Prenatal Development
- Length and Weight – 5 mm and 0.02 g (25.4 mm equal 1 inch; 28.35 g equal 1 ounce).
- Somite formation begins (eventual vertebrae and mucles).
- Neural tube formation begins.
- Eyes and ears begin to form.
- Heartbeat begins.
- Trachea and lungs begin to form.
- Intestinal tract, liver, and pancreas begin to form.
- Yolk sac is completely formed.
- Allantois, which is an important factor of the developing umbilical cord.
- Limbs begin to form.
- Length and Weight – 28 mm and 2.7 g.
- Nail beds, hair follicles, and sweat glands begin to form.
- Axial and appendicular carilage begins to form.
- Rudiments of the axial muscles have completed formation.
- CNS and PNS organization begins.
- Growth of the cerebrum begins.
- Taste buds and olfactory epithelium begin formation.
- Thymus, thyroid, pituitary, and adrenal glands begin formation.
- Basic heart structure, major blood vessels, lymph nodes and ducts are complete.
- Blood formation begins in the liver.
- Extensive bronchial branching into the mediastinum begins.
- Diaphragm is complete.
- Intestinal subdivisions, villi, and salivary glands begin formation.
- Kidney formation begins.
- Mammary glands begin formation.
- Calcification of bones begins.
- Fingers and toes are apparent on limbs.
- Facial features more distinct.
- Length and Weight – 78 mm and 26 g.
- Epidermal layers begin to appear.
- Ossification centers spread (bony matter).
- Rudiments of appendicular muscles are completed.
- Basic spinal cord and brain structure begin to form.
- Thymus and thyroid glands have completed formation.
- Tonsils and blood formation in the bone marrow begin.
- Gallbladder and pancreas have complete formation.
- Definitive gonads (sex organs), ducts, and genitalia are distinguishable.
- Facial features are distinct, but eyes remain closed.
- Nails develop on fingers and toes.
- Length and Weight – 133 mm and 150 g.
- Hair and sebaceous glands begin to form.
- Sweat glands have completed formation.
- Articulations (joints) begin.
- Facial and palatal organization is completed.
- Fetus begins movement.
- Rapid expansion of cerebrum begins.
- Basic eye and ear structure is completed.
- Peripheral receptor formation begins.
- Migration of lymphocytes to lymphatic organs begins.
- Blood formation in the spleen begins.
- Degeneration of embryonic kidneys begins.
- Length and Weight – 185 mm and 460 g.
- Keratin and nail production begins.
- Myelination of spinal cord begins.
- Tonsils are completely formed.
- Nostrils are completely formed and open.
- Intestinal subdivisions are complete.
- Skeletal muscles become active.
- Body begins to catch up to head in growth.
- Body is covered with fine hair (lanugo).
- Length and Weight – 230 mm and 823 g.
- Perineal muscles are completely formed.
- Layering of the cortex is completed.
- CNS tract begins formation.
- Adrenal glands are completely formed.
- Spleen, liver, and bone marrow are completely formed.
- Formation of the alveoli begins.
- Epithelial organization is complete.
- All glands are completely formed.
- Eyelashes and eyebrows form.
- Length and Weight – 270 mm and 1492 g.
- Keratinization and nail and hair formations begin.
- Eyelids open and the retina is sensitive to light.
- Pituitary gland is completely formed.
- Intestinal plicae (ridges or folds) are completed.
- Testes begin to descend.
- Length and Weight – 310 mm and 2274 g.
- Epiphyseal plate formation begins.
- Taste receptors are complete and functional.
- Pulmonary branching and alveolar formation is complete.
- Production of pulmonary surfactant begins.
- Nephron formation is complete.
- Descent of the testes is now complete, or by the time of birth.
- Length and Weight – 346 mm and 2912 g.
- All body systems are complete, and only growth now takes place.
- Lanugo is shed.
- Hair changes in consistency and distribution.
- Formation and growth of epiphyseal plates continue.
- Muscle mass and control increase.
- Myelination, layering, and CNS tract formation continues.
- Cardiovascular system changes at birth, and the immune system becomes operative.
- Alveoli inflate at birth (breathing begins).
- Heartbeat
- over 100 bpm (beats per minute) – scores 2
- below 100 – scores 1
- no heartbeat – scores 0
- Respiration
- strong, vigorous cry – scores 2
- weak cry – scores 1
- no respiratory effort – scores 0
- Muscle tone
- spontaneous, active motion – scores 2
- some motion – scores 1
- no muscle tone – scores 0
- Reflex response to stimulation on soles of feet
- a cry in response – scores 2
- a grimace – scores 1
- no response – scores 0
- Colour
- healthy coloration – scores 2
- cyanotic (bluish) extremeties – scores1
- cyanosis of trunk and extremeties – scores 0
See also: University of Michigan Health System - Prenatal Development
This page was updated in July 2007.
