Finally, when we control for mother fixed effects in columns (3), (6), and (9), the estimates for CO become even larger. Ozone in the second trimester now has a statistically significant negative effect at the 10% level on birth weight and gestation.
For PM10 the first trimester in the low birth weight regression is statistically significant at the 10% level.
This pattern of results across specifications suggests the importance of controlling for both maternal and neighborhood fixed effects to account for confounding factors.
It also suggests that in New Jersey, conditional on other observable characteristics of mothers, mothers in more polluted areas have unobserved characteristics that make them more likely to have healthy infants.