What are dizygotic twins? Dizygotic twins are what the average person would call a fraternal twin. A woman’s body typically only releases one egg once a month but sometimes more than one is released, that is how dizygotic twins come about. Dizygotic twins share the same prenatal environment. The average person would believe that a pair of dizygotic twins would be the least common type of twin but in fact they are more common than identical twins.
In order to understand how a teratogen can affect not only one fetus let alone a set of twins, one must understand what a teratogen is and the process of fetal development.
Prenatal development occurs in a series of trimesters. The first trimester consists of the egg maturing and the fetus developing body structure as well as organ system development. The first trimester is the first ten to twelve weeks of pregnancy. The second trimester is where the fetus starts to present some sort of movement. The second trimester lasts from weeks thirteen to week twenty-six. Lastly, the third trimester, the final stretch of pregnancy, is where the uterus increases in size and one gives birth.
Throughout the prenatal development process if some sort of substance harms the fetus it is known as a teratogen. The substance that can cause harm can range from anything like environmental factors, for example, pollution or radiation and can further extend to factors like cigarettes, alcohol, or drugs that may cause harm. Although teratogens don’t always cause harm to the fetus, in larger and longer periods of time it can cause harm.
To specify how alcohol affects a pair of dizygotic twins, one must understand the effects of Fetal Alcohol Syndrome (FAS) in a singular fetus. If the mother abuses alcohol while pregnant, upon birth the baby will develop or have a lack of development in physical features as well as learning capabilities. In some cases babies can have a wide variety of under-developed features such as a smaller head size, shorter than average height, low body weight, difficulty with attention especially in a school setting which can lead to learning disabilities as well as delays in language and speech, and poor memory. In a set of dizygotic twins, FAS can affect one and not the other. A study found that “one twin had prenatal growth delays, neonatal withdrawal symptoms, and delays in both motor and cognitive function during the first year of life. The catch-up growth occurred during the postnatal period for the affected twin while the other twin was normal at the end of the follow-up at age 17 months”. In that study, they concluded that exposure to alcohol during the second half of pregnancy rather than the first half, greatly increases the risk for brain damage but not lasting postnatal growth.
The answer to if a teratogen such as alcohol can have an effect on both dizygotic twins would be no, one twin can be effected while the other be a healthy baby with no long-term effects, that doesn’t always give someone the green light to abuse alcohol while pregnant and the baby not have FAS.
References
Riikonen, R. S. (1994, November). Difference in susceptibility to teratogenic effects of alcohol in discordant twins exposed to alcohol during the second half of gestation. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/7702696.
Basics about FASDs | CDC. (n.d.). Retrieved from https://www.cdc.gov/ncbddd/fasd/facts.html.