Prenatal Development

First Month:

• 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 muscles).
• 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.

Second Month:

• Length and Weight: 28 mm and 2.7 g.
• Nail beds, hair follicles, and sweat glands begin to form.
• Axial and appendicular cartilage 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.

Third Month:

• 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.

Fourth Month:

• 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.

Fifth Month:

• 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).

Sixth Month:

• 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.

Seventh Month:

• Length and Weight: 270 mm and 1492 g.
• Keratinization, 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.

Eighth Month:

• 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.

Ninth Month:

• Length and Weight: 346 mm and 2912 g.
• All body systems are complete; only growth now takes place.
• Lanugo is shed.

Early Postnatal Development:

• 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).

Apgar Score is an overall assessment of an infant made one minute after birth, and repeated at five minutes, if the first score is low. The highest possible score is 10, and infants who score under 5 require immediate medical attention. There are five categories of assessment:

• 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 colouration – scores 2
  • cyanotic (bluish) extremeties – scores1
  • cyanosis of trunk and extremeties – scores 0