Week five of pregnancy
The development of body form is the major event that happens this week. At the beginning of this week, the embryo is oval
to round in appearance. Three days later, the embryo becomes longer and pear-shaped. By the end of this week, the embryo lengthens
further, taking on the shape of a slipper. The embryo is now visible to the naked eye. The head becomes distinguishable from
the body and the embryo looks like a miniature seahorse!
During week five, the embryo undergoes a process that creates three “germ layers,” from which all tissues and organs will
develop:
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ectoderm layer: leads to the development of skin, hair, nails, and
nervous system
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mesoderm layer: leads to development of major structural parts of the body including bones, cartilage, and muscles
-
endoderm layer: leads to development of the cells that line the organs
Early development of the central nervous system
At the end of week five, a set of cells in the ectoderm layer starts to become taller. This set of cells forms a structure
called the neural plate. A groove forms along the middle of the neural plate, which is flanked on either side by two folds.
The folds fuse together to begin formation of another structure called the neural tube. The neural tube will become an integral
part in the development of the central nervous system. The neural plate will give rise to other types of cells, which will
eventually form the nerves extending from the brain and spine, and other structures.
Early development of the cardiovascular system
Blood vessels start to appear in week five, first in the yolk sac and other tissues surrounding the embryo, and then in the
embryo itself. The blood vessels unite with other blood vessels to form the beginnings of the cardiovascular system.
Toward the end of week five, two heart tubes form which are joined to the blood vessels in the embryo and the surrounding
tissues. By the end of this week, the heart tubes fuse to form one heart in the shape of a tube. The heart starts beating
this week! The embryo is now able to make its own blood cells.
Before the end of week five, tiny blood vessels called capillaries begin to develop. These are very helpful to the exchange
of oxygen and other nutrients between the mother and her embryo.
Early development of other structures
By the end of week five, the mesoderm layer undergoes a lot of activity. The mesoderm divides into special sections called
somites, which will give rise to the skeleton, muscles, and skin. The somites look like a series of little bumps along the
embryo’s back.
Isolated spaces start to form between the layers of the mesoderm. These spaces will eventually become the body cavities surrounding
the heart, lungs, and organs in the abdomen.
Week six of pregnancy
At the beginning of week six, the embryo is slipper-shaped and straight. One major event that occurs this week is the folding
of the embryo into a curled up position. The embryo grows rapidly this week, especially its brain and spinal cord, and this
is what causes the body to fold. Folding of the embryo produces head and tail folds. By the end of this week, the embryo measures
about 6 mm (one-quarter inch) or about the width of your little fingernail.
This week, folds at the top end of the embryo thicken to form the beginnings of the brain. By the middle of the week, the
neural tube is formed. The heart is quite prominent at the front of the body. The neural tube undergoes a number of processes
that give rise to certain nerves and tissues of the central nervous system, and other structures such as the skeletal and
muscular components of the head and neck.
Openings in the neural tube close. If these openings do not close properly, they can lead to serious abnormalities called
neural tube defects, such as spina bifida. The front of the brain, called the forebrain, is now very prominent in the head.
The embryo is very curved, especially its long tail.
Little buds of cells appear where the arms and legs will start to develop. The inner ear begins to develop. Other structures
involved with the formation of the head and neck are visible. The earliest stages of development of the lenses of the eyes
begin at this time. Little buds appear which will develop into the lungs, as well as the stomach, liver, and pancreas.
Week seven of pregnancy
This week, changes in body form are minor compared to those that happened last week. The embryo continues to grow in a curved
position.
Brain development is very rapid this week, and therefore the head grows faster than any other part of the body. The face,
nose, ears, and eyes begin to develop. The arm and leg buds continue to form, and the ends of the upper arms become paddle-shaped.
The embryo may start to twitch its trunk, arms, and legs this week, but the movements will be too slight for the mother to
feel. The umbilical cord becomes clearly visible.
Believe it or not, the embryo goes through two sets of kidneys during its early development. They both form early in pregnancy
and eventually disappear. Around week seven, a third, “real” set of kidneys starts to form.
Week eight of pregnancy
This week, there is rapid growth of the back of the brain, called the hindbrain. This causes the head to become much larger
than the rest of the body, and more bent over.
There is rapid development of the upper limbs this week. Elbow and wrist regions become clearly visible, and little buds at
the ends of the limbs begin to form, which will become the fingers. A couple of days later, the lower limbs begin to develop
in a similar fashion.
Swellings on the sides of the head close to the neck indicate the beginnings of the outer ear. The eyes continue to develop
on the sides of the head. The eyes are easy to notice because they have formed a retinal pigment, which is a substance that
gives colour to a part of the eye called the retina.
In week eight, the abdominal cavity is not big enough to hold the rapidly growing intestinal loops, and therefore the loops
get pushed out of the abdomen for a while.
Prior to this week, it was impossible to distinguish between the sex organs of the male and female embryo. In week eight,
these sex organs start to develop into either ovaries for females or testes for males.
What concerns are there in these early weeks of pregnancy?
The first few weeks of pregnancy are a delicate time when miscarriage is common. Miscarriage is also referred to as spontaneous
abortion. At this stage in pregnancy, miscarriage is usually caused by abnormalities within the embryo or problems with implantation.
If you have a miscarriage during this time, chances are that you will not realize it.
Another concern during this time is a condition known as an ectopic pregnancy. An ectopic pregnancy occurs when the embryo
implants outside the uterus, usually in the fallopian tube. An ectopic pregnancy is often related to a problem with the transport
of the embryo through the fallopian tube.
Because the central nervous system, heart, limbs, eyes, and ears are in their critical stages of development this month, exposure
to harmful substances called teratogens during this time may lead to miscarriage or birth defects.
