In Tanzania, the estimated delivery date (EDD) is mainly determined by the last menstrual period (LMP), presumably leading to inaccurate estimates with a falsely high proportion of post-term and preterm deliveries. The study aimed to compare EDD determination by LMP and ultrasound in pregnant Tanzanian women and examine how the dating method affects the proportions born preterm, at term, and post-term and assess how maternal characteristics relate to wrongly EDD determination by LMP. 1123 women attending antenatal care in Moshi, Tanzania were included. Gestational age (GA) at birth was calculated according to LMP and ultrasound separately. The distribution of preterm (GA<259 days), term (GA 259-294 days), and post-term (GA >294 days) births was examined separately for LMP and ultrasound. According to LMP, 17.0% of deliveries were preterm and 17.6% post-term. According to ultrasound, 7.7% of deliveries were preterm and 3.4% post-term. Unplanned pregnancy was associated with an increased odds ratio of 3.06 (95% CI: 1.91-4.91) for preterm delivery when LMP was used for EDD determination instead of ultrasound. Parity 3+ was associated with an increased odds ratio of 2.12 (95%CI: 1.16-3.89) for post-term delivery when EDD was assessed by LMP. LMP based EDD increased the preterm birth rate two times and the post-term birth rate five times compared to ultrasound-based EDD.