Changes During Embryonic and Fetal Development | CHAPTER 9 | Anatomy and Physiology

Embryonic

Period
1-4 weeks

• Primary germ layers and notochord develop. Neurulation occurs.
• Brain development begins.
• Blood vessel formation begins and blood forms in yolk sac, allantois, and chorion.
• Heart forms and begins to beat.
• Chorionic villi develop and placental formation begins.
• The embryo folds. The primitive gut and limb buds develop.
• Eyes and ears begin to develop, tail
• forms, and body systems begin to form.

• Brain development continues.
• Limbs become more distinct and digits appear.
• Heart becomes four-chambered.
• Eyes are far apart and eyelids are fused.
• Nose develops and is flat.
• Face is more human-like.
• Ossification begins.
• Blood cells start to form in liver.
• External genitals begin to differentiate.
• Tail disappears.
• Major blood vessels form.
• Many internal organscontinue to develop.

Fetal Period

9-12 weeks

• Head constitutes about half the length of the fetal body, and fetal length nearly doubles.
• Brain continues to enlarge.
• Face is broad, with eyes fully developed, closed, and widely separated.
• Nose develops a bridge.
• External ears develop and are low set.
• Ossification continues.
• Upperlimbs almost reach final relative length but lower limbs are not quite as well developed.
• Heartbeat can be detected.
• Gender is distinguishable from external genitals.
• Urine secreted by fetus is added to amniotic fluid.
• Red bone marrow, thymus, and spleen participate in blood cell formation.
• Fetus begins to move, but its movements cannot be felt yet by the mother.
• Body systems continue to develop.

• Head is relatively smaller than rest of body.
• Eyes move medially to their final positions, and ears move to their final positions on the sides of the head.
• Lower limbs lengthen.
• Fetus appears even more humanlike.
• Rapid development of body systems occurs.

• Head is more proportionate to rest of body. Eyebrows and head hair are visible.
• Growth slows but lower limbs continue to lengthen. Vernix caseosa (fatty secretions of sebaceous glands and dead epithelial cells) and lanugo (delicate fetal hair) cover fetus.
• Brown fat forms and is the site of heat production.
• Fetal movements are commonly felt by mother (quickening).

• Head becomes even more proportionate to rest of body.
• Weight gain is substantial, and skin is pink and wrinkled.
• By 24 weeks, lung cells begin to produce surfactant.

• Head and body are more proportionate and eyes are open.
• Toenails are visible.
• Body fat is 3.5% of total body mass and additional subcutaneous fat smoothes out some wrinkles.
• Testes begin to descend toward scrotum at 28 to 32 weeks.
• Red bone marrow is major site of blood cell production.
• Many fetuses born prematurely during this period survive if given intensive care because lungs can provide adequate ventilation and central nervous system is developed sufficiently to control breathing and body temperature.

• Skin is pink and smooth.
• Fetus assumes upside down position.
• Pupillary reflex is present by 30 weeks.
• Body fat is 8% of total body mass.
• Fetuses 33 weeks and older usually survive if
• born prematurely.

• By 38 weeks, circumference of fetal abdomen is greater than that of head.
• Skin is usually bluish-pink, and growth slows as birth approaches.
• Body fat is 16% of total body mass.
• Testes are usually in scrotum in full-term male infants.
• Even after birth, an infant is not completely developed; an additional year is required, especially for complete development of the nervous system.