1084
UNIT 5
Continuity
28
Table 28.1
Developmental Events of the Fetal Period
TIME
CHANGES AND ACCOMPLISHMENTS
8 weeks
(end of
embryonic
period)
Head nearly as large as body; all major brain regions present; first brain waves in brain stem
Liver disproportionately large and begins to form blood cells
Limbs present; digits are initially webbed, but fingers and toes are free by the end of this interval
Ossification just begun; weak, spontaneous muscle contractions occur
Cardiovascular system fully functional (heart has been pumping blood since the fourth week)
All body systems present in at least rudimentary form
Approximate crown-to-rump length: 22 mm (0.9 inch); weight: 2 grams (0.06 ounce)
9–12 weeks
(month 3)
Head still dominant, but body elongating; brain continues to enlarge, shows its general structural
features; cervical and lumbar enlargements apparent in spinal cord; retina of eye is present
Skin epidermis and dermis obvious; facial features present in crude form
Liver prominent and bile being secreted; palate is fusing; most glands of endodermal origin are
developed; walls of hollow visceral organs gaining smooth muscle
Blood cell formation begins in bone marrow
Notochord degenerating and ossification accelerating; limbs well molded
Sex readily detected from the genitals
Approximate crown-to-rump length at end of interval: 90 mm
13–16 weeks
(month 4)
Cerebellum becoming prominent; general sensory organs differentiated; eyes and ears assume
characteristic position and shape; blinking of eyes and sucking motions of lips occur
Face looks human and growth of the body beginning to outpace that of the head
Glands developed in GI tract; meconium is collecting
Kidneys attain typical structure
Most bones are now distinct and joint cavities are apparent
Approximate crown-to-rump length at end of interval: 140 mm
17–20 weeks
(month 5)
Vernix caseosa (fatty secretions of sebaceous glands) covers body; lanugo (silklike hair) covers skin
Fetal position (body flexed anteriorly) assumed because of space restrictions
Limbs reach near-final proportions
Quickening occurs (mother feels spontaneous muscular activity of fetus)
Approximate crown-to-rump length at end of interval: 190 mm
21–30 weeks
(months
6 and 7)
Period of substantial increase in weight (may survive if born prematurely at 27–28 weeks, but
hypothalamic temperature regulation and lung production of surfactant are still inadequate)
Myelination of spinal cord begins; eyes are open
Distal limb bones are beginning to ossify
Skin is wrinkled and red; fingernails and toenails are present; tooth enamel is forming on deciduous
teeth
Body is lean and well proportioned
Bone marrow becomes sole site of blood cell formation
Testes reach scrotum in seventh month (in males)
Approximate crown-to-rump length at end of interval: 280 mm
30–40 weeks
(term) (months
8 and 9)
Skin whitish pink; fat laid down in subcutaneous tissue (hypodermis)
Approximate crown-to-rump length at end of interval: 360 mm (14 inches); weight: 3.2 kg (7 lb)
8 weeks
12 weeks
16 weeks
At birth
previous page 1118 Human Anatomy and Physiology (9th ed ) 2012 read online next page 1120 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off