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Stages of
Pregnancy
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Pregnancy Begins: Fertilization |
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The sperm fertilizes the egg (ovum) at the time
of ovulation (release of the egg from the ovary),
which typically occurs around day 14 of the
menstrual cycle. Only one sperm can enter the egg:
The male or female sex of the baby is determined at
this moment. Once fertilization occurs, the
egg/sperm cell, called a zygote, begins to divide
into multiple cells that start the development of
differing tissues and organs. At this time, the
developing baby is called an embryo. The embryo
attaches to the wall of the uterus (womb) about 6
days after fertilization.
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End of Week Two |
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The mother's blood supply (identified by the dark
red blood vessels) can be seen beginning to surround
the embryo; eventually it will connect with vessels
the embryo forms to develop the placenta. The embryo
(purple oval structure) is contained within the
amniotic cavity and is connected to the yolk sac
(brown oval structure) by a narrow yolk stalk. The
yolk sac helps deliver nutrients to the embryo and
fosters blood vessel development.
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End of Week Three |
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During week 3, there is further development of
the mother's blood supply. The yolk sac has enlarged
but is too small to be seen until after Week 20. The
primitive development of the embryo can be seen
within the amniotic cavity. At this point, a heart
bulge has begun to form (seen on the right end of
the embryo), and the brain is beginning to form just
above the "heart."
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End of Week Four |
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By the beginning of Week 4, the
embryo's heart produces a large "chest"
bulge and begins to pump blood. The embryo's head is
very large compared to the rest of the body. A tail is
present that shrinks continuously and is gone within
the next 2 to 3 weeks. Also present are the beginnings
of the arms; the legs begin to be visible by the end
of this week, when the eyes and ears can also be
identified. At the end of Week 4, the embryo is less
than 1/7 of an inch long.
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End of Week Five |
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The changes in the developing embryo are minor
during this week. The head continues to enlarge due
to the rapid growth of the brain. The arms are
shaped like paddles and the legs like flippers. At
the end of Week 5, the embryo is still less than
half an inch long.
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End of Week Eight |
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Between Weeks 5 and 8, the embryo begins to take
final form. Fingers start to form during Week 6, and
the arms and legs develop elbows and knees by the
end of Week 8. The early shape of the nose is also
distinguishable. The embryo's sex cannot yet be
physically identified. At the end of Week 8, the
embryo is just over an inch long.
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End of Week Ten |
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From the beginning of Week 9 onward,
the baby is called a fetus instead of an embryo.
During Week 9, the sex organs (genitals) develop male
or female characteristics, but the sex of the child
cannot be determined by ultrasound until several weeks
later. All of the organs have begun to form by Week 9;
they continue to grow and develop throughout the
pregnancy. The fetus now grows rapidly and is almost
two and a half inches long by the end of Week 10.
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Estimated Due Date
(40 weeks) |
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During the second and third trimester, all of the
major body organs continue to develop until birth.
The expected date of delivery is 38 weeks after
fertilization or 40 weeks after the last menstrual
period. The factors that trigger labor are not
completely understood, but several body hormones are
associated with the start of contractions. The first
stage of labor is the dilation of the cervix,
followed by delivery of the baby and then the
placenta (after-birth), some 15 minutes later.
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