If sought early, antenatal care (ANC) – health care given to women during pregnancy to ensure healthy outcomes of themselves and their newborns  – can be most efficient in averting adverse pregnancy outcomes [2, 3]. This is a doorway to early detection and management of potential complications associated with pregnancy, and consequently reduces potential maternal and newborn morbidity and mortality [4–7].
Maternal deaths are caused by complications of abortion, obstetric complications such as haemorrhage, dystocia, eclampsia, sepsis and infections such as tuberculosis and HIV-1, to mention a few [8, 9]. Records reveal that haemorrhage and hypertensive disorders account for the largest share of the deaths in developing countries . However, most of these deaths are avertable. For example, proper ANC utilization and skilled attendance at birth have been reported to reduced maternal deaths [4, 7]. Also access to emergency obstetric care, adequate nutrition and basic health services considerably lessen the risk of maternal death . Evidence shows moreover that access to essential obstetric care would results into 52% decline in the current global maternal deaths .
Unintended pregnancies (mistimed and unwanted) pose important public health risks, and their pernicious consequences have been documented in many studies [13–16]. For example, existing evidence shows presence of a relationship between unintended childbearing and several adverse health outcomes such as maternal depression [17–21], anxiety , poor psychological wellbeing  and poor utilization of ANC or delivery care [15, 16]. Generally, it has been established that women who experience an unintended pregnancy are less likely than women with intended pregnancies to seek care [15, 23]. Most of these studies however were conducted in developed countries while such evidence is limited and sometimes inconsistent in developing countries .
The World Health Organization (WHO) recommends that adequate care for a normal pregnancy that has no complications should comprise four ANC visits, with the first occurring within the first trimester . The first visit should occur before 12 weeks of gestation but not later than 16 weeks, and afterwards at 24-28 weeks, 32 weeks and 36 weeks [25, 26]. Where any complication is detected, frequent visits and if necessary an admission may be advised . Timely execution of the first ANC visit allows more time for more such visits thus guaranteeing timely identification and mitigation of potential pregnancy complications  and enables a woman receive a wide range of health promotion and disease curative and preventive services . This includes but not limited to treatment of anemia, diagnosis and treatment of malaria, tuberculosis (TB), sexually transmitted infections (STIs) and tetanus toxoid (TT) immunization. During ANC visits, women also receive counseling on and promotions of skilled attendance at birth, postpartum care for women and newborns and prevention of mother-to-child transmission (PMTCT) of human immunodeficiency virus (HIV) .
The situation of ANC in Tanzania is such that, nationwide, the coverage of ANC utilization at least once on the course of pregnancy is near universal. The recent Tanzania Demographic and Health Survey (TDHS)  shows that 96% of the Tanzanian women who gave birth five years preceding the survey sought ANC at least once from skilled providers. While this is the case, the first visit (initiation) is often delayed  and subsequent visits drop considerably, resulting in incomplete doses of the ANC services such as TT immunization, intermittent preventive treatment of malaria during pregnancy (IPTp) and counseling on birth preparedness. The report shows further that 43% of the women made at least 4 ANC visits recommended by WHO and only 15% made the first ANC visit during the first trimester. The median gestational age at first ANC visit among Tanzanian women was estimated at 5.4 months.
Although women are positive about ANC , the health system’s framework for ANC delivery in Tanzania is challenged by factors such as shortages of trained staff, inadequate supply of drugs and equipments  and poor implementation of the guidelines of the focused antenatal care (FANC) . While these may affect ANC utilization on one hand, individual factors such as maternal age, maternal education  and parity [31, 32] are as well acknowledged on the other. Many more studies reporting factors affecting ANC utilization are available [33–38].
Overall, factors affecting ANC utilization are well understood and clearly documented in the literature. The literature classifies ANC utilization in three groups as (1) any ANC (2) ANC initiation and (3) adequate number of ANC visits . Of these groups, the first holds prominent recognition among researchers in both developed and developing countries while the second and third are inadequately researched especially in developing countries. A few studies like a recent one from Kenya on utilization of maternal services among young women (15-24 year-olds) found that place of residence, household wealth, education, ethnicity, parity, marital status and age at birth were associated with both timing of first ANC visit and type of delivery assistance received . One qualitative study from Tanzania reports fear of wild animals on a way to a clinic and lack of money as reasons for late ANC initiation . Another study from Tanzania on timing of ANC initiation found that later ANC enrollment was associated with ethnicity, perceived poor quality of care, late recognition of pregnancy and not being supported by husband or partner . However, a comprehensive review of the predictors of timing of ANC initiation is lacking, since pregnancy intentions – categorized as intended (wanted then), mistimed (wanted later) and unwanted (not wanted at all) – is yet to be recognized as a possible predictor of timing of ANC initiation. Our study therefore responds to this knowledge gap with the following objectives: (1) to quantify the extent of mistimed and unwanted pregnancies, (2) to determine the level of early ANC initiation as a proportion of women initiating ANC in the first trimester, (3) to assess the association between mistimed pregnancy and timing of ANC initiation and (4) to assess the association between unwanted pregnancy and timing of ANC initiation among women of reproductive age who gave birth in the past two years in three districts (i.e. Rufiji, Kilombero and Ulanga) in Tanzania.