Multifactorial inheritance means that many factors are involved in causing a birth defect. The factors are usually both genetic and environmental, where a combination of genes from both parents, in addition to unknown environmental factors, produce the trait or condition. Often one gender (either males or females) is affected more frequently than the other in multifactorial traits. There appears to be a different threshold of expression, which means that one gender is more likely to express the condition over the other gender. For example, hip dysplasia is nine times more common in females than males.
Multifactorial traits do recur in families, because they are partly caused by genes. The chance for a multifactorial trait or condition to happen again depends upon how closely the family member with the trait is related to you. For example, the risk is higher if your brother or sister has the trait or disease, than if your first cousin has the trait or disease. Family members share a certain percentage of genes in common, depending upon their relationship. For example:
|Degrees of relationship||Percentage of Genes in Common||Example|
|First Degree Relative||50 percent||Parents, children, siblings|
|Second Degree Relative||25 percent||Aunts, uncles, nieces, nephews, grandparents|
|Third Degree Relative||12.5 percent||First cousins|
Now, if we look at a multifactorial condition, such as a neural tube defect (NTD), the chance for this birth defect to be seen in a future pregnancy would be:
As you can see, the more genes you have in common with the family member, the higher the chance for you to have a child with a similar defect.
Examples of multifactorial traits and diseases include: height, neural tube defects, and hip dysplasia.
ONTDs occur in children without a prior family history of these defects in over 90 percent of cases. ONTDs result from a combination of genes inherited from both parents, coupled with environmental factors. Some of the environmental factors include uncontrolled diabetes in the mother, and use of certain medications that are available by prescription only. ONTDs are seen five times more often in females than males. Once a child has been born with an ONTD in the family, the chance for an ONTD to happen again in a future pregnancy is increased to 4 percent. It is important to understand that the type of neural tube defect can differ the second time. For example, one child could be born with anencephaly, while the second child could have spina bifida and not anencephaly.
The neural tube closes 28 to 32 days after conception, before many women are aware they are pregnant. Folic acid is a B vitamin found to reduce the chance for neural tube defects to occur. For this reason, the American College of Medical Genetics (ACMG) and the CDC recommend that all women in their reproductive years take a multivitamin containing folic acid. However, do not take more than one multivitamin per day.
If a couple has had a previous child with an ONTD, a larger amount of folic acid is recommended. The CDC recommends that a woman take 4.0 mgs (4,000 mcgs) of folic acid one month before becoming pregnant (before conception). To obtain this amount of folic acid, you must get a prescription from your physician or health care provider.
ONTDs can be diagnosed before birth by measuring a protein called AFP (alpha-fetoprotein) present in the amniotic fluid around the baby. Fetal ultrasound during pregnancy can also give information about the possibility of an ONTD, but is not 100 percent accurate, since some babies with an ONTD may look the same on ultrasound as those without these defects. Measurement of the AFP, and other biochemical markers from amniotic fluid, is over 95 percent accurate for detecting ONTDs. Small or closed defects (which do not leak spinal fluid) may not be picked up by this test.
For all women who are pregnant who have not previously had a child with an ONTD and do not have a family history of ONTDs, the American College of Obstetrics and Gynecology (ACOG) recommends that a blood test be offered between 15 to 20 weeks, to measure AFP (and other biochemical markers) to determine whether a pregnancy is at increased risk for an ONTD. Although this test (sometimes called maternal serum screening, the double screen, triple, or quadruple screen) does not tell a couple for certain whether their baby has an ONTD, it will determine which pregnancies are at greater risk, so that additional testing will be offered.
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