Unit A Reproduction

amniotic
fluid
placenta
umbilical
cord
fetus
uterus
Figure 3 The fetus receives nutrition
from the mother via the placenta and
the umbilical cord.
Figure 4 By the end of the second
trimester, facial features are present
and internal systems are maturing.
Figure 5 In the third trimester, the fetus
fills the uterus. With so little space, the
fetus no longer moves around as much.
By the end of the eighth week, the embryo now has the beginnings of all the
organs and is called a fetus. The fetus looks human, but it is only the size of a
lima bean. It is now receiving nutrition from the mother through the placenta
(Figure 3). The placenta is shaped like a disk and grows to 20 to 30 cm in
diameter. It contains blood vessels from the mother and blood vessels from the
fetus. Nutrients and oxygen diffuse from the mother’s blood into the fetus’s
blood. Wastes from the fetus diffuse into the mother’s blood. Because the
mother’s and fetus’s blood vessels are so close to each other, diffusion is possible.
The mother’s blood does not enter the circulatory system of the fetus. The fetus’s
circulatory system is connected to the placenta through the umbilical cord.
By the end of the third month, muscles and bones are forming. The fetus
can flex arm and leg muscles, and may hiccup. The heart is beating and
blood is circulating. Sex organs are forming, and gender can be determined.
The head is almost half the size of the body. The fetus is about 12 cm long,
with a mass of around 50 g.
Since the mother’s gain in mass is about 1 kg, she may not look different.
There are usually signs of pregnancy, however, such as no menstruation,
enlarged breasts, and sometimes nausea. A blood test done by a health-care
provider can confirm a pregnancy. The fetus is most susceptible to birth
defects during the first trimester. A birth defect is a structural or
physiological problem in the fetus that develops before birth.
The Second Trimester
Months four, five, and six are a time of intense growth for the fetus. The fetus
grows from 8 to 30 cm long by the end of the sixth month. The heartbeat can
be detected with a stethoscope. The fetus is sucking and swallowing amniotic
fluid, which will be excreted in urine. The mother now looks pregnant, as her
abdomen swells. Movement of the fetus can be felt by the mother, first as a
flutter but soon as pokes and kicks as the legs and arms develop.
At 24 weeks, the fetus resembles a tiny infant (Figure 4). Fingers and toes
have elongated. Facial features are more developed, and eyelashes are visible.
The fetus is now covered with a fine hair. Practice breathing also occurs,
although no gas exchange occurs. Oxygen reaches the fetus through the
blood vessels in the umbilical cord.
The Third Trimester
During the last three months (Figure 5), the fetus puts on most of its mass as it
continues to develop in preparation for birth. The organ systems are functioning
properly. The fetus has sleeping and waking patterns and reacts to stimuli from
the outside world. The brain is developing rapidly. The bones are developed but
still soft. The eyelids are open. The fetus fills the uterus and the mother’s belly
may undulate as the baby changes position. During the last month, the fetal hair
disappears. The fetus turns so that the head is pointing down in preparation for
birth. The lungs continue to mature right up to birth. At 37 weeks, the fetus is
considered full term. The size of the fetus ranges from 45 to 55 cm and can be
up to 4.5 kg in mass, although the average size is 53 cm and 3.4 kg at birth.
118 Unit A Reproduction
NEL