The contraceptive effect of breastfeeding remains essential to controlling fertility in many developing regions of the world. The extent to which this negative effect of breastfeeding on ovarian activity is sensitive to ecological conditions, notably maternal energetic status, has remained controversial. We assess the relationship between breastfeeding duration and postpartum amenorrhea (the absence of menstruation following a birth) in 17 World Fertility Surveys and 284 Demographic Health Surveys conducted between 1975 and 2019 in 84 low- and middle-income countries. We then analyze the resumption of menses in women during unsupplemented lactation. We find that a sharp weakening of the breastfeeding–postpartum amenorrhea relationship has globally occurred over the time period analyzed. The slope of the breastfeeding–postpartum amenorrhea relationship is negatively associated with development: higher values of the Human Development Index, urbanization, access to electricity, easier access to water, and education are predictive of a weaker association between breastfeeding and postpartum amenorrhea. Low parity also predicts shorter postpartum amenorrhea. The association between exclusive breastfeeding and maintenance of amenorrhea in the early postpartum period is also found in rapid decline in Asia and in moderate decline in sub-Saharan Africa. These findings indicate that the effect of breastfeeding on ovarian function is partly mediated by external factors that likely include negative maternal energy balance and support the notion that prolonged breastfeeding significantly helps control fertility only under harsh environmental conditions.