BACKGROUND: Migration, driven by factors like poverty, violence, and natural disasters, is a key social determinant of health. While international migrants often have worse perinatal outcomes, research on perinatal health differences between internal migrants and non-migrants remains limited. We aimed to determine whether the offspring of women who migrate within Brazil experience poorer perinatal outcomes than those of non-migrants, according to the Human Development Index (HDI) of their municipalities of origin and destination. METHODS: We used the CIDACS Birth Cohort, consisting of women applying for social programmes in the Unified Registry for Social Programmes Cadastro Único linked with live births and mortality registries. We included live births conceived from March 2010 to February 2018. Internal migrants were women who changed their state of residence from registration in CadUnico to the birth of the child. We derived risk ratios (RR) of migration's effect according to HDI of residence before and after migration using logistic regression. FINDINGS: We included 10,184,021 births in the study, with 5.7% of these births from women who were internal migrants. The offspring of women who migrated to municipalities with equal/higher HDI (80% of migrations), exhibited a decreased risk of preterm births (RR: 0.94, 95% CI: 0.93-0.95), low birth weight (RR: 0.94, 95% CI: 0.92-0.95) and small for gestational age (RR: 0.92, 95% CI: 0.91-0.93), but higher risk of congenital abnormalities (RR: 1.14, 95% CI: 1.10-1.18). The offspring of women who migrated to municipalities with lower HDI had delayed access to healthcare and worse outcomes except for a lower risk of low birth weight (RR: 0.94, 95% CI: 0.92-0.96). INTERPRETATION: Offspring of those migrating to municipalities with equal/higher HDI tend to have better perinatal outcomes, whereas migrants to lower HDIs have a similar pattern to non-migrant women. FUNDING: NIHR, Wellcome Trust, Royal Society.