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 cle ar. We hypothesize that growth restriction of 1 monochorionic twin is due t o fetal under-nutrition. STUDY DESIGN: We measured plasma amino acid concentrations by high performa nce liquid chromatography in maternal venous and fetal umbilical cord venou s blood samples that were collected at birth from gestational age-matched m onochorionic twins (n = 14) with a birth weight discordance of greater than or equal to 20%. The concordant monochorionic twins with birth weight diff erences 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 t he co-twins and concordant monochorionic twin pairs. Fetal concentrations o f the nonessential amino acids taurine (P < .001), serine (P < .01), glycin e (P < .01), tyrosine (P < .01), and aspartic acid (P < .01) were lower in the intrauterine growth-restricted twin than the co-twin or concordant mono chorionic twins. No such differences were found between concordant monochor ionic twin pairs. Maternal amino acid concentrations were similar between d iscordant and concordant groups. CONCLUSION: Concentrations of certain essential and nonessential amino acid s in the intrauterine growth-restricted twins were lower than the co-twins. These differences support the hypothesis that intrauterine growth-restrict ion that affects 1 of the monochorionic twins is due to the impaired placen tal transport of amino acids rather than intertwin transfusion of blood.