Background: Antenatal care (ANC) is crucial for the health of the mother and the unborn child as it delivers highly–effective health interventions that can prevent maternal and newborn deaths and morbidity. In 2002, the World Health Organization (WHO) recommended a minimum of four ANC visits for a pregnant woman with a positive pregnancy during the entire gestational period. Tanzania has sub-optimal adequate (four or more) ANC visits and the trend has been fluctuating over time. An understanding on the factors that have been contributing to the fluctuating trend over years is crucial in increasing the proportions of pregnant women attaining adequate ANC visits in Tanzania.Methods: The study used secondary data from Tanzania Demographic Health Survey (TDHS) from 2004 to 2016. The study included 17976 women aged 15-49 years. Data were analyzed using stata version 13. Categorical and continuous variables were summarized using descriptive statistics and using the weighted proportions. A Poisson regression analysis was done to determine factors associated with adequate antenatal. A multivariable Poisson decomposition analysis was done to determine factors associated with changes in adequate ANC visits among pregnant women in Tanzania from 2004 to 2016.Results: The overall proportion of women who had adequate ANC visits in 2004/05, 2010 and 2015/16 was 62%, 43% and 51% respectively. The identified determinants of adequate ANC visits were: early ANC initiation, zones, having more than one child, being aged 20 years and above, wanting pregnancy later, belonging to a richer and richest wealth quintile, having secondary and higher education, reporting distance to health facility not a big problem and watching TV at least once per week. Comparing 2004/05 and 2010 surveys, changes in the population structure contributed 4.2% of changes in adequate antenatal care visits while changes in coefficients contributed 95.8% of the changes. Also, comparing 2015/16 to 2010 surveys, changes in population structure and coefficients contributed 66.2% and 33.8% respectively. Conclusion: Early ANC initiation has greatly contributed to increased proportion of pregnant women who attain four or more ANC visits overtime. More effort should focus on pregnant women to initiate ANC in the first trimester in order to increase proportion of women with adequate ANC visit.