In many countries, women fall pregnant much sooner than desired and have more pregnancies and bear more children than they want. Pregnancies are usually associated with an array of negative health, economic, social and psychological outcomes for women and children. Preventing unintended pregnancies through family planning is an integral component in limiting associated consequences and population growth. However, despite widely available contraceptives, the demand for family planning among married women is still high. A quantitative study was conducted to explore the prevalence and determinants of unintended pregnancy among married women seeking antenatal care at Bwaila District Hospital in Lilongwe, Malawi. Permission was granted by the relevant authorities to conduct the study. Between July and August 2019, 413 married pregnant women were interviewed. The women gave written consent before joining the study and all the completed questionnaires were personally collected daily. Descriptive statistics were used to analyse the data while nonparametric tests were used to explore associations amongst the variables. The mean age of the participants was 23 years and 43.1% of the participants reported that their most recent pregnancy was unintended. Multivariate analysis revealed that young age (18–25 years), low education level and preference for the sex of the child were significant determinants of unintended pregnancy. Inconsistent use of family planning methods increased the risk of unintended pregnancy. Unintended pregnancy among married women indicates the need for improving education among women to delay marriage to the universal minimum of 18 years, and increasing access to long-term family planning methods to avoid failure and discontinuation of contraceptives.