Whether or not one of the fetuses stops developing
What are the risks for defects?
Essentially a fetus in a pregnancy involving twins or higher is not at a greater
risk for genetic problems if ICSI treatment was not performed. In spite of this, however,
more defects and genetic problems are associated with twin or higher order pregnancies
than with singleton pregnancies. There are a number of reasons for this:
- In a singleton pregnancy each fetus has approximately a 3% risk of defects and mental retardation. In a twin pregnancy the risk
increases to 6% (because there are two fetuses). For each disease there is a double
risk in twins and a triple risk in triplets, etc.
- In any pregnancy certain defects can arise from disrupted tissue development in
the critical stage of its formation (organization of cells in the tissue) because
of a specific event such as disrupted blood flow (to the tissue or to the entire
embryo), infection, or metabolic or vitamin deficiency. Twins are at increased risk
for impaired blood flow in the fetal blood circulation because sometimes there is
an exchange of blood between the two fetuses, blood clots can pass from one to the
other, there may be pressure by the sibling on blood vessels, etc. There is an increased
risk for defects secondary to and as a result of these conditions. Such defects
are called disruptions, as genetically everything is normal, but ischemic damage
(decreased/impaired blood perfusion) to the tissue leads to a developmental disorder
expressed as a defect or necrosis in the tissue. For instance, there is an increased
two umbilical blood vessels instead of 3, as a result of clotting and obliteration
of one artery. Usually the umbilical cord contains 2 arteries that transfer blood
from the placenta to the fetus, and thereby "feed" the fetus, and 1 vein that returns
the blood from the fetus to the placenta, but in this situation the cord contains
1 artery and 1 vein. This is common in twins and higher multiple gestations and
may often be associated with several anomalies. Such a situation is referred to
as the VACTER association ("V" - Vertebra, "A" - Anal atresia, "C" - Cardiac defect,
"T - E" - Tracheo-Esophageal atresia, and "R" - Radial and Renal anomalies). This
association represents a combination of defects that are associated with environmental
influence in the first trimester, mainly involving defects in one of the kidneys,
the anal orifice, the esophagus, vertebrae, thumbs, heart, etc. These occur more
commonly in twin pregnancies. Intestinal blockages and defects such as missing parts
of the extremities, mainly digits, etc., also occur in cases with clotting events
in fetal life and are more common in twins.
- There is a risk of delayed growth. The more severe this is, the greater the reduction
in brain growth. Beyond a certain limit this increases the risk for mental retardation,
even though there is no actual defect.
- The effect of the fetuses pressing against one another can be to cause deformities
such as recesses in the skull at the pressure site. These usually resolve after
- Premature birth and birth complications are more common in twin pregnancies, and
this affects the final outcome.
Should I undergo a scan for nuchal translucency? A test to measure the alpha-fetoprotein
Regarding chromosome analysis in multiple gestations, as mentioned, there is no
increased risk for chromosomal problems among fetuses of these pregnancies as compared
to the fetus in a singleton pregnancy.
However, these pregnancies are usually "precious", and are at increased risk during
It is therefore important that the decision whether or not to perform either of
these tests is made on clear medical grounds.
The problem is that in multiple gestations it is difficult to evaluate the risk
for Down syndrome based on
the blood biochemistry screening (alpha-fetoprotein).
The test is performed on the mother's blood, meaning that even if one of the fetuses
is abnormal, the test results may still be normal because of the results of the
other fetus or fetuses. On the other hand,
nuchal translucency testing is reliable, because it directly evaluates the
risk for each of the fetuses separately. Because of this, the nuchal translucency
test has become an important test in multiple
Most physicians believe that if the result of the nuchal translucency test indicates
that the risk for a chromosomal problem is low, there is no reason to calculate
the risk for Down syndrome from the maternal blood biochemistry screening test in
the first trimester of pregnancy. However, performing this first trimester screening
test does not eliminate the need to test the level of alpha-fetoprotein in the maternal
blood between the 16th and the 18th week. This test, which is intended to identify
defects in the closure of the spinal column ( neural tube defects), also gives reliable results
in multiple gestations. However, if one of the fetuses stops developing (spontaneous
intrauterine reduction or death after week 11) but one or more fetuses remain, the
biochemical screening test loses its value completely, both for evaluating the risk
for Down syndrome and regarding defects associated with a high alpha-fetoprotein
level (neural tube defects).
What happens if a defect or a severe disease is found in one of the fetuses?
In a multiple gestation in which a defect is found in one of the fetuses, selective
"reduction" of the affected fetus can be carried out. This procedure will usually
not harm the healthy fetuses.