Unit A Reproduction

Birth
About nine months or 266 days after fertilization, the muscles of the mother’s
uterus contract regularly (every 15 to 20 minutes, and lasting at least 40 seconds).
The birth process, called labour, has begun. Hormones control the process,
which is described in Figure 6.
A The cervix begins to dilate, or open up. The
membrane surrounding the baby is forced into
the vagina (now called the birth canal).
Did You
KNOW?
All Pregnancies Are Not Equal
Pregnancy is also called gestation.
The gestation period is often related
to the size of the mammal. Here are
some mammals and their gestation
periods in weeks:
• elephant—88
• horse—48
• grizzly bear—30
• lion—17
• wolf—9
• squirrel—3.5
B The amniotic membrane breaks, and amniotic
fluid lubricates the canal. This is called the
“breaking of the water.”
LEARNING TIP
Imagine the birth process (Figure 6) and
the explanations (A to D) happening
visually. Share your mental images
with a partner.
C Once the cervix has widened to 10 cm, uterine
contractions push the baby’s head into the
birth canal. The baby’s head is followed by the
rest of the body.
D When the head and shoulders are free of the
birth canal, the rest of the baby slips out easily.
The baby is born. A short while later, the
placenta, often called the “afterbirth,” is pushed
out of the uterus. Once the baby is breathing on
his or her own, the umbilical cord is cut and tied
off to prevent bleeding. The “belly button” is the
scar from the umbilical cord.
Figure 6 The birth process
NEL
4.3 Pregnancy: Fertilization to Birth 119