Childbirth (also called labour, birth, partus or parturition) is the
culmination of a HYPERLINK "http://en.wikipedia.org/wiki/Human" \o
"Human" human HYPERLINK "http://en.wikipedia.org/wiki/Pregnancy" \o
"Pregnancy" pregnancy or HYPERLINK
"http://en.wikipedia.org/wiki/Gestation" \o "Gestation" gestation
period with the delivery of one or more newborn HYPERLINK
"http://en.wikipedia.org/wiki/Infant" \o "Infant" infants from a
HYPERLINK "http://en.wikipedia.org/wiki/Woman" \o "Woman" woman 's
HYPERLINK "http://en.wikipedia.org/wiki/Uterus" \o "Uterus" uterus .
The process of human childbirth is categorized in 3 stages of labour.
The first stage accomplishes the shortening and then the dilation of the
cervix. It is deemed to have started when the cervix is 3 cm dilated,
and ends with full dilation. Contractions begin in the first stage of
labour although they may be irregular and sporadic at first. The second
stage, often called the pushing stage, starts when the cervix is fully
dilated and ends with the expulsion of the fetus. In the third stage,
the HYPERLINK "http://en.wikipedia.org/wiki/Placenta" \o "Placenta"
placenta detaches from the uterine wall and is expelled through the
birth canal. HYPERLINK "http://en.wikipedia.org/wiki/" \l "cite_note-0"
\o ""  Preceding the onset of labour is a period called the latent
phase. This phase may last many days, and the contractions are an
intensification of the HYPERLINK
"http://en.wikipedia.org/wiki/Braxton_Hicks_contractions" \o "Braxton
Hicks contractions" Braxton Hicks contractions that start around 26
weeks gestation. Latent phase ends with the onset of active first stage
The mechanics of birth
Because humans are bipedal with an erect stance and humans have
relatively the biggest head and shoulders to the size of the pelvis of
any species, humans fetuses are adapted to make birth possible.
The erect posture causes the weight of the abdominal contents to thrust
on the pelvic floor, a complex structure which must not only support
this weight but allow three channels to pass through it: the urethra,
the vagina and the rectum. The relatively large head and shoulders
require a specific sequence of manoeuvres to occur for the bony head and
shoulders to pass through the bony ring of the pelvis. If these
manoeuvres fail, the progress of labour is arrested. All changes in the
soft tissues of the cervix and the birth canal are entirely dependent on
the successful completion of these six maneuvers:
Engagement of the fetal head in the transverse position. The baby is
looking across the pelvis at one or other of the mother's hips.
Descent and HYPERLINK "http://en.wikipedia.org/wiki/Flexion" \o
"Flexion" flexion of the fetal head
Internal rotation. The fetal head rotates 90 degrees to the HYPERLINK
&redlink=1" \o "Occipito-anterior (page does not exist)"
occipito-anterior so that the baby's face is towards the mother's
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