Placental transport rather than maternal concentration of amino acids regulates fetal growth in monochorionic twins: Implications for fetal origin hypothesis

Objective: The cause of discordant growth in monochorionic twins is not clear. We hypothesize that growth restriction of 1 monochorionic twin is due to fetal under-nutrition. Study Design: We measured plasma amino acid concentrations by high performance liquid chromatography in maternal venous and fetal umbilical cord venous blood samples that were collected at birth from gestational age–matched monochorionic twins (n = 14) with a birth weight discordance of ≥20%. The concordant monochorionic twins with birth weight differences of ≤10% constitute a control group. Results: In the intrauterine growth-restricted twins, fetal concentrations of essential amino acids valine (P< .01), leucine (P< .01), isoleucine (P< .01), phenylalanine (P< .01), and L-arginine (P< .05) were lower than the co-twins and concordant monochorionic twin pairs. Fetal concentrations of the nonessential amino acids taurine (P< .001), serine (P< .01), glycine (P< .01), tyrosine (P< .01), and aspartic acid (P< .01) were lower in the intrauterine growth-restricted twin than the co-twin or concordant monochorionic twins. No such differences were found between concordant monochorionic twin pairs. Maternal amino acid concentrations were similar between discordant and concordant groups. Conclusion: Concentrations of certain essential and nonessential amino acids in the intrauterine growth-restricted twins were lower than the co-twins. These differences support the hypothesis that intrauterine growth-restriction that affects 1 of the monochorionic twins is due to the impaired placental transport of amino acids rather than intertwin transfusion of blood. (Am J Obstet Gynecol 2001;185:1239-46.)