• In the menstrual cycle, estrogens produced by the develop-
ing follicle stimulate the growth of the uterine lining, and
progesterone converts the endometrium into a form that is
ready to receive the fertilized egg. Progesterone produced
by the corpus luteum maintains the lining after ovulation.
When the corpus luteum degrades, the uterine lining is shed
as the menses, and the cycle repeats. The female reproduc-
tive cycle—which includes coordinated hormonal changes,
the ovarian cycle, and the menstrual cycle—lasts approxi-
mately 28 days.
Fertilization Requires the Egg and Sperm to
Get Very Close to One Another 484
• Sperm can survive for only about 48 hours after ejaculation,
and the egg remains viable for only about 24 hours. Sperm
require about 6 hours of transit time from their delivery site.
Therefore, timing of sperm delivery is important for success-
ful fertilization.
• Numerous contraceptive products are available to help
prevent pregnancy. These products are designed to disrupt
the fertilization or implantation processes. Sterilization pre-
vents the egg and sperm from passing through the tubing
in their respective reproductive systems. Barrier methods,
as shown, and chemical contraceptives prevent sperm from
reaching the egg after ejaculation. Hormonal contraceptives
disrupt the female reproductive cycle. Implantation-inhibit-
ing contraceptives alter the endometrium and prevent the
embryo from attaching to the uterus.
M e t h o d s o f c o n t r a c e p tio n a n d th e ir e f f e c t iv e n e s s r a te s
— b a r r ie r m e t h o d s • T a b le 1 6 .1
Pregnancy Lasts from Fertilization to
Delivery 488
• Once deposited in the vagina, sperm travel through the
cervical mucus, along the walls of the uterus, and into the
uterine tubes. If they encounter a secondary oocyte, they
will attempt to penetrate its outer layers and plasma mem-
brane. Once a single sperm penetrates the zona pellucida,
this region swells outward to block the activity of other
sperm. Fertilization occurs when the nuclei of the sperm and
oocyte fuse and the oocyte becomes a zygote.
• During the first week of development, the zygote divides
many times and changes to a hollow sphere of cells called a
blastocyst. During the second week, the blastocyst implants
into the uterine wall, where it develops connections to the
maternal blood supply to receive nutrients, exchange gases,
and eliminate wastes via the formation of chorionic villi.
Later, as shown, the placenta and umbilical cord develop
from the chorionic villi and endometrium to support the
growing embryo.
C o n n e c tio n s b e t w e e n t h e e m b r y o a n d t h e m o th e r
• F ig u r e 1 6 .1 3 b
Chorionic villi
Yolk sac
fluid in
• During the first 8 weeks of development, the blastocyst
changes size and shape into an embryo. Some key changes
include gastrulation, the formation of three layers of cells
(germ layers); all organs will form from various germ layers.
Another key event is neurulation, in which the nervous
system develops from the ectoderm (one of the three germ
• From 8 weeks onward, the developing organism is a fetus.
By the 12th week, all organs have developed. The fetus con-
tinues to grow; it is considered full term at 38 weeks.
Summary 505
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