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Multiple Gestations

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 frequency of 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 birth.

  • 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 level? Amniocentesis?


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 amniocentesis or chorionic villus sampling.

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 gestations.

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.
 
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Comments (2)


Tuesday, April 05, 2011 11:00 AM
(1) John  says:


My Twins

My wife and I are expecting twins. She is 39 and just finished her first trimester. We received her tests results and the screening risk for Down Syndrome is 1:12. My question is if we are having twins does therisk actually become 1:6? Thanks, John


Monday, April 25, 2011 2:15 PM
(2) Beata  says:


About the twins

Hi,I'm also having twins I'm 35 yr old and 17 weeks preagnant % like your wife my test came back 1:6 for each baby for down syndrome the spina bifida was fine .Today is Monday am having the amnio done on Wenesday am very scared .I wish u guys luck .Good Bles to you both and the babies .Beata





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