| Unit 1: The First Week | |
| Day 0 | – | – |  Embryonic period begins |
| 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 | – | – | Zygote divides into two blastomeres (24 – 30 hours from start of fertilization) |
| Day 4 | – | – | Free floating blastocyst |
| | | Inner cell mass |
| | | See where the back and chest will be |
| Day 6 | – | – | Embryo attaches to wall of uterus |
| 1 week | – | – | Placenta begins to form |
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| Unit 2: 1 to 2 Weeks | |
| 1 week, 1 day | – | – | Positive pregnancy test |
| 1 week, 2 days | – | – | Cells in womb engorged with nutrients |
| 1 week, 5 days | – | – | Implantation complete |
| | | Yolk sac |
| | | |
| Unit 3: 2 to 3 Weeks | |
| 2 weeks, 1 day | – | – | Rostral-caudal orientation |
| 2 weeks, 2 days | – | – |  Three types of blood-forming cells in yolk sac |
| 2 weeks, 4 days | – | – |  Foregut, midgut, and hindgut |
| | |  Brain is first organ to appear |
| 2 weeks, 6 days | – | – |  Beginnings of the heart can be seen |
| | | Forebrain, midbrain, and hindbrain |
| | | Three main divisions of brain |
| 3 weeks | – | – | Blood and blood vessels |
| | | |
| Unit 4: 3 to 4 Weeks | |
| 3 weeks, 1 day | – | – |  Respiratory outgrowth |
| | | Circulatory system function begins |
| | | Heart begins beating |
| | | Tubular heart begins folding |
| | | Umbilical arteries |
| | | Umbilical veins (right and left) |
| | | Body cavities |
| 3 weeks, 3 days | – | – | Liver |
| | | Membrane between future mouth and throat may begin to rupture |
| 3 weeks, 5 days | – | – | First part of pancreas |
| | | Lung bud |
| | | Descending aorta |
| | | Lowermost spinal cord formation begins |
| | | Neural tube closes (lower back) |
| | | 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 |
| | | Pancreas: Ventral pancreas |
| | | Pharynx |
| | | Small & large intestines |
| | | Lungs begin filling chest cavity |
| | | Trachea |
| | | Circulatory system "well established" |
| | | Functioning two-chamber heart |
| | | Heart rate (about) 113 beats/min |
| | | Fourth ventricle |
| | | Cervical flexure |
| | | Limb buds - the first sign of arms and legs |
| | | Umbilical cord emerging |
| | | Upper and lower limb buds |
| Unit 5: 4 to 5 Weeks | |
| 4 weeks, 4 days | – | – |  Nose: Nasal pits |
| | | Brain: Cerebral hemispheres appear and begin rapid growth |
| 4 weeks, 5 days | – | – | Blood vessels penetrate diencephalon |
| | | Brain with five main sections |
| | | First nerve fibers |
| 5 weeks | – | – | Lobar pattern mimics adult pattern |
| | | Pacemaker cells |
| | | Head is one third of entire embryo |
| | | |
| Unit 6: 5 to 6 Weeks | |
| 5 weeks, 2 days | – | – | All cranial nerves identifiable |
| 5� weeks | – | – |  Initial tooth formation |
| 5� weeks - 6 weeks | – | – |  Subtle movement begins |
| 5 weeks, 6 days | – | – | Primordial vermiform appendix |
| 6 weeks | – | – | Face withdraws from light touch around mouth |
| | | Blood forming in liver |
| | | Tooth buds (primary teeth) |
| | | Intestines fill base of umbilical cord |
| | | External ears |
| Unit 7: 6 to 7 Weeks | |
| 6 weeks, 2 days | – | – | Elbow regions sometimes identifiable |
| | | Humerus, radius, and ulna |
| | | Submandibular gland primordia |
| | | Brainwave activity has begun |
| 6� weeks | – | – | The hands begin to move |
| | | Bones first form in the collar bones and lower jaw |
| 6 weeks, 5 days | – | – | Arms point forward |
| | | Beginnings of occipital and sphenoid bones |
| 6 weeks, 6 days | – | – | Cloacal membrane ruptures |
| 7 weeks | – | – | Head rotates |
| | | Leg movements |
| | |  Ovaries |
| | |  Hiccups |
| | | The heart has four chambers and is nearly complete. |
| | | The heart rate peaks at 165 to 170 beats per minute. |
| 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 |
| | | Eyelids growing rapidly |
| 7� weeks | – | – | Fingertips thicken |
| | | EKG pattern similar to adult |
| 7 weeks, 4 days | – | – | The fingers are free |
| 7 weeks, 5 days | – | – | Bone-forming cells emerge |
| | | Hands can reach one another and fingers can overlap |
| 8 weeks | – | – | Complex response to touch |
| | | More frequent hand-to-face contact |
| | |  Urine production and release |
| | | Occasional breathing motions begin |
| | | Blood supply to the brain closely resembles adult pattern |
| | |  Ear drum |
| | | "The hindbrain "presents striking resemblance to that of the newborn." |
| | | Brain represents 43% of embryo |
| | | Right- and left-handedness emerges |
| | | Embryonic Period Ends |
| Unit 9: 8 to 9 Weeks | |
| 8� weeks | – | – | Eyelids completely fused |
| 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 |
| | |  Thyroid gland weighs 2 grams |
| | | Face, hands, and feet sense light touch |
| Unit 10: 9 to 10 Weeks | |
| 9 weeks - 10 weeks | – | – | My weight will rise more than 75% this week |
| 9 weeks, 4 days | – | – | Yawns |
| 10 weeks | – | – | 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) |
| | | |
| Unit 11: 10 to 11 Weeks | |
| 10� weeks | – | – | Volar and plantar pads regress |
| 11 weeks | – | – | 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 |
| Unit 12: 11 to 12 Weeks | |
| 12 weeks | – | – | Many different hormones are present in pituitary gland |
| | | Upper limbs reach final proportion |
| | | Bowel movements |
| | | Liver: Bile production begins |
| | | There are taste buds all over the mouth |
| | | Head circumference 10 cm |
| | | |
| Unit 13: 3 to 4 Months | |
| 13 weeks | – | – | Teeth are growing |
| | | Most of body sensitive to touch |
| 14 weeks | – | – | Girls move their jaws more than the boys do |
| | | Light touch to mouth evokes turn toward stimulus |
| | | Cerebellum resembles adult structure |
| | | Fat deposits in cheeks |
| 15 weeks | – | – | Body fat emerges throughout the body |
| 16 weeks | – | – | Quickening |
| | | Fat deposits upper & lower limbs |
| Unit 14: 4 to 5 Months | |
| 18 weeks | – | – | Cream-like substance protects skin |
| | | Sweat glands |
| 19 weeks | – | – | Melanin production |
| | | Daily cycles in biological rhythms |
| 20 weeks | – | – | All skin layers and structures |
| | | Hearing and responding to sound begins |
| | | Hearing and responding to sound begins |
| | | Head circumference 20 cm |
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| Unit 15: 5 to 6 Months | |
| 20 weeks - 24 weeks | – | – | Eyelids separate, eyes open and close |
| 21 weeks - 22 weeks | – | – | If born prematurely from this point on, survival is possible |
| 23 weeks | – | – | Brain weight 100 grams |
| Unit 16: 6 to 7 Months | |
| 25 weeks | – | – | Intestinal lining contains all adult cell types |
| | | The ability to taste |
| 26 weeks | – | – | Additional fat deposits decrease wrinkles |
| | | The ability to smell has arrived |
| 26 weeks - 38 weeks | – | – | Brain weight increases 400% to 500% |
| 27 weeks | – | – | Pupils react to light |
| | | |
| Unit 17: 7 to 8 Months | |
| 30 weeks | – | – | Breathing motions are common even though there is no air in the womb |
| | | Head circumference 30 cm |
| 32 weeks | – | – | Esophagus: Lower esophagus muscles functional |
| Unit 18: 8 to 9 Months | |
| 32 weeks - 36 weeks | – | – | Prenatal food affects newborn taste preferences |
| 35 weeks | – | – | Firm grip |
| 36 weeks | – | – | Brain weight 300 grams |
| Unit 19: 9 Months to Birth | |
| 37 weeks | – | – | Fetus drinks an estimated 15 oz (or 450cc) of amniotic fluid/day |
| 38 weeks | – | – | Heart beats 54 million times before birth |
| | | Brain weight 350 grams |
| | | Fetus initiates labor |
| | | Head circumference 35 cm |
| | | Time to be born! |
Growth Parameters