BACKGROUND: Pre-term birth (< 37 weeks' gestation) is a major cause of neonatal mortality, with very pre-term (< 32 weeks' gestation) and extremely pre-term (< 28 weeks' gestation) infants facing the highest risks. While socioeconomic disparities in pre-term birth are well-documented, relatively little is known about inequities among the highest risk subgroups. METHODS: Using data from the RECAP Pre-term Project across six high-income countries, we analyzed socioeconomic inequality in the incidence of extreme pre-term birth (EP) and extremely low birth weight (ELBW) among very pre-term/very low birth weight (VP/VLBW) infants. We measured inequality using concentration indices across cohorts and two adjusted measures of horizontal inequity (HI1 and HI2), to estimate the contributions of socioeconomic factors (e.g., parental education, ethnicity) to the outcomes. RESULTS: Results showed that the incidence of EP in the Netherlands (HI2 = 0.171) and Norway (HI2 = 0.210) was higher among more socioeconomically advantaged infants born at VP/VLBW, but higher among more socioeconomically disadvantaged infants in New Zealand (HI2 = -0.020). Incidence of ELBW was higher among disadvantaged infants in Germany (HI2 = -0.046). Parental education was the strongest driver of these effects, though ethnicity and socioeconomic status moderated these effects. CONCLUSIONS: Counterintuitively, EP/ELBW were concentrated among advantaged groups in some countries, possibly reflecting survival bias or unequal access to neonatal care. The study highlights the need for targeted policies addressing inequities within high-risk pre-term populations and underscores methodological challenges in assessing disparities among vulnerable subgroups.