Correspondence E Petridou Department of Hygiene and Epidemiology Athens University Medical School Goudi 115-27 Athens Greece Modulation off birthweight through gestational age and ffetal growth E Petridou,*t D Trichopoulos,t K Revinthi,* D Tongf and E Papathomat */6epartment of Hygiene and Epidemiology, Athens University Medical SchooUAthensJGreecej' tDepartment of Epidemiology, Harvard School of Public Health, Boston, USA and :tDepartment of Neonatalogy, Neonatal Unit, Alexandra Hospital, Athens, Greece Accepted for publication 4 August 1995 Child: care, health and development VOLUME 22 NUMBER I 1996 PAGES 37-53 Summary Several factors are known to affect birthweight and their effects are variously mediated through gestationai duration or through fetal growth conditional on this gestation, in order to quantify independent associations of birthweight condi- tional and unconditionai on gestational age, all 2538 mothers of singieton babies delivered during 1993 in two Maternity Hospitais in Athens were inter- viewed and their obstetric records abstracted. Birthweight was modelled as outcome variable through multipie regression inciuding 32 potentially predictive factors. The regression model was fitted with and without gestational age as an additional independent variable in order to apportion birthweight associations into those independent of, or mediated through, gestationai length. The factors studied were found to be classifiable into the foilowing categories: factors associated with birthweight mostly through increases in gestationai duration, either positively (age at menarche, long menstrual cycies, parity 4 or higher), or negatively (single motherhood, maternal age, tobacco smoking); those associated with birthweight mostly through increase of birthweight condi- tional on gestationai duration, either positiveiy (maie gender, short menstrual cycles, maternal pre-pregnancy weight, anaemia, oedema) or inversely (em- ployment during pregnancy, stiilbirth, primiparity, pregnancy induced hypertension, coffee drinking); and those associated with birthweight through © 1996 Blackwell Science Ltd 37