• Pregnancy, or gestation, is the carrying of
developing young within the female reproductive tract.
• Human pregnancy averages 266 days (38 weeks)
from fertilization (also called conception in
• Human development begins with fertilization in the oviduct.
• Cleavage produces
• a blastocyst whose inner cell mass becomes the embryo and
• the trophoblast, the outer cell layer, which
• attaches to the uterine wall and
• forms part of the placenta.
Figure 27.15ab Trophoblast Cavity Uterine cavity Blastocyst Cleavage starts Fertilization of mature egg by sperm Oviduct Secondary oocyte Ovulation Ovary Blastocyst (implanted) Endometrium Uterus Uterine cavity
Figure 27.15c
Endometrium Uterine cavity
Trophoblast Multiplying
cells of
trophoblast (contribute to future placenta)
Embryo
Future yolk sac
The embryo and placenta take shape during
the first month of pregnancy
• Four extraembryonic membranes develop.
1. The amnion
• surrounds the embryo and
• forms a fluid-filled amniotic cavity that protects the embryo.
2. The yolk sac,
• in reptiles, stores yolk, and
3. The allantois
• contributes to the umbilical cord,
• forms part of the urinary bladder, and
• in reptiles, stores embryonic waste.
4. The chorion
• contributes to the placenta and
• The placenta is a composite organ consisting of
chorionic villi closely associated with the blood
vessels of the mother’s endometrium.
• The placenta is a vital organ with both embryonic
Human pregnancy is divided into three
trimesters
• For humans, pregnancy, the period of
development from conception to birth, is divided
First trimester
• The first trimester is the time of the most radical change for both mother and embryo.
• During this time, the embryo is particularly
Figure 27.16-0
TRIMESTER 1 TRIMESTER 2
TRIMESTER 3
Gill pouches (primitive
gill-like structures) Placenta
Limb buds Tail Umbilical cord Amnion Umbilical cord Placenta
Most radical changes for mother and embryo; embryo particularly susceptible to radiation, drugs, alcohol.
The fetus continues to grow; its eyes can open, teeth form, and bones begin to harden. The placenta begins to secrete progesterone and stops secreting hCG. The corpus luteum degenerates.
The fetus grows rapidly and gains strength. The mother’s abdominal organs become squeezed, causing frequent urination, digestive troubles, and backaches. Babies born prematurely—as early as 24 weeks—may survive, but they require special medical care.
Timeline of Human Development Conception
Week 5: Highly organized multicellular embryo about 7 mm long, with notochord and coelom. Brain and spinal cord taking shape. Gill pouches will develop into parts of throat and ear.
Week 8: All major structures present in rudimentary form. The embryo is about 4 cm long. Fetus can move its limbs and head and make facial expressions.
Week 14: Fetus is about 6 cm long. Features have been refined, and the fetus now appears more human.
Week 20: The fetus is about 19 cm long, weighs about 0.5 kg, and has eyebrows, eyelashes, fingernails, and toenails and is covered with fine hair. It may be quite active and “kick.” The mother’s abdomen is markedly enlarged. With limited space, the fetus bends into the fetal position.
Week 40 (newborn): The circulatory system and
Figure 27.16-1
TRIMESTER 1
Gill pouches (primitive
gill-like structures) Placenta
Limb buds
Tail Umbilical cord
Most radical changes for mother and embryo; embryo particularly susceptible to radiation, drugs, alcohol.
Timeline of Human Development Conception
Week 5: Highly organized multicellular embryo about 7 mm long, with notochord and coelom. Brain
Week 8: All major structures present in rudimentary form. The embryo is about 4 cm long. Fetus can move its limbs and head and make
Second trimester
• During the second trimester, the fetus continues to
grow and has increasingly human features, but the changes are not as dramatic as those changes of the first trimester.
• The placenta begins to secrete progesterone (and
the corpus luteum stops secreting progesterone), which helps maintain the placenta, and at the
Figure 27.16-2
TRIMESTER 2
Umbilical cord
Placenta
The fetus continues to grow; its eyes can open, teeth form, and bones begin to harden. The placenta
begins to secrete progesterone and stops secreting hCG. The corpus luteum degenerates.
Week 14: Fetus is about 6 cm long. Features have been refined, and the fetus now appears more human.
Third trimester
• The third trimester is a time of rapid growth as the
fetus gains the strength it will need to survive
outside the protective environment of the uterus.
• Babies born prematurely, as early as 24 weeks,
may survive, but require special medical care after birth.
• As the fetus grows and the uterus expands around
Figure 27.16-3
TRIMESTER 3
The fetus grows rapidly and gains strength. The mother’s abdominal organs become squeezed, causing frequent urination, digestive troubles, and backaches. Babies born prematurely—as early as 24 weeks—may survive, but they require special medical care.
Week 40 (newborn): The circulatory system and
Childbirth is induced by hormones and other
chemical signals
• The series of events that expel an infant from the uterus is called labor.
• Hormonal changes induce birth.
• Estrogen makes the uterus more sensitive to
oxytocin.
• Oxytocin acts with prostaglandins to initiate labor.
• The cervix dilates to about 10 cm.
• The baby is expelled by strong uterine contractions.
• The induction of labor involves positive feedback, a type of control in which a change triggers
mechanisms that amplify that change.
• Oxytocin and prostaglandins cause uterine
contractions that in turn stimulate the release of
more oxytocin and prostaglandins.
• The result is a steady increase in contraction
Figure 27.17a
Estrogen from ovaries
Induces oxytocin receptors on uterus
Oxytocin
from fetus and mother’s pituitary
Stimulates uterus to contract
Stimulates
placenta to make Prostaglandins
Stimulate more P
• Labor occurs in three stages:
1. dilation of the cervix,
2. expulsion, delivery of the infant, and
Figure 27.17b
Placenta
1 2 3
Umbilical cord Uterus Cervix
Uterus Placenta (detaching)
Umbilical cord
Dilation of the cervix Expulsion: delivery of the infant
Reproductive technologies increase our
reproductive options
• New techniques can help many infertile couples.
• About 15% of couples wanting children experience infertility, the inability to conceive.
• Drug therapies can help address problems of impotence
(erectile dysfunction) and induce ovulation.
• Assisted reproductive technologies (ART) require eggs to be harvested from the ovaries, fertilized, and returned to a woman’s body.
• In vitro fertilization (IVF) is the most common assisted
Figure 27.18-0
Implantation
Collected egg
Collected
Zygote
In vitro fertilization
