900
UNIT SIX
The term “Siamese twins” comes from Chang
and Eng, born in Thailand, then called Siam, in
1811. They were joined by a ligament from the
navel to the breastbone, which surgeons could
easily correct today. Chang and Eng lived for
sixty-three years, and each married.
in a manner seen only four times before. They are
the result of incomplete twinning, which occurred
during the first two weeks of gestation. The girls
have shared tissue derived from ectoderm, meso-
derm, and endoderm, so the partial twinning event
must have occurred before the three germ layers
were established, at day fourteen.
P
atty Hensel’s pregnancy was uneventful.
An ultrasound scan revealed an appar-
ently normal fetus, although at one medi-
cal exam, Mike Hensel thought he heard two
heartbeats.
A cesarean section was necessary because
the baby was positioned bottom-F
rst. To every-
one’s amazement, the baby had two heads and
two necks, yet appeared to share the rest of the
body, with two legs and two arms in the cor-
rect places, and a third arm between the heads.
The ultrasound had probably imaged the twins
from an angle that superimposed one head on
the other. Patty, dopey from medication, recalls
hearing the word “Siamese” and thinking she had
given birth to cats. She had delivered conjoined,
or Siamese, twins.
The baby was two individuals, named Abigail
and Brittany. Each twin had her own neck, head,
heart, stomach, and gallbladder. Remarkably, each
also had her own nervous system. The twins shared
a large liver, a single bloodstream, and all organs
below the navel, including the reproductive tract.
They had three lungs and three kidneys.
Abby and Brittany were strong and healthy.
Doctors suggested surgery to separate the twins.
Aware that only one child would likely survive
surgery, Mike and Patty chose to let their daugh-
ters be. As teens, Abby and Brittany are glad their
parents did not choose to separate them, because
they would have been unable to walk or run, as
they can today. They enjoy kickball, volleyball, bas-
ketball, and cycling. Like any teen girls, they have
distinctive tastes in clothing and in food (F
g. 23D).
Conjoined twins occur in 1 in 50,000 births,
and about 40% are stillborn. They can be attached
in any of several ways. The Hensel twins were joined
23.2
CLINICAL APPLICATION
Joined For Life
(f
g. 23.27)
. At the same time, dilation of the cervix refl
exly
stimulates an increased release of oxytocin from the poste-
rior pituitary gland.
As labor continues, positive feedback stimulates abdom-
inal wall muscles to contract, helping to propel the fetus
through the cervix and vagina to the outside.
Table 23.5
summarizes some of the factors promoting labor.
Figure
23.28
illustrates the steps of the birth process.
Following birth of the fetus, the placenta, which ini-
tially remains inside the uterus, separates from the uter-
ine wall and is pushed by uterine contractions through the
birth canal. This expelled placenta, called the
afterbirth,
is
myometrium due to the decline in progesterone secretion,
stimulation by oxytocin aids
labor
in its later stages.
During labor, muscular contractions force the fetus
through the birth canal. Rhythmic contractions that begin at
the top of the uterus and travel down its length force the
contents of the uterus toward the cervix.
The fetus is usually positioned head downward, so
labor contractions force the head against the cervix. This
action stretches the cervix, which elicits a refl
ex that stimu-
lates still stronger labor contractions. Thus, a
positive feed-
back system
operates in which uterine contractions produce
more intense uterine contractions until effort is maximal
FIGURE 23D
Abby and Brittany Hensel are conjoined twins, the result of incomplete
twinning during the F
rst two weeks of prenatal development.
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