During the study years, the infant mortality rate declined in all regions of the World Health Organization; one of the main reasons for this decrease could be the perinatal mortality rate. According to the results of Ebrahimvandi’s study [14], reduced PMR by 0.9 deaths per 1000 people could reduce IMR by 56% in some regions, and PMR reduction was itself affected by increased prenatal care. The trend of IMR is declining in all areas, but there is a long way to go, as in some countries preventable deaths account for more than three quarters of infant deaths, with more than half of these deaths being prevented by caring for women during pregnancy and around childbirth [15]. The pattern of infant mortality has varied with interventions conducted in developed countries. Developed areas have reduced the impact of preventable factors on infant mortality by providing appropriate health services and essential education. Due to the increasing number of registered deaths and improved diagnoses of abnormalities that lead to termination of pregnancy, live births will occur in newborns who are severely underweight, which accounts for most of the deaths of an infant less than 1 year. For this reason, the main cause of death in these regions is congenital defects and chromosomal abnormalities [16]. IMR in different areas may have completely different causes, depending on the number of deaths under 1 year of age, specific interventions are needed to control them, in areas where deaths occur within 0 to 27 days causes are mostly related to pregnancy, childbirth care, and genetic abnormalities, whereas deaths over 28 days are mainly related to environmental factors and maternal literacy level [16]. In general, premature births, which are affected by congenital malformations and chromosomal abnormalities, are among the main causes of IMR in underdeveloped and developing areas [17,18,19]. Of course, sudden death syndrome and unintentional injuries in low economic and cultural status societies have a large impact on this rate [18]. Social factors, such as racial inequalities in employment, and educational services [20], childhood marriage, sexual injustice, cultural mutation, and religion cause psychological problems, are directly related to IMR. Therefore, screening, diagnosis, and home-based services can sometimes be the primary strategy to reduce IMR in each region [21]. In other studies, marriage status, gender, birth weight, and multiple births have been identified as predictors of IMR. Also, infants born from unmarried women, double or multiple births, and boys with a birth weight of fewer than 2500 g, had a higher mortality rate [17, 22]. The factors listed above may overlap with each other.
The reason for this decrease in mortality can be attributed to the greater sensitivity of health care providers as the only intervention [23] Another study specifically examining the impact of these cares shows that women who received care from their first month of pregnancy only had one in fifteen infant mortalities in comparison to women who received no care during their pregnancy. Receiving care is affected by women’s awareness and age; the lower the age of the pregnant woman, the less likely she is to seek and receive pregnancy care. Therefore, the youngest women are the highest risk group [17, 22].
Environmental factors such as smoking are effective factors on the incidence of diseases, so infant mortality is higher among smoker mothers. Maternal smoking increases the risk of respiratory infections and impedes the immune response of the infant [17]. Besides, smoking cessation by pregnant women, vaccination, reduced preterm deliveries, increased age of childbirth, economic growth in countries, and an increase in the health sector’s share of the countries’ income were known as other IMR reduction factors [14, 24, 25]. In a similar study conducted in European countries, a slower downward trend is occurring than in the past, but there are fluctuations in the different countries that may be influenced by birth cohort effect or period effect [17].
Successful countries in rapid control of the IMR have cited rapid economic progress as a reason for their success. In addition to reducing IMR, interventions in different areas change the pattern of mortality, which requires continuous monitoring and modifications of interventions to the desired level. Different effective factors can affect the IMR; these factors include the development of public health programs such as vaccination programs, Human Development Index (HDI), level of women empowerment, income inequality, GDP per capita, social inequalities, socioeconomic status, social economic and environmental hygiene. So a decreased trend of IMR in different areas may be because of improvement of the mentioned index such as development of vaccination programs, improvement of the nutritional status, increase in women educational level and women empowerment, and improvement of the human development index and GDP per capita [26, 27].
Despite the reduction in IMR in all regions, the IMR reduction rates varied significantly in different regions of the World Health Organization. Reducing mortality from infectious diseases is easier and faster than reducing the deaths due to congenital disorders, childhood cancers, and complications of preterm childbirth. So a higher percentage of deaths in Africa and Southeast Asia had occurred due to infectious diseases compared to regions such as Europe [28,29,30].
Also, the mentioned variation in IMR may be due to differences in some indicators such as vaccination coverage, HDI, level of women empowerment, income inequality, GDP per capita, social inequalities, socioeconomic status, and environmental hygiene. There is an inverse association between IMR and HDI; in other words, a region with a higher HDI index has a lower IMR. A similar association exists between IMR and the level of women empowerment, income inequality, and GDP per capita; social inequalities are other effective factors. Lower income and social inequality can decrease the level of IMR. So the lower amount of IMR in the Americas and Europe region can be because of a better HDI index, an increase in the number of educated women, lower income, and social inequality in mentioned areas [27].
In considering other effective factors on the difference of IMR between different WHO areas, ethnical factors can be regarded. Evaluation of the effect of maternal age on IMR showed that children born of mothers under 20 years had the highest and children born of mothers between 30 and 34 years had the lowest IMR [16]. In regions such as Europe where effective measures to reduce IMR had been started earlier, the marginal effectiveness of many effective measures to reduce IMR in those regions had declined in 1990 (the first year of the study). Hence, IMR would decrease in those regions with a decreasing rate. (Any action taken in different parts of the world to reduce IMR might have an increasing marginal effectiveness trend in the first stage so that in the first years, the number of reduced deaths as a result of that action in a specific population would increase each year compared to the previous one. After several years, the marginal effectiveness of the carried out action would reach the highest and then become decreasing.) But in regions such as Africa where the IMR-mitigating measures had started at a later stage, there was an increased efficiency of the actions at the beginning of the study. In other words, the IMR rate was increasingly reduced during 1990–2003, and the number of deaths in each year was lower than in the previous one. Since 2005, the marginal effectiveness of the measures taken in Africa has been declining, too, but the IMR is still numerically higher than in other regions.
Studies have shown that infant deaths in rural areas are significantly higher than in urban areas. And with the same interventions, the intensity reduction of deaths in rural areas is higher than in urban areas. Mortality due to congenital malformations in rural areas 20% higher than in urban areas [31, 32]. The quality of care provided is directly related to the development of the area, and this is due to the difference in infant mortality rates in villages relative to developed cities or countries compared to developing or less developed ones. The most direct cause of this discrepancy can be found in perinatal care (respiratory system diseases are the most important) followed by timely diagnosed malformations [31]. Given the decline in the IMR reduction rate in all the regions, maintaining this reduction in future years requires more investment in this area, so that, in addition to carrying out the current measures, countries must invest in programs such as improving maternal health behaviors (smoking, using alcohol and drugs during pregnancy) to control chronic diseases in mothers and increasing maternal education. It is the economic and social development of countries that can be a precondition for taking such measures [12, 33, 34].
