The Routine 18-20 Week Ultrasound Examination
In the state of Victoria a routine ultrasound examination is
offered to most patients between 18-20 weeks , and is it is loosely
referred to as the "18 week ultrasound". The purpose of this
examination is to:
- To determine the viability of the pregnancy. In fact the
heart rate may be measured as early as 6 weeks gestation.
- To determine the number of gestation sacs ie. singleton,
twins , triplets and higher multiples.
- To determine the size and the gestation of the pregnancy.
Several measurements are taken of the baby and include the
head size, the abdominal circumference and the femur length.
- To assess the fetal anatomy. This involves a detailed
examination of the fetal head ,brain, face, lips, heart,
stomach, lungs, abdominal wall, kidneys, bladder, spine
arms, legs, hands, and feet. Several landmarks within individual
organ systems are noted and recorded on video film for archiving.
Only upon request is fetal gender disclosed (assuming it
can be seen).
- Assessment of the position of the placenta.
- Assessment of the liquor volume .
- Assessment of the pelvic anatomy and the cervix.
This examination is expected to detect the majority of major
fetal malformations. It is important to appreciate however
that such an examination does not detect all abnormalities.
Many congenital heart abnormalities are complex and escape
diagnosis at the 18 weeks ultrasound examination. Also in
many instances the view of the fetus may he hampered by the
fetal position at the time of examination. Also ,the tissue
interposed between the ultrasound probe and the baby absorbs
the ultrasound waves, so if a particular patient is overweight
the fatty tissue of the abdominal wall may make visualisation
of the fetus difficult. In these circumstances the patient
may be rebooked for further assessment of the fetus.
Finally in certain circumstances fetal abnormalities may
not be evident on ultrasound despite adequate views. This
may be explained by the natural history of the condition where
the abnormality only becomes evident in later pregnancy or
where there are in fact no structural changes in the baby
(eg. cerebral palsy, biochemical abnormalities and some chromosomal
abnormalities).
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