Objective: To examine trends (2008-2018) in socioeconomic inequalities in birth outcomes among infants born in Colombia, a middle-income country with high levels of inequality emerging from a long internal armed conflict. Methods: Using birth certificate data covering virtually all births in Colombia, we analysed the outcomes of low birth weight, an Apgar score <7 at 5 minutes after birth and the number of prenatal visits among full-term pregnancies. Maternal education and health insurance schemes were used as socioeconomic position (SEP) indicators. Inequalities were estimated using the prevalence/mean of the outcomes across categories of the SEP indicators and calculating the relative and slope indices of inequality (RII and SII, respectively). Findings: Among the 5,433,265 full-term singleton births analysed, there was a slight improvement in the outcomes analysed over the study period. We observed a general pattern of social gradients and significant relative (RII) and absolute (SII) inequalities for all outcomes across both SEP indicators. Trends over the period 2008-2018 showed that inequalities in low birth weight related to maternal education increased while those in Apgar score <7 decreased, as measured by the RII and SII. Relative inequalities across health insurance schemes increased for the two birth outcomes but decreased for the number of prenatal visits. Conclusion: The lack of a consistent improvement in the magnitude of inequalities in birth outcomes over an 11-year period is a worrying issue because it could aggravate the cycle of inequality. The findings of our analysis emphasize the importance of policies aimed at providing access to quality education and providing a health care system with universal coverage and high levels of integration.