Labor • Figure 16.17
a.
Positive feedback mechanism that governs uterine
b. Stages of
labor
contractions during
labor
Baby's body stretches
cervix more
Interruption of cycle:
Birth of baby decreases
stretching of cervix,
thus breaking the
positive feedback cycle.
Urinary bladder
Vagina
Ruptured
amniotic sac
Rectum
Stage
1.
Dilation
• Extends from onset of labor to complete dilation of cervix (6-12 h)
• Regular uterine contractions
• Ruptured amniotic sac
• Completed dilation of cervix (10 cm)
Placenta
Stage
2.
Expulsion
extends from complete dilation of cervix to
delivery of the baby
( 1 0
min to several hours).
Uterus
Placenta
Umbilical cord
Stage 3. Placental stage
extends from the time of delivery
of the baby to the delivery of the placenta. (5-30 minutes or more).
Uterine contractions constrict blood vessels torn during delivery
and expel the placenta.
Giving Birth Requires a Complex Series of Hormonal Changes
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