Guillén and colleagues1 conducted a systematic review of studies performed in high-income countries on the effect of loss to follow-up on neurodevelopmental outcomes for extremely preterm infants. This study showed substantial variation in neurodevelopmental impairment (NDI) rates among cohorts, with the worst rates reported mostly from studies in the United States. The authors demonstrated that this can be partly explained by higher loss to follow-up of healthy patients among US cohorts.