ffects of these other risk factors and potentially confounding factors. These factors include child's sex (boy, girl), professional assistance at delivery (no, yes), birth order (1, 2, 3, 4+), child's birth size (below average, average, above average), mother's age at childbirth (13–17, 18–24, 25–34, 35–48), mother's body mass index (BMI) (<18.5, 18.5–24.9, 25.0+ kg/m2), mother's education (no education, some primary, secondary or higher), household wealth index (highest, fourth, middle, second, lowest), household access to safe drinking water (yes, no), availability of a hygienic toilet (yes, no), cooking fuel type (low pollution fuel, high pollution fuel), ethnic group (Hausa/Fulania, Igbo, Yoruba, others) residence (urban, rural) and geographic division (North central, North East, North West, South East, South South, and South West).
We used univariable and multivariable survival regression procedure with Weibull hazard function in Stata version 10 to examine the relationship of multiple birth status and other factors on infant mortality. A number of unadjusted hazard regression models were used to assess the unadjusted effect of multiple births and different risk factor and confounding factor, and a full adjusted model to assess the adjusted effect of multiple births controlling for all other factors that were significant in the unadjusted analyses (p < .05). In our analysis, weights were used to restore the representativeness of the sample, in which certain categories of respondents were over-sampled and non-response rates varied from one geographical area to another. Results were presented as hazard ratios (HR) with 95% confidence intervals (CI).
The sample distribution of children born in the five years preceding the 2003 Nigeria Demographic and Health Survey (NDHS) by multiple birth status and other selected characteristics are shown in Table 1 (see additional file 1). About 4% of children are of multiple births. Fifty-one percent of all births were boys and 49% were girls.
About one third received professional assistance at delivery. Twenty-one per cent of children were first order births, and 49% were fourth or higher order births. Only 9% of the births were to mothers aged 13–17, about 36% and 40% were to mothers' aged 18 – 24 and 25–34 years respectively, and the remaining 14% to mothers aged 35–48 years. Only 12% of the births were to undernourished mothers (BMI < 18.5 kg/m2), more than half (51%) of the births were to illiterate mothers. Consistent with higher fertility in the poorer households, 44% of children were born in the poorest 40% households, and 17% were born in the richest 20% households. Only (26%) of children were born in households with safe sources of drinking water. Most of the children (87%) were in households without a hygienic toilet facility. Similarly, most of the children (79%) were in households using high pollution fuels (firewood or straw) for cooking. Seventy-one per cent of births were in rural areas. By geographic division, more than one-third (35%) of the births were in North West, 24% were in North East, 14% were in North Central, 13% were in South South, 9% were in South West, and only 6% were in South East
On average, more than one in every 10 children born in Nigeria (101 per 1000 live births) does not survive to their first birthday. The infant mortality rate was very high among the multiple birth children – 236 per 1000 live births compared with 95 per 1000 live births among the singletons. The infant mortality rate was also higher among boys (107) than among girls (95). Mothers who received professional assistance at delivery were associated with lower infant mortality rate. There was a U-shaped relationship between infant mortality rate and childbirth order. Children were born to the older mothers (35–48 years old) were at greater risk of infant mortality. As expected infant mortality rate was strongly negatively associated with mother's educational status.
The probability of death before 12 months of age for children born in the poorest 20% households was greater than c