Elusive exclusivity: Comparing cross-sectional and longitudinal data on exclusive breastfeeding

Background: Exclusive breastfeeding maximizes health outcomes but is difficult to measure, and cross-sectional data, used in many studies, may be unreliable. We aimed to compare “exclusivity rates” when collected by 2 different methods in the same infants at 4 months. Exclusivity was defined as only breast milk, with no formula or solid foods. Methods: We collected feeding data on infants born at an inner city Boston hospital between 2008-2010. We recorded all feeds in week 1; then collected feeding data by weekly (month 1) and monthly (months 2-4) phone calls to the mother. At 4 months, we calculated an “exclusive breastfeeding rate” based on cross sectional data (a single answer gained from maternal 24 hour recall at 4 months) and another based on longitudinal data (all data collected since birth for each infant). Results: According to cross-sectional, maternal 24 hour-recall at 4 months, 13% (33/248) of 4 month olds were exclusively breastfed at 4 months. However, according to longitudinal data, only 6% (16/248) of 4 month olds were exclusively breastfed; 48% (17/33) were recorded as having received non-breast milk feeds in the previous 4 months. Conclusion: Exclusive breastfeeding rates may be significantly over-reported in cross sectional studies based on 24 hour recall, leading to over-estimated rates of exclusivity. True rates of exclusivity may be far lower, as our longitudinal data did not record all feeds and probably missed other non-breastmilk feeds. This has major implications for health-related outcomes, especially in areas where exclusivity is paramount.