Background and Aim: There is growing evidence that green spaces may be beneficial for health. However, the effects on pregnant individuals and fetuses, including the underlying mechanisms, have been underexplored. We examined the association between neighborhood greenness and fetal growth, and the role of air pollution in these relationships (interaction and mediation) in a Massachusetts-based cohort. Methods: Our study consisted of 9,446 pregnancies that delivered at Beth Israel Deaconess Medical Center, Massachusetts in 2011-2016. Neighborhood greenness was estimated using normalized difference vegetation index (NDVI) with 270 m resolution. Fetal growth parameters included birth weight (BW), and ultrasound measures of biparietal diameter (BPD), head circumference (HC), femur length, and abdominal circumference (AC). Ultrasound parameters were classified into three distinct gestational periods: 16-23 weeks, 24-31 weeks, and 32+ weeks. We used distributed lag models to estimate the time-varying association between monthly NDVI and fetal growth. All models were adjusted for sociodemographic characteristics, and time trends. For models in which we identified an association, we tested for potential interaction with particulate matter ≤2.5 µm (PM2.5) and nitrogen dioxide (NO2) —predicted from well-validated ensemble models— and assessed for mediation by these pollutants. Results: We found that higher NDVI was positively associated with BPD, HC, AC, and BW. For example, a 0.1 NDVI increase sustained up until week 32 was associated with a higher mean AC z-score of 0.03 (95% CI: 0.01, 0.06), and a higher mean BW z-score of 0.02 (95% CI: 0.01, 0.03). We found no evidence for interaction or mediation by air pollution. Conclusions: Residential greenness was associated with larger fetal growth measures. Our effect decomposition suggests that the association of greenness with fetal growth is independent from associations with local air pollution. Keywords: greenness, air pollution, fetal growth, ultrasound, pregnancy, interaction, mediation