Figure 4 .

Illustration of immortal time where exposure is diagnosis of preeclampsia and the outcome is stillbirth. The star represents time of preeclampsia diagnosis and the arrow indicates when stillbirth occurs. Solid lines represent unexposed person-time and dashed lines exposed person-time. In (a), the mother is not diagnosed with preeclampsia for the duration of pregnancy. In (b), the mother is diagnosed, and the time before diagnosis occurs is the immortal time, because if she had experienced a stillbirth prior to the diagnosis, as in (c), she would have been in the unexposed, or no preeclampsia, group. Therefore, pregnant woman (b) could not have experienced a stillbirth prior to her preeclampsia diagnosis while maintaining her status in the exposed group. The time before exposure occurs should be considered unexposed, whereas the time after exposure should be counted as exposed person-time, as in (d)