Previous studies of postpartum amenorrhea (PPA) demonstrated distinct subgroups of women with short and long durations of amenorrhea. This phenomenon was attributed to cases where breastfeeding is absent because of pregnancy loss or infant death, or confusion of postpartum bleeding with resumption of menses. We explored these ideas using data from an 11-month prospective study in Bangladesh in which 858 women provided twice-weekly interviews and urine specimens for up to 9 months; 300 women were observed while experiencing PPA. The resulting exact, interval-censored, or right-censored durations were used to estimate parameters of two-component mixture models. A mixture of two Weibull distributions provided the best fit to the observations. The long-duration subgroup made up 84% (+/- 4% SE) of the population, with a mean duration of 457 (+/- 31) days. The short-duration subgroup had a mean duration of 94 (+/- 17) days. Three covariates were associated with the duration of PPA: women whose husbands had high-wage employment had a greater probability of falling in the short-duration subgroup; women in the long-duration subgroup whose husbands seasonally migrated had shorter periods of PPA within the subgroup; and mothers in the short-duration subgroup who gave birth during the monsoon season experienced a shortened duration of PPA within the subgroup. We conclude that the bimodal distribution of PPA reflects biological or behavioral heterogeneity rather than shortcomings of data collection.
Copyright 2006 Wiley-Liss, Inc.