accompanied by bleeding, because vascular tissues are dam-
aged in the process. However, continued contraction of the
uterus compresses the bleeding vessels, minimizing blood
loss. Oxytocin stimulates this contraction. Breastfeeding also
contributes to returning the uterus to its original, prepreg-
nancy size, because suckling by the newborn stimulates the
mother’s posterior pituitary to release oxytocin.
A positive feedback mechanism propels the birth
Fetal head is forced
Fetus is moved
Reflex is elicited
that causes stronger
Factors Contributing to the
1. As the time of birth approaches, secretion of progesterone declines,
and its inhibiting eF
ect on uterine contractions lessens.
2. Decreasing progesterone concentration stimulates synthesis of
prostaglandins, which initiate labor.
3. Stretching uterine tissues stimulates release of oxytocin from the
posterior pituitary gland.
4. Oxytocin stimulates uterine contractions and aids labor in its later
5. As the fetal head stretches the cervix, a positive feedback mechanism
results in stronger and stronger uterine contractions and a greater
release of oxytocin.
6. Positive feedback stimulates abdominal wall muscles to contract
with greater and greater force.
7. The fetus is forced through the birth canal to the outside.
An infant passing through the birth canal can stretch and tear the
perineum (the tissues between the vulva and anus). Before the birth is
complete, a physician may make an incision along the midline of the
perineum from the vestibule to within 1.5 centimeters of the anus.
This procedure, called an
ensures that the perineal tissues
are cut cleanly rather than torn, which aids healing.
Stages in birth. (
) ±etal position before labor, (
) dilation of the cervix, (
) expulsion of the fetus, (
) expulsion of the placenta.