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ULTRASOUND in Pregnancy Translated by
Informative text by the S.I.E.O.G. (Italian
society of echography Gynecological Obstetrician)
What
is ultrasound? Ultrasound
is a technique used to visualize internal organs by emitting the high frequency
sound waves (too high for human hearing) which pass through the body tissues.
Ultrasound
uses the same principles as the sonar used by ships to detect
submarines, and works in the following way. The probe placed on a woman's
abdomen sends out sound waves into the body. The sound waves, which are partly
reflected off the abdominal and uterine wall create echoes. When the waves reach
the foetus, they send out more echoes or backward waves which are displayed as
images on the screen. Ultrasound enables
us to observe the foetus in detail while it is still in the uterus.
What
we can see with ultrasound? By
measuring the length of the foetus, during the first two or three months of
pregnancy, the size of the foetus may be compared with the standard
parameters of gestational age, determined on the bases of the last menstrual
period. Ultrasound performed during the first three months of pregnancy enables
us to determine gestational age precisely, because sometimes a woman does not
remember the exact date of her last menstrual
period or her periods
are not regular. By the end of the second month the heart beat is visible with
ultrasound as well as the foetal movements and the number of foetuses. From the
fourth month on, the head, abdomen and femur measurements are taken and compared
with the reference
values. In this way the
size of the foetus is compared to the
expected size at that stage of pregnancy. In this period the site of placental
insertion, the quantity of amniotic liquid and some internal organs may be seen
with ultrasound. From
the fourth month the baby's gender can be determined, although it may depend on its position. This is
why it is not possible to determine the gender, even at the end of pregnancy,
if the baby is not in
the right position.
Is
it possible to observe the internal organs of the foetus? Yes,
it is. Besides the structures which are usually measured (head, abdomen, femur),
the bladder, kidneys, stomach and the intra-skull structures can also be
visualized. The other parts of the foetus's body, such as the heart, can also be
visualized with ultrasound, but detailed examination of certain organs is
carried out only in the event of a specific indication. The
possibility of discovering an anomaly depends on its size, the foetal position
in the uterus, the quantity
of amniotic liquid, the thickness of the mother's
abdominal wall and
the gestational age. For these reasons and because of the intrinsic limits of
the method, some lesions, even very important ones, may not be
detected.
Is
ultrasound harmless for the foetus? Ultrasound
has been used in obstetric practice for more than twenty five years with no
known harmful effects on the foetus, even after a considerable period. For
this reason, following the current procedures, the use of ultrasound for
diagnostic purposes, is considered risk-free.
Is
there a recommended number of ultrasounds that should be performed
during pregnancy? In
a normal pregnancy ultrasound should be performed three times, in the first,
second (20-22 weeks) and third trimester (30-34 weeks). If the consultant
advises it, it may even be performed at other times. If advised by the
ultrasound operator it may be necessary to repeat the ultrasound several times
during the same session.
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