Fertility Determinants and Mathematical Model for the Timing of its Convergence to Replacement Level in Nigeria

Abstract

High level of female fertility and accelerated population growth have been consistently reported in Nigeria. Population growth stability is achieved when fertility converges to a Replacement Level (RL) attained when the Total Fertility Rate (TFR) is on average of 2.1 children per woman. The unrestraint population growth can lead to a population explosion that might constitute a challenge to achieving sustainable development goals (SDGs). Shifts in age pattern of fertility are central to modelling fertility convergence to RL. However, poor quality of fertility data from the vital registration system in Nigeria is a major limitation to the estimates of the fertility pattern. Therefore, this study used indirect demographic techniques to examine fertility determinants and develop a mathematical model for the timing of its convergence to RL. Nigeria Demographic and Health Survey data sets of the weighted sample of 2003 (n1=7620), 2008 (n2=33385), 2013 (n3=38948) and 2018 (n4=41821) were analyzed. Each survey of the secondary data set was a cross-sectional population-based design and a two-stage cluster sampling technique was used to select women aged 15-49 years. Fertility was measured using the information on the history of the selected women’s full birth. Analyses were conducted using Bongaarts’ revised proximate determinants model with a focus on Postpartum Infecundity-(Ci), Sexual Exposure-(Cm), Contraceptive use-(Cc) and abortion rate-(Ca). Das Gupta five-factor and Oaxaca Blinder decomposition were used to examine the fertility determinants. Age Specific Fertility Rate (ASFR) was modelled with the assumption of uniformity in the percentage contribution of TFR by the observed and the standard ASFR. Annual changes in the age patterns of fertility were employed to develop a model that predict the timing of fertility convergence to replacement level. Mean children ever born per woman in Nigeria was 3.0±3.2, 3.1±3.1, 3.1±3.0 and 3.1±3.0 in 2003, 2008, 2013 and 2018, respectively. The adjusted estimate of TFR was highest in 2003 (6.1) and least in 2018 (5.6). In 2003 and 2018, Ci’s fertility-inhibiting effect (0.69 and 0.70) was highest, and the smallest was Ca (0.94 and 0.93). Decomposition analysis showed that the change observed in TFR between 2003 and 2018 was attributed to Cc-(63%) and Cm-(43%). Risk difference (RD) of high fertility between uneducated-educated women was highest in South-East (RD=56.9; 95%CI=49.1-64.8) and least in North-East (RD=15.0; 95%CI=9.9-20.1). For rural-urban differentials, South-West has the highest RD (12.7; 95%CI=10.2-15.3) and lowest in South-East (RD=1.9; 95%CI=-0.8-4.6). Also, Poor-Rich differentials, RD were highest in South-West (RD=15.9; 95%CI=11.5-20.4) and the least in the North-West (RD=15.9; 95%CI=11.5-20.4). The model developed for predicting the timing of fertility convergence to RL is f x  f x  t T o rep t ( ) ( ) The timing of fertility convergence to RL in Nigeria was projected as the year 2089. Nigeria might not attain fertility replacement level until the next seventy years (2020-2089) if the prevailing fertility pattern persists. Women’s education, sexual exposure and contraceptive use are pertinent to fertility reduction to replacement level in Nigeria. Increasing educational opportunities for girls and access to family planning services for women of reproductive age will facilitate quick achievement of replacement level fertility in Nigeria