1. 23.1: Introduction
A sperm and a secondary oocyte unite, forming a zygote, and the journey of prenatal development begins
After 38 weeks of cell division, growth and specialization into distinctive tissues and organs, a new human being enters the world
Humans grow, develop and age
Growth is an increase in size
Development, which includes growth, is the continuous process by which an individual changes from one life phase to another
Prenatal period is from fertilization to birth
Postnatal period is from birth to death

2. 23.2: Pregnancy
Pregnancy is the presence of a developing offspring in the uterus
It consists of three periods, or trimesters, each three months long

3. Fertilization
Fertilization is the union of an egg cell (secondary oocyte) and a sperm cell
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Corona radiata
First polar body
Second meiotic
spindle
Cytoplasm of secondary
oocyte
Zona pellucidav
Cell membrane
of secondary
oocyte
Acrosome containing
enzymes
Nucleus containing
chromosomes 3 1 4 5 2

4. Hormonal Changes During Pregnancy
Secretion of hCG maintains the corpus luteum
The corpus luteum secretes estrogens and progesterone
The placenta secretes large amounts of estrogens and progesterone
Estrogens and progesterone stimulate and maintain the uterine lining, inhibit FSH and LH, inhibit uterine contractions, and enlarge the reproductive organs
Relaxin from the corpus luteum inhibits uterine contractions and relaxes the pelvic ligaments
Placental lactogen stimulates breast development
Aldosterone promotes sodium retention
PTH maintains calcium concentrations in the blood

6. Other Changes During Pregnancy
Growth of the uterus can displace abdominal organs and disrupt meals, including the development of heartburn and increased urinary frequency
More oxygen is needed and more waste is excreted causing increases in blood volume, cardiac output, breathing rate, and urine production
To obtain adequate nutrition, intake must be sufficient to supply needed vitamins, minerals and proteins
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(a)
(b)
(c)
a: © A. Tsiara/Photo Researchers, Inc.; b: © Omikron/Photo Researchers, Inc.; c: © Petit Format/Nestle/Photo Researchers, Inc.

7. b: © Carroll Weiss/Camera M.D. Studios
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Actual length
Actual
length
4 weeks
5 weeks
Actual
length
(a)
Actual
length
6 weeks
7 weeks
(b)
b: © Carroll Weiss/Camera M.D. Studios

8. Mandibular process Lens Developing eye Developing ear Maxillary
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Mandibular
process
Lens
Developing
eye
Developing ear
Maxillary
process
Elbow
Forebrain
Handplate
Hindlimb
Paddle-shaped
forelimb
(a) 35 ± 1 day (10–12 mm)
(b) 37 ± 1 day (12.5–15.75 mm)
External acoustic
meatus
Midbrain
Pigmented
eye
External ear
External ear
Heart prominence
Wrist
Digital rays
Toe rays
Paddle-shaped
foot plate
(d) 45 ± 1 day (22–24 mm)
(c) 40 ± 1 day (16–21 mm) –
Eyelid
Ear
Webbed fingers
Notches
between
toe rays
(e) 49 ± 1 day (28–30 mm)
Fingers
separated
Toes
separated
Fan-shaped
webbed toes
(f) 52 ± 1 day (32–34 mm)
(g) 56 ± 1 day (34–40 mm)

9. © Donald Yaeger/Camera M.D. Studios
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(a) When physical structures develop
Reproductive system
Ears
Eyes
Upper and lower limbs
Heart
Central nervous system 1 2 3 4 5 6 7 8 9
Month
(b) When different teratogens disrupt development
Accutane
Diethylstilbestrol
Thalidomide 1 2 3 4 5 6 7 8 9
Month
© Donald Yaeger/Camera M.D. Studios

11. Birth Process
Pregnancy terminates with the birth process called parturition
The process is complex as noted in Table 23.5

12. 12 Amniotic sac Symphysis pubis Urinary bladder Ruptured amniotic sac
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Amniotic sac
Symphysis
pubis
Urinary
bladder
Ruptured
amniotic
sac
Urethra V
agina
Placenta
Cervix
Rectum
(a)
(b)
Uterus
Placenta
Placenta
Umbilical
cord
(c)
(d) 12