If it is necessary, amniocentesis is usually conducted
for prenatal evaluation around 16-18 weeks. By this
time your uterus is large enough and the amniotic
fluid is sufficient for testing. A procedure at this
time gives a woman enough time to decide on terminating
the pregnancy if she desires. Ultrasound is used to
locate a pocket of fluid where the fetus and placenta
are not in the way. The part of the abdomen above
the uterus is cleaned; skin is numbed and a needle
is passed through the wall into the uterus. Fluid
is withdrawn from the abdominal cavity (area around
the baby) with a syringe. About an oz (30ml) of amniotic
fluid is required to perform the various tests. Fetal
cells that float in the amniotic fluid can be grown
in cultures and can be used to identify fetal abnormalities.
There are about 400 abnormalities a child can be born
with - amniocentesis identifies about 40 (10%) of
them, including the following:
• Chromosomal problems particularly Down syndrome
• Fetal sex, if sex specific problems such as
hemophilia or Duchenne muscular dystrophy must be
identified
• Skeletal diseases
• Fetal infections such as herpes or rubella
• Central nervous system diseases
• Hematologic (blood) diseases
• Inborn errors of metabolism (chemical problems
or deficiencies of enzymes)
Risks from amniocentesis include injury to the fetus,
placenta or umbilical cord, infection, miscarriage,
or premature labor. The use of ultrasound to guide
the needle helps prevent such complications but doesn't
guarantee to eliminate all risk. There can be bleeding
from the fetus to the mother, which can pose a problem
because it is hard to tell fetal blood from maternal
blood and both blood types may be different. This
is a particular risk to an RH-negative mother carrying
an RH-positive baby. An RH-negative mother should
receive RhoGAM at the time of amniocentesis to prevent
bleeding. Fetal loss from this procedure is estimated
to be less than 3%. The procedure should be done by
someone qualified and with the necessary experience.
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