2.2. Model for infant mortality
In order to examine infant mortality conditional on health at birth, we modify the birth outcomes model to capture the fact that birth outcomes are a one-time occurrence but mortality is a continuously updated outcome. 
For example, the risk of death is highest in the first week or two of life and drops sharply thereafter. 
Therefore,we estimate a weekly hazard model with time-varying covariates to account for a varying probability of survival and levels of pollution over the infants’ first year of life. 
To do this, we treat an infant who lived for n weeks as if they contributed n person-week observations to the sample. 
The dependent variable is coded as in the period the infant dies, and 0 in all other periods. 
Each time-invariant covariate (such as birth parity) is repeated for every period, while the time-varying covariates (such as pollution and weather) are updated each period.