Birthweight discordant female twins and their offspring: is the intergenerational influence on birthweight due to genes or environment?

STUDY QUESTION Does the intergenerational influence on birthweight and birth length remain within female dizygotic and monozygotic twin pairs? SUMMARY ANSWER The intergenerational influence on birthweight and birth length remained within dizygotic but not within monozygotic twin pairs. WHAT IS KNOWN ALREADY Low birthweight is associated with increased morbidity and mortality in both the short and long term; therefore it is important to understand determinants of fetal growth. There is a known intergenerational association between parents' and offspring's size at birth. STUDY DESIGN, SIZE, DURATION This is a register-based cohort study with a nested within-twin-pair comparison. The study is retrospective, but based on prospectively collected information. The study population included 8685 monozygotic and like-sexed dizygotic female twins born in Sweden from 1926 to 1985, who had given birth to their first infant between 1973 and 2009. PARTICIPANTS/MATERIALS, SETTING, METHODS This study is set in Sweden and used data from the Swedish Twin Register and the Swedish Medical Birth Register. We used generalized estimating equations to obtain regression coefficients with 95% confidence intervals (CI) for the outcomes: offspring birthweight and birth length. To control for genetic and shared environmental factors, we performed within-twin-pair analyses in 1479 dizygotic and 1526 monozygotic twin pairs. MAIN RESULTS AND THE ROLE OF CHANCE In the cohort of both dizygotic and monozygotic twins, there was an association between mother's and offspring's size at birth. Within-dizygotic twin pairs, a 500-g increase from the twin pair's mean birthweight was associated with increased offspring birthweight [70 g (95% CI: 35-106)] and birth length [0.22 cm (95% CI: 0.07-0.38)]. The corresponding increase in birth length of 1 cm was estimated to increase offspring's birthweight by 26 g (95% CI: 12-40) and birth length by 0.11 cm (95% CI: 0.04-0.17). Within-monozygotic twin pairs there were no such associations. LIMITATIONS, REASONS FOR CAUTION This study is limited to twins who themselves or whose co-twin voluntarily responded to questionnaires. WIDER IMPLICATIONS OF THE FINDINGS The intergenerational influence on size at birth is suggested to be due to direct or indirect genetic factors.