]. These problems are particularly serious among high-risk pregnancies and births, and in developing countries where the health-care system is still struggling to provide basic public health and maternal and child health care to their population[3,4]. In such countries, adequate health-care services for managing high-risk pregnancy and delivery are usually available at the referral levels such as regional and national hospitals[5,6]. However, access to these facilities remain limited owing to factors such as distance, transportation cost and medical fees; specifically for the poor women and women who live in the rural and remote areas.
Multiple births are relatively rare events, but contribute substantially to mortality in both neonatal and post-neonatal periods. Yoruba of western Nigeria is considered "land of twins". Almost 5 percent of all Yoruba births produce twin, compared with just around 1.2 percent for Western Europe and 0.8 percent for Japan. Previous studies have associated high infant mortality with multiple births in both developing [7-14] and developed [15-22] countries; specifically those with birth defects, premature birth and low birth-weight. To the best of our knowledge no study has examined the effects of multiple births on infant mortality in Nigeria. Therefore, the aim of this study was to examine if infants of multiple births have disproportionately higher risk of mortality than infants of singleton births.
This study uses data from the 2003 Nigeria Demographic and Health Survey (NDHS) . It is based on information of 6219 children born within five years prior to the survey. The NDHS collected demographic, socio-economic, and health data from nationally representative sample of 7620 women aged 15–49 years in 7864 households included in the survey. The state was stratified into 36 states and the Federal Capital Territory (FCT) of Abuja within the six geopolitical regions. Methods used in the NDHS have been published elsewhere .
Briefly, each domain is made up of enumeration areas (EAs) established by a general population and housing census in 1991. The sampling frame was a list of all EAs (clusters). Within each domain, a two-stage sample was selected. The first stage involved selecting 466 clusters (primary sampling units) with a probability proportional to the size, the size being the number of households in the cluster. The second stage involved the systematic sampling of households from the selected clusters.
This study is based on an analysis of existing survey data with all identifier information removed. The survey was approved by the Ethics Committee of the ORC Macro at Calverton in the USA and by the National Ethics Committee in the Ministry of Health in Nigeria. All study participants gave informed consent before participation and all information was collected confidentially.
Each woman interviewed in the survey was asked to provide a detailed history of all her live births in chronological order, including whether a birth was single or multiple, sex of the child, date of birth, survival status, age of the child on the date of interview if alive, and if not alive, age at death of each live birth. These data from the birth histories were used to calculate infant mortality rate, defined as the probability of dying before completing 12 months of age, using a synthetic cohort life table. The rate was expressed as deaths per 1000 live births.
The multiple birth status was analysed as not multiple-birth (singleton) and multiple-birth (twin, triplet, quadruplet, or higher order). Each multiple birth child was analysed as an individual child, and the clustering effect of each group of multiple births was included in the analysis.
Because child survival is correlated with pregnancy care, delivery assistance, maternal nutrition, household living conditions, and other child, mother, and household characteristics and socio-economic factors that can also affect morbidity and mortality in children, the association between multiple birth status and infant mortality were estimated after adjusting for the e