In Tanzania, the most recent Demographic and Health Survey conducted in 2010 indicated that neonatal mortality estimates declined from 32 deaths per 1000 in 2001–2005, to 26 deaths per 1000 in 2006–2010 [3, 18]. This indicates 19% improvement in neonatal survival for the period from 2001 to 2010 . Our findings indicates that the neonatal mortality rate was 32 per 1000 live birth during the period of 2004 to 2009 and declined from 36 per 1,000 live births in 2005 to 29 per 1,000 live births in 2009. The possible explanation for this decline could be attributed to different interventions related to maternal and newborns such as Integrated Management of Childhood Illness (IMCI) , and increased malaria control efforts, which lead to the decline of malaria morbidity in Kilombero and Ulanga .
The higher neonatal mortality rate among infants born to teenage mothers in our study corresponds with previous studies [21–23]. Usually adolescent mothers face financial and social problems that lead to less provision of child care . Also, physiological immaturity of teenage mothers such as small uterus or narrow bony pelvis and lack of social experience on caring newborn can lead to more neonatal deaths . Some scholars have argued that the neonatal deaths observed in teenage pregnancies might have been attributable to socio-demographic factors .
The association between birth order and increased neonatal death might be an artefact of overrepresentation of mothers with poor outcomes in their previous birth as reported by previous authors . Previous studies also have shown that women who experience pregnancy loss or poor pregnancy outcome tends to go for next pregnancy after a short time to replace the previous pregnancy loss in order to achieve the desired family size, i.e. selective fertility or reproductive compensation [27–29]. This is an important challenge in reproductive epidemiology, as women who experienced poor outcomes are more likely to continue for the next pregnancy compared with those who had favourable birth outcomes. This results to overrepresentation of high risk group (women) in the subsequent pregnancies. Therefore, outcome of previous pregnancy is an important determinant for neonatal survival in the subsequent pregnancy including through shortening of the birth interval, it also influences the length of interval between pregnancies. The increased neonatal mortality among infants in the second birth order and among women with short birth interval could be explained by the effect of selective fertility.
Previous studies have showed that women who experienced preterm birth, delivery of low birth weight infants and neonatal death in their first pregnancy are more likely to have similar adverse outcome in subsequent pregnancies [26, 30]. Since all these factors are associated with increased risk of neonatal death especially in young maternal age, the observed high risk of neonatal mortality in our study may in part be explained by recurrence of these factors in successive pregnancies .
In the present study, short birth interval was associated with increased risk of neonatal death compared with long birth interval [9, 10]. It is generally accepted that if closely spaced births were delayed, particularly in countries where mortality and fertility are still high, child mortality levels would fall . Birth intervals increase mortality of children in two ways: Children born after a short interval are likely to have mothers in poor health, and such children tend to have low birth weight and increased chances of neonatal mortality . On the other hand, women with short intervals between two pregnancies have insufficient time to restore their nutritional reserves, a situation which is thought to adversely affect fetal growth and thereby increases risk of neonatal deaths.
Our study also showed that a mother with no partner at the time of birth had increased risk of neonatal death as compared to her counterpart who had a partner co-resident. Our findings are consistent with previous studies that assessed the impact of father’s involvement on child development, functioning and quality of life [32, 33]. Evidence from the previous studies indicated that mothers with partner were more likely to provide their children with a healthy environment and nutritious food than mothers without partner, even when other conditions are similar [34, 35].
We also found that neonatal mortality was higher for male newborns than females. Our results are in agreement with other studies that showed males had a higher odds of dying than females during the first month of life [36–39]. This increased hazard for newborn males may also be due to the large proportions of neonatal deaths occurring in the first week of life, which is the time when gender differences in neonatal mortality are most pronounced . Biological factors that have been implicated with this increased risk of neonatal death in male infants include immunodeficiency  increasing the risks of infectious diseases in males, late maturity  resulting in a high prevalence of respiratory diseases in males, and congenital malformations of the urogenital system. Also higher mean birth weight in males as compared to females , which leads to more difficult births and more asphyxia and birth trauma, leading to higher neonatal mortality.
We found no evidence to suggest that delivery in health facilities is protective to the newborns. This observation concurs with a previous study in the same area  but is contrary to the expected neonatal survival gains conferred through institutional delivery . In Tanzania, lack of safe delivery facilities, shortage of skilled providers as well basic equipment and supplies remain critical . This situation is reflected in the observed high and slightly stable neonatal mortality rate despite some increase in the facility-based. Lack of adequate maternal and neonatal care at health facilities in time critical has been argued to be linked to deaths within the first day of life . Findings from facility based studies in parts of north-eastern Tanzania that assessed unmet need for emergency obstetric care blamed poor quality of care for the negative maternal outcomes and high perinatal mortality . Quality of delivery services and variations in newborn care practices were not included in these analyses but could affect the risk of neonatal deaths. A recent systematic review indicated that over three quartets of intrapartum-related deaths occurred in settings with weak health systems . Scarcity of skilled providers, poor infrastructure and substandard quality of care are some of the critical components of such health systems that constrain progress in maternal and newborn survival .
Strengths and limitations
This study utilized huge datasets from Health and Demographic Surveillance System which are continuously registered vital demographic events in a geographical defined area. Large sample size provided our study with a sufficient power to provide accurate statistical analysis across sub groups in the study population. On the other hand, findings from Health and Demographic Surveillance Systems data provides information to policy makers and program manager which can be translated into policy and practice.
This study has some limitations that need to be considered in interpreting the findings. First, self-reporting of neonatal deaths may result to under estimation of true neonatal mortality due to underreporting particularly for deaths that happened within the first day of life. Secondly, misclassification of stillbirth and early neonatal death, the demarcation between intrapartum stillbirth and early neonatal death is problematic, this leads to potential overestimation of early neonatal death as some stillbirths are regarded as early neonatal. Third, there are other possible factors associated with neonatal survival that were not available in the HDSS dataset, such as environmental, genetic factors, gestational age and birth weight.