摘要：Objectives. We examined age, period, and cohort (APC) effects on temporal trends in stillbirths among Black and White women in the United States. Methods. We conducted a cohort study of Black and White women who delivered a singleton live-born or stillborn infant during 1981 through 2000. We analyzed stillbirth rates at 20 or more weeks of gestation within 7 age groups, 4 periods, and 10 “central” birth cohorts after adjusting for confounders. Results. In both racial groups, women younger than 20 years or 35 years or older were at increased risk of stillbirth; risks decreased over successive periods in all age groups. Birth cohort had no impact on stillbirth trends among Blacks and only a small, nonsignificant effect among Whites. Analyses of various APC combinations showed that Blacks were at a 1.2- to 2.9-fold increased risk for stillbirth relative to Whites. Attributable fractions for stillbirth because of age, period, and cohort effects were 16.5%, 24.9%, and 0.1%, respectively, among Black women and 14.5%, 36.2%, and 2.1%, respectively, among White women. Conclusions. Strong effects of age and period were observed in stillbirth trends, but these factors do not explain the persistent stillbirth disparity between Black and White women. During the past few decades, rates of stillbirth at 20 or more weeks of gestation have declined substantially in Western countries. In the United States, for instance, the stillbirth rate declined by 52% between 1970 (14.0 per 1000 total births) and 1998 (6.7 per 1,000 total births), with the rate of decline more evident during earlier than more recent decades. 1 , 2 However, disparities in rates of stillbirth remain, with Black women in the United States at an approximately 2-fold greater risk of delivering a stillborn fetus than White women. 2 In assessments of stillbirth trends, the goal of an age, period, and cohort (APC) analysis is to disentangle the complex associations of maternal age, historical trends (period effects), and life course factors (maternal birth cohort effects). Several studies have shown that risks of stillbirth increase at the extremes of maternal age 3 , 4 and decrease at successively more recent time periods. 2 However, these studies have inevitably failed to shed light on how differences in stillbirth rates vary by maternal age across generations. An APC analysis might help provide an understanding of how age, period, and birth cohort have affected temporal trends in stillbirth. For instance, trends in stillbirths that are strongly associated with maternal age would support the hypothesis of a biological effect related either to aging or to selective fertility. 5 A strong period effect would suggest general improvements in maternal health, access to pre-natal care, antepartum surveillance, or other aspects of obstetric management, including increases in rates of routine labor induction or cesarean section. A period effect might also raise the question of changes in the registration of stillbirths, particularly at very short lengths of gestation (20–25 weeks). On the other hand, the presence of a cohort effect, although rarely examined, might lead to an understanding of the effects of persistent or cumulative (maternal) exposure to risk factors during the fetal period or childhood on stillbirth trends.
