NORMAL HUMAN EMBRYONIC DEVELOPMENT

LO #3: Fetal period epochs as seen in ultrasound

The transition from EMBRYONIC to FETAL period occurs at 7 weeks AFTER FERTILIZATION, w/c corresponds to 9 weeks  AFTER LMP.

  • Fetus size: 24 mm length

  • Significant developments: Most organ systems have developed


12 GESTATIONAL WEEKS

The uterus is usually palpable just above the symphysis pubis.

  • Fetus size: Crown rump 5-6 cm

    • Period of rapid growth 

  • Significant changes: 

    • Appearance of centers of ossification (bone formation)

    • Differentiation of fingers & toes

    • Development of skin & nails

    • Appearance of hair rudiments

    • Definitive signs of gender

  • Movements: spontaneous


16 GESTATIONAL WEEKS

  • Fetus size: Crown rump 12 cm; Weight ~150 g

    • Period of slow growth

    • Clinically, beyond 13 weeks, biparietal diameter, head & abdominal circumference. & femur length is measured

    • Fetal weight is also measured from a combination of the abovementioned

  • Significant changes: 

    • In female fetus, uterus is formed & vaginal canalization begins (by 18 weeks)

    • In male fetus, testicles start to descend (by 20 weeks)

  • Movements: Beginning of eye movement (16-18 weeks), w/c coincides w/ midbrain maturation


20 GESTATIONAL WEEKS

The midpoint of pregnancy from LMP estimation.

  • Fetus size: Weight >300 g

    • Substantial linear weight increase

  • Significant changes: 

    • Formation of brown fat (adipose tissue that regulates body temperature/keeps fetus warm)

    • Less transparent skin

    • Entire body is covered by downy lanugo & appearance of some scalp hair

  • [not seen] Significant changes: 

    • Development of cochlear function (22-25 weeks, continues until 6 mos after delivery)

  • Movements: ~ every minute; active 10-30% of the day


24 GESTATIONAL WEEKS

Most fetuses born at this time will die since the terminal sacs, w/c are required for gas exchange, are not yet formed.

  • Fetus size: Weight ~700 g

  • Significant changes: 

    • Wrinkled skin w/ fat deposition

    • Large head

    • Recognizable eyebrows & eyelashes

  • [not seen] Significant changes: 

    • Surfactant secretion by secretory type II pneumocytes

    • Canalicular lung development period; enlargement of bronchi & bronchioles, development of alveoli

    • Development of the neural pain system

    • Sites for hemopioesis are the fetal liver & spine

  • Movements: Eyes open (by 26 weeks)


28 GESTATIONAL WEEKS

Normal fetus born at this time has a 90% chance of survival.

  • Fetus size: Crown rump 25 cm; Weight ~1,100 g

  • Significant changes: 

    • Thin, red skin covered w/ vernix caseosa (white, creamy, naturally occurring biofilm covering the skin)

  • [not seen] Significant changes: 

    • The major site for hemopoiesis is the bone marrow

  • Movements: Isolated eye blinking

 

32 & 36 GESTATIONAL WEEKS

Normal fetuses born at this time have a 100% chance of survival.

  • Fetus size: 

    • @32 weeks - Crown rump 28 cm; Weight ~1,800 g

    • @36 weeks - Crown rump 32 cm; Weight ~2,800 g 

    • ~2,500 g for Filipinos

  • Significant changes: 

    • @32 weeks - Wrinkled, red skin 

    • @36 weeks - Rotund body due to subcutaneous fat deposition; Fuller facies (face)


40 GESTATIONAL WEEKS

This is considered term. The fetus is fully developed.


Fetus size: Crown rump ~36 cm; Weight ~3,500 g