BACKGROUND: Antenatal care (ANC) is crucial for the health of the mother and unborn child as it delivers highly effective health interventions that can prevent maternal and newborn morbidity and mortality. 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 of the factors that have been contributing to the fluctuating trend over years is pivotal 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 14. Categorical and continuous variables were summarized using descriptive statistics and weighted proportions. A Poisson regression analysis was done to determine factors associated with adequate ANC visits. To determine factors associated with changes in adequate ANC visits among pregnant women in Tanzania from 2004 to 2016, multivariable Poisson decomposition analysis was done. 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 increase in the proportion of women attaining adequate ANC from 2010 to 2015/16 was mainly, 66.2% due to changes in population structure, thus an improvement in health behavior. While 33.8% was due to changes in the mother's characteristics. Early initiation of first ANC visit had contributed 51% of the overall changes in adequate ANC attendance in TDHS 2015/16 survey. CONCLUSION: Early ANC initiation has greatly contributed to the increased proportion of pregnant women who attain four or more ANC visits overtime. Interventions on initiating the first ANC visit within the first twelve weeks of pregnancy should be a priority to increase proportion of women with adequate ANC visit.