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The objective was to test the fetal origins hypothesis by examining the association between fetal growth and systolic blood pressure in a large cohort of adult male twins of known zygosity. This cohort study is based on a record linkage of routinely collected data from the Swedish Medical Birth Registry, the Swedish Military Conscription Registry and a mailed questionnaire. It comprises 886 pairs of same-sex male twins born in Sweden in 1973-79 for whom data were available on size at birth and systolic blood pressure, height and weight at age 17-19 years. Based on self-reported similarity of appearance in childhood, 384 twin pairs were classified as monozygotic (MZ), 269 as dizygotic (DZ) and 233 as of uncertain zygosity (XZ). Differences in systolic blood pressure within and between twin pairs were analysed in relation to birthweight and birthweight-for-gestational-age z-score using random effects linear models. Within twin pairs, a 1 kg difference in birthweight was associated with a difference of -1.30 mmHg in systolic blood pressure [95% confidence interval -4.15, +1.54] for MZ twins and +0.14 [-3.49, +3.76] for DZ twins; for all twins combined it was -0.21 [-2.13, +1.71]. Between twin pairs, a 1 kg difference in birthweight was associated with a difference of -2.68 mmHg in systolic blood pressure [-4.95, -0.42] for MZ twins and +0.28 [-2.35, +2.91] for DZ twins; for all twins combined -1.68 [-3.15, -0.22]. All these estimates included adjustment for age, year and conscription centre of examination, gestational age and height and weight at conscription. These results provide little support for the fetal origins hypothesis. The estimates of the within-pair effect in MZ twins and their wide confidence intervals (including zero) cannot exclude the existence of common genetic mechanisms, but are also not inconsistent with an in utero programming effect. This study highlights some methodological problems with twin studies.