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Two blood vessels in the umbilicus(single umbilical artery)
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What is this and what is its significance?
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Usually the umbilical cord has two arteries that bring blood to the fetus and one
vein that returns the blood to the placenta. However, in 1% of pregnancies there
is one vein and only one artery. This can arise as a result of clotting and obliteration
of the second artery, and it is therefore more common in twin pregnancies, in which clots in blood vessels
are more common. Even though the fetus is apparently being nourished by only one
artery, in 99% of cases this will not affect its nutrition and its growth will be
normal.
A single umbilical artery in itself is not a defect, because the umbilical cord
is not an organ of functional importance once the fetus is born. However, it is
an indication that various fetal defects may be present and tests should be performed
in order to look for these. If any defects are found, further investigation is necessary
because of an increased risk of genetic and chromosomal syndromes.
In the great majority of cases there are no additional defects and the fetus's growth
is normal. In these cases, the extra risk for additional problems is less than 1%
in excess of the regular risk in pregnancy (3% - 4%).
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The extra risk is mainly of
intrauterine growth retardation, defects in the structure of the kidneys
and urinary tract, heart
defects, cleft lip and palate, defects in the structure of the vertebrae,
defects in the digestive system (anal or esophageal atresia, which cause obstruction),
and defects in the structure of the thumb. These can all be detected by ultrasound.
Some reports in the literature show that even when there are no other abnormal findings
there is still an increased risk of an associated chromosome disorder in cases where
there is a single umbilical artery. According to some studies there is a threefold
increase in the risk for Down syndrome, whereas other studies have shown that in
the absence of additional defects demonstrated by ultrasound there is no significant
increase in the risk for Down syndrome
in particular and chromosome disorders in general.
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What should be done if a single umbilical artery is diagnosed?
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- A thorough 3rd level ultrasound scan should be performed between the 22nd and the
24th week of gestation (and/or between the 15th and the 16th week) in order to look
for additional defects. This should be aimed at looking specifically for the defects
mentioned above, including those of the heart.
- Fetal echocardiography
can also be performed in the 24th week.
- The rate of growth of the fetus should be monitored.
- Amniocentesis
(or chorionic
villus sampling) is undoubtedly the only test that can categorically rule
out chromosome disorders. However, it is neither medically indicated nor recommended
that every pregnant woman should undergo amniocentesis - only those who are at high
risk of having a specific problem that can be examined by this test (see information
sheet titled: Why is there no medical recommendation for amniocentesis
in all cases?). Most opinions hold that in the majority of the cases where
the single umbilical artery is an isolated finding, the weighted risk is not in
the high-risk range for which amniocentesis is recommended. However, it should be
noted that at the present time there are no accurate statistical data that allow
for a precise calculation of the weighted risk taking the results of all the tests
into account.
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For practical purposes:
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The "gold-standard" threshold for recommendation of amniocentesis is a risk of Down
syndrome greater than 1:386 based on the results of the biochemical marker screening
tests, and when the weighted risk is equal to or higher than this amniocentesis
is generally recommended. However, the presence of a single umbilical artery may
statistically increase the risk of Down syndrome. Therefore in these cases some
physicians suggest that this should be integrated with the results of the biochemical
screening tests and the new threshold for recommending amniocentesis is a risk for
Down syndrome of greater than 1:1000 in the biochemical marker screening tests.
If other abnormal findings are also present, these guidelines are insufficiently
established and the woman should be referred for genetic counseling.
In genetic counseling,
the necessity for amniocentesis can be assessed. It is important to recommend counseling
in all cases where other signs or defects are present, because the risk of an associated
chromosomal or other syndrome is then significantly increased, and the advisability
of performing further tests should be discussed.
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