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 greater than or equal to 20%. The concordant monochorionic twins with birth weight differences of greater than or equal to 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 <0.1), 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.