 19: 9 Months to Birth | |
| 37 weeks | – | – | Fetus drinks an estimated 15 oz (or 450cc) of amniotic fluid/day |
| 38 weeks | – | – | Air breathing begins |
| | | By term, the typical umbilical cord measures 20 to 24 inches (50 to 60 cm) |
| | | 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! |
Growth Parameters