ABSTRACT
During pregnancy, the maternal organism undergoes a number of remarkable physiological adjustments, many of them involving the absorption, distribution, and metabolism of ingested compounds. Gastrointestinal motility is generally slowed, presumably reflecting an inhibitory action of progesterone on smooth muscle, and the extent of absorption tends to increase. The trans-placental gradient for most amino acids is maintained such that the fetal plasma concentration is typically one and one-half to two times the maternal concentration. Lemons and associates measured whole blood concentrations of twenty two amino acids over the last third of gestation in a chronically catheterized unstressed fetal lamb preparation. Maternal intravenous administration of phenylalanine led to a significant accumulation of radioactivity in the fetal circulation, compared with only minimal transfer to the maternal circulation when the compound was given to the fetus, findings interpreted as indicating an active placental transport mechanism from mother to fetus.
