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To investigate the hypothesis that the genotype at nucleotide A(−20)C in the 5′ flanking region of the angiotensinogen gene, which lies within a sequence with high homology to an oestrogen response element, affects plasma angiotensinogen levels in pregnancy.Prospective observational studySeventy-two healthy pregnant women were recruited in the second half of pregnancy from hospital and primary care antenatal clinics in Nottingham, UK. Plasma angiotensinogen concentrations were measured by radioimmunoassay of angiotensin I generated from endogenous angiotensinogen in the presence of excess human renin. DNA was extracted from peripheral venous blood, and angiotensinogen genotype determined at A(−20)C, G(−6)A and Met235Thr. Associations between genotype and plasma angiotensinogen concentration were assessed by analysis of variance.Women homozygous for the −20C allele had the lowest mean plasma angiotensinogen concentration of 1.7 ± 0.3 μmol/l. Women homozygous for −20A had significantly higher plasma angiotensinogen concentrations (2.6 ± 0.1 μmol/l), and intermediate levels (2.0 ± 0.1 μmol/l) were observed in women heterozygous for A(−20)C (P = 0.002, ANOVA). The polymorphisms at nucleotide −6 and codon 235 were in almost complete linkage disequilibrium, and nucleotide −20C was found only in a subset of −6 A/235Thr alleles.The low plasma angiotensinogen levels associated with the −20C/-6A/235Thr haplotype in pregnant women contrast with the high concentrations associated with the 235Thr allele in the non-pregnant state. A possible explanation lies in the presence of a motif with high homology to an oestrogen response element between the TATA box and transcription initiation site. Previous in vitro studies of reporter gene constructs have demonstrated that the A(−20)C polymorphism affects oestrogen responsiveness. The results of this study support the hypothesis that the oestrogen response element of the angiotensinogen gene is of functional importance in pregnancy, and that oestrogen responsiveness in pregnancy is influenced by the genotype at nucleotide −20.