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INTRODUCTION: Antenatal Care (ANC) is an important component of maternal health and covers a wide range of activities with huge potential benefits for positive pregnancy out comes. However, large proportions of women do initiate ANC early resulting in adverse consequences.
Data for this study came from NDHS 2013 which was a nationally representative sample of women in reproductive age group (15-49 years). A national sample of 40,320 households from 904 primary sampling units (PSU) was selected. All women aged 15-49 who were usual members of the selected households or who spent the night before the survey in the selected households were eligible for individual interviews. As with previous Demographic and Health Surveys, 2013 NDHS was to provide reliable information among others on maternal and child health, childhood and adult mortality levels. The survey provides reliable estimates for key indicators at national levels as well as for urban and rural areas for the 36 states and the Federal capital territory (FCT). Nigeria is administratively divided into 36 states and the FCT. The states are regrouped into six geo-political zones (North West, North Central, North East, South West, South East and South South). Each state is subdivided into local government areas (LGAs), of which there are 774 and each LGA is further subdivided into smaller (secondary and tertiary) localities. During the 2006 census, each locality was subdivided into enumeration areas (EAs), however, the EAs in Nigeria are small in size with an average of 211 inhabitants (48 households), so the 2013 DHS included several EAs per DHS cluster (with a minimum cluster size of 80 households). The NDHS sample was stratified and selected independently in three stages from sampling frame. Each state was stratified into urban and rural areas. In the first stage 893 localities were selected with probability proportionate to size and with independent selection in each sampling stratum. In the second stage, one EA was randomly selected from most of the selected localities with an equal probability selection. In a few larger localities, more than one EA was selected, giving a total of 904 EAs that were selected. Household listing operation of selected 904 EAs was done before the main survey, drawing a location map and a detailed sketch map and recording on the household forms all occupied residential households found in EA with address and the name of household head. Where a selected EA had less than 80 household, a neighboring EA from the selected locality was added to the cluster and listed completely. This list of households served as the sampling frame for the selection of households in the third stage. Finally, in the third stage a fixed number of 45 households were selected in every urban and rural cluster through equal probability systematic sampling based on the newly updated household listing. The sample allocation features an equal size allocation with minor adjustments. Among the 904 clusters, 372 were urban and 532 rural; each with a total number of households of 16, 740 and 23, 940 respectively. This gives overall total households sampled at 40, 680. Since 2013 NDHS used a 3 stage stratified cluster sampling technique, it means sampling weights based on sampling probability will be required for any data analysis to ensure the representativeness of the survey results at both national and domain levels. For a detailed description of the sampling procedures, distribution of population, EAs by state and the survey questionnaires see NDHS report (26). The primary outcome variable for this study is delayed ANC (defined as having the first ANC in second or third trimester). From 2013 Nigeria DHS, the following potential factors associated with delay in seeking ANC as predictor variables were identified: pregnancy intention; maternal age at birth of last child; maternal and spousal level of education; maternal and spousal occupational statuses; religion; birth order; household wealth level; level of participation in household decision making; exposure to source of health information (through the three media channels of radio, television and newspaper/magazine); type of marriage; ethnicity; geopolitical zone; place of residence and insurance cover. The first stage of the statistical analysis involved an examination of associations between the outcome variable and various socio-demographic factors; this involved conducting chi-squared tests. Further, a test of collinearity was conducted to determine if the variables in the model are correlated with one another and the variable strongly related with the outcome variable was retained. Collinearity was further tested with variance inflation factor (VIF) and any variable that overshoots the threshold was dropped from the model. Finally, polytomous logistic regression was carried out to determine significant factors related to timing of initiation of antenatal care. This model was selected since the outcome variable has more than two categories in contrast binary logistic regression where the outcome variable has two categories. Therefore, to assess strength of the relationship between the independent variable and the dependent variables, relative risk ratio was estimated using initiation of antenatal care within the first three months as the outcome base. The analysis was conducted using Stata v13. This study is a secondary analysis of the 2013 NDHS, so does not require ethical approval. We were 2015 DHS Fellows, we registered and requested for access to NDHS datasets from DHS on-line archive and received approval to access and download the de-identified DHS data files. This was a pre-requisite for the Fellowship training programme and research work we conducted during the period.
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