Associations of maternal quitting, reducing and continuing smoking during pregnancy on longitudinal fetal growth:evidence from Mendelian randomization and parental negative control studies

Background: Maternal smoking during pregnancy is an established risk factor for low infant birth weight, but evidence on critical exposure windows and timing of fetal growth restriction is limited. Here we investigate the effects of maternal quitting, reducing and continuing smoking during pregnancy on longitudinal fetal growth by triangulating evidence from three analytical approaches to strengthen causal inference.Methods and findings: We analysed data from 8621 European liveborn singletons in two population36 based pregnancy cohorts (the Generation R Study, the Netherlands 2002-2006 (n = 4682) and the Born in Bradford study, United Kingdom 2007-2010 (n= 3939)) with fetal ultrasound and birth anthropometric measures, parental smoking during pregnancy and maternal genetic data. Associations with trajectories of estimated fetal weight (EFW) and individual fetal parameters (head circumference (HC), femur length (FL), abdominal circumference (AC)) from 12/16 to 40 weeks (wks)’ gestation were analysed using multilevel fractional polynomial models. We compared results from 1). confounder adjusted multivariable analyses; 2). a Mendelian Randomization (MR) analysis using maternal rs1051730 genotype as an instrument for smoking quantity and ease of quitting; and 3). a negative control analysis comparing maternal and paternal smoking associations. In multivariable analyses, women who continued smoking during pregnancy had a smaller fetal size than non-smokers from early gestation (16-20 wks) through to birth (p value for each parameter < 0.001). Fetal size reductions in continuing smokers followed a dose-dependent pattern (compared to non-smokers differences in mean EFW (95% CI) at 40 wks’ gestation were -144 g (-182 to -106), -215 g (-248 to -182) and -290 g (-334 to -247) for light, moderate and heavy smoking, respectively). Overall, fetal size reductions were most pronounced for FL. The fetal growth trajectory of women quitting smoking in early pregnancy was similar to that of non-smokers, except for a shorter FL and greater AC around 36-40 wks’ gestation. In MR analyses, each genetically determined 1-cigarette-per-day increase was associated with a smaller EFW from 20 wks’ gestation to birth in smokers (p = 0.01, difference in mean EFW at 40 wks= -45.4 g (-80.7; -10.2)) and a greater EFW from 32 wks’ gestation onwards in non-smokers (p = 0.03, difference in mean EFW at 40 wks = 25.6 (4.6; 46.7)). There was no evidence that paternal smoking was associated with fetal growth. Study limitations include measurement error due to maternal self-report of smoking and the modest sample size for MR analyses resulting in unconfounded estimates being less precise. The apparent positive association of the genetic instrument with fetal growth in non-smokers suggests that genetic pleiotropy may have masked a stronger effect in smokers.Conclusions: A linear dose-dependent effect of maternal smoking on fetal growth was observed from the early second trimester onwards, while no major growth deficit was found in women quitting smoking early in pregnancy except for a shorter FL during late gestation. These findings reinforce the importance of smoking cessation advice in preconception and antenatal care and show that smoking reduction can lower the risk of impaired fetal growth in women who struggle to quit