Small for gestational age by grandmaternal smoking during pregnancy: Tohoku Medical Megabank Project Birth and Three-Generation Cohort Study

Small-for-gestational age (SGA) is an important global public health issue because of its increasing prevalence and long-term effects. Maternal smoking is a known risk factor for SGA; however, the effect of grandmaternal smoking on the risk of SGA in grandchildren SGA remains unclear. In this study, we examined whether grandmaternal smoking during pregnancy was associated with small birth weight, length, and head circumference for gestational age. Data were obtained from 23,730 pregnant women and their offspring from the Tohoku Medical Megabank Project Birth and Three-Generation Cohort Studies. A total of 1,130 grandmaternal-maternal-child triads were identified. Grandmaternal smoking during pregnancy was defined by the Maternal and Child Health Handbook owned by the mothers at birth mothers when they were born. Birth outcomes of grandchildren were obtained from medical records and converted to SGA using the 10^th percentile for weight, length, and head circumference. A multivariate logistic regression and propensity scores were used for the analysis. Prevalence of <10th percentile for birth weight, length, and head circumference in grandmaternal smokers were 10.2%, 2.0%, and 10.2%, respectively. Grandmaternal smoking during pregnancy was associated with the lower grandchild's birth weight (odds ratio (OR) [95% (CI)]: 2.86 [1.05-7.82]) and remained consistent when adjusted by propensity score (OR [95% CI]: 2.87 [1.04-7.92]). Grandmaternal smoking should not be ignored when assessing the SGA risk. Future work should consider the complex mediating relationship between smoking and growth restriction across generations.