How does a fetus get fetal alcohol syndrome? What modifies the fetus's growth?

Alcohol injestion. Any amount of alcohol while pregnant can cause fas. We do not know the amount needed. Sometimes we see only fetal alcohol effect, not syndrome. Usually, if a mom injests significant alcohol, her nutrition is poor which delays fetal growth.
Binge drinking, Having 4 drinks during a party, or having 1/2 oz, of absolute alcohol/day (1 beer, 1 glass of wine or 1 mixed drink) can cause fetal alcohol spectrum disorders. There's no known safe amt. Or time to drink when pregnant. The brain starts forming 3 weeks post-conception often before pregnancy diagnosis.It's important to stop drinking to prevent further damage. See www.Fasdcenter.Samhsa.Gov.
FAS. A fetus gets fetal alcohol syndrome from alcohol that the mother drinks during pregnancy. That's why it is recommended that pregnant women do not drink.
Toxic effect. When ethanol is broken down in the body, byproducts including acetaldehyde can have a direct toxic effect on tissue formation. There is some suggestion that some are genetically more susceptible than others. The severity of FAS correlates directly with the intensity of alcohol exposure.