Comparison of longitudinal and cross-sectional analysis: maternal age and stillbirth ratio.

Previous studies have reported a U or J shaped association between stillbirth ratio and maternal age and have led to attempts to determine the causes behind the association. These analyses have generally been based on cross-sectional data. This study contrasts the descriptions of the association resulting from cross-sectional and the more appropriate longitudinal analysis of the same body of data. It is concluded that cross-sectional analysis substantially underestimates the maternal age of which stillbirth ratio reaches a minimum for each birth order examined. Additionally, the basic shape of the relationship is sometimes altered. Weighted logistic polynomial regression is used to describe the associations. Traditional interpretations of the results of a cross-sectional analysis have been that women who postpone childbearing until the late twenties thereby increase their risk of stillbirth. A consequence of the present findings is the rejection of the claim that women who choose employment during the period of their early twenties automatically incur elevated risk of stillbirth in their postponed pregnancies.