Effects of maternal socio-demographic characteristics on the birth weight distribution in Greece: a quantile regression analysis

Georgia Verropoulou, University of Piraeus
Cleon Tsimbos, University of Piraeus

The present study aims at exploring and quantifying effects of the socio-demographic characteristics of the mother across the distribution of birthweight in Greece. For the purposes of the analysis nationwide vital registration micro-data are used. The data include 103,266 single live births registered in 2006 to women aged 20 or higher. The response variable is birth weight, recorded in grams. The method involves application of quantile regression models. Births occurring before the 37th week of gestation (preterm) are examined separately from those occurring at term as they form distinct and independent distributions while they are also influenced by different risk factors. The effects of the explanatory variables are estimated at 6 different quantiles of the response variable (0.05, 0.10, 0.25, 0.50, 0.75 and 0.90). Estimates are then compared with results based on OLS models. The results reconfirm that preterm and term births form distinct birth weight distributions. Moreover, effects of most factors differentiate across the distributions in both instances. More specifically, for most predictors the effect is greater at the lower tail, i.e. among low birth weight babies. For instance, illegitimate live births at full term have lower weight by 130 grams at the 0.05 quantile compared to legitimate babies while the difference is only 60 grams at the 0.90 quantile. By contrast, the OLS indicates a difference of 82 grams. Hence, the method has an advantage over OLS, which assumes a constant effect. The findings indicate that female sex, primiparity, illegitimacy status, age of the mother over 35, past history of stillbirths and deceased children, living in big metropolitan areas, and being a Greek citizen are associated with a lower birth weight while tertiary education has a protective effect. Among preterm births parity, prior stillbirth and deceased child and education have a more pronounced effect.