Figure 4.

Parity-specific unadjusted smoothed rates and fitted adjusted relative rates of PE for primiparous (a,b) and multiparous (c,d) pregnancies in Norway. The smoothed curve in (a), and (c) is based on a moving average of outcomes that uses a window encompassing $\pm 30\text{d}$ from each day of the year, thus estimating the fraction with PE among pregnancies conceived at each day of the year. The fitted relative rates shown in (a), and (c) are based on the first harmonic (sine and cosine) fit separately to each of the two parity categories, primiparous (b), and parous (d), with models that are adjusted for the covariates maternal age, maternal age squared, education, smoking (in three categories), marital status, and region (in three categories). The curve shown is based on entering the parity-specific mean for all covariates, centering the estimated log hazard ratio and then exponentiating it.