Objective: To identify maternal and pregnancy-related physiological and pathological variables associated with fetal growth and birthweight in Ireland and to develop customized birthweight centile charts for the Irish population that will aid in appropriate identification and selection of growth-restricted fetuses requiring increased antenatal surveillance.Study design: Prospectively collected outcome data of 11,973 consecutive ultrasound-dated singleton pregnancies between 2008 and 2009 from six maternity units in Ireland (Dublin, Galway, Limerick and Belfast) were included for analysis. Maternal weight and height at booking, parity and ethnicity were recorded and combined with birthweight, fetal gender and pregnancy outcomes. Coefficients were derived by backward multiple regression using a stepwise backward elimination approach.Results: A total of 11,973 ultrasound-dated singleton pregnancies were included in the analysis. Over 90% of women (n = 10,850) were of Irish or European descent, 3.4% (n = 407) were African or African Caribbean, 1.7% (n = 208) were Indian; 42.2% (n = 5057) were nulliparous, 32.8% (n = 3923) had one previous delivery after 24 weeks' gestation, 15.6% (n = 1872) had two previous deliveries and 9.4% (n = 1121) had three or more previous deliveries. Mean term birthweight for a standard Irish mother was 3491grams. Babies of all other ethnic origins were smaller than their Irish counterparts. African Caribbean, Bangladeshi, Indian and Pakistani babies were on average 237 g, 196 g, 181 g and 181 g lighter, respectively, when compared to the average Irish offspring. Pathological factors significantly affecting term birthweight were pre-gestational diabetes (+137 g; p < 0.001), smoking (-225 g; p < 0.001), pregnancy-induced hypertension (-37.6 g; p = 0.009) and maternal obesity (-41.6 g; p = 0.012).Conclusion: Birthweight in this Irish maternity population is subject to similar influences to those observed in studies from the UK, Sweden, USA and Australasia. The derived coefficients can be used for customized assessment of fetal growth potential in Ireland. The implementation of these customized centile charts and their free online availability will aid clinicians in Ireland in the interpretation of fetal weight estimation. (C) 2012 Elsevier Ireland Ltd. All rights reserved.