Alcohol (ethanol) use during pregnancy can produce a wide spectrum of effects in the developing embryo/fetus that are dependent on the maternal drinking pattern. The effects of chronic ethanol exposure on the developing conceptus are reviewed with primary focus on ethanol teratogenesis, manifesting in the human as the fetal alcohol syndrome or fetal alcohol effects. The effects of acute ethanol exposure on the near-term fetus are described, including suppressed fetal breathing movements, electrocorticographic (ECoG) activity and electrooculographic (EOG) activity. The ethanol-induced suppression of fetal breathing movements is a very sensitive index of acute exposure of the near-term fetus to ethanol, and appears to involve a direct mechanism of action rather than an indirect mechanism involving suppression of electrocortical activity. The disposition of ethanol and its pharmacologically active proximate metabolite, acetaldehyde, and the activity of alcohol dehydrogenase and aldehyde dehydrogenase in the near-term maternal-fetal unit are described, and a pharmacokinetic model is proposed. The effects of short-term ethanol exposure on the near-term fetus include the development of tolerance to the ethanol-induced suppression of fetal breathing movements, low-voltage ECoG activity and EOG activity. The development of tolerance occurs more rapidly to the latter two fetal biophysical activities. The mechanism of tolerance development appears to be pharmacodynamic (functional) in nature, as there is no increase in the rate of ethanol elimination from the maternal-fetal unit. The role of prostaglandins (PGs) in the mechanism of the ethanol-induced suppression of fetal breathing movements is described. In the near-term fetus, there is a direct relationship between fetal blood ethanol concentration and fetal plasma PGE2 concentration, and an inverse relationship between the incidence of fetal breathing movements and each of fetal plasma and fetal cerebrospinal fluid (CSF) PGE2 concentrations. Indomethacin, a PG synthetase inhibitor, selectively blocks and reverses the ethanol-induced suppression of fetal breathing movements. These data support the postulates that the ethanol-induced suppression of fetal breathing movements is mediated by increased PGE2 concentration in the near-term fetus and that the ability of indomethacin to antagonize the ethanol-induced suppression of fetal breathing movements is due to its biochemical action to decrease fetal PGE2 concentration. Current and future research activities are directed towards determining whether the ethanol-induced suppression of fetal breathing movements is due to increased PGE2 concentration in the lower brain stem (lower pons/medulla) of the near-term fetus resulting either from increased local synthesis of PGE2 or from increased PGE2 synthesis at a site(s) outside the central nervous system and consequent transfer of PGE2 from the fetal systemic circulation into the lower brain stem.
