The above was from a 1993 study, where sixteen pairs of twins, all exposed to alcohol prenatally, were evaluated. They represented all available twins of mothers who confirmed  alcohol use during pregnancy and were on the patient rolls of the study authors.

In two pairs, one twin had fetal alcohol syndrome (FAS), while the other had fetal alcohol effects (FAE). In 2 other pairs, one twin had no diagnosis while one had FAE.

More information on this study can be found here.

In 2019 a, Twin study confirms virtually identical prenatal alcohol exposures can lead to markedly different fetal alcohol spectrum disorder outcomes- fetal genetics influences fetal vulnerability

The purpose of the study was to compare the prevalence and magnitude of pairwise discordance in FASD diagnoses across monozygotic twins, dizygotic twins, full-siblings, and half-siblings sharing a common birth mother.

The results showed:

As genetic relatedness between siblings decreased from 100% to 50% to 50% to 25% across the four groups (9 monozygotic, 39 dizygotic, 27 full-sibling and 9 half-sibling pairs, respectively), the prevalence of pairwise discordance in FASD diagnoses increased from 0% to 44% to 59% to 78%. Despite virtually identical PAE, 4 pairs of dizygotic twins had FASD diagnoses at opposite ends of the fetal alcohol spectrum—Partial Fetal Alcohol Syndrome versus Neurobehavioral Disorder/Alcohol-Exposed.

Not all fetuses are equally vulnerable to the adverse effects of prenatal alcohol exposure. Risk is not just dependent on timing and level of exposure. Fetal genetics plays an important role. As demonstrated in this study, despite virtually identical prenatal alcohol exposures, two fetuses can experience vastly different FASD outcomes.

Therefore the Study concluded:

Despite virtually identical PAE, fetuses can experience vastly different FASD outcomes. Thus, to protect all fetuses, especially the most genetically vulnerable, the only safe amount (of alcohol) to drink is none at all.

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