International Journal of

     Introduction
Perinatal and neonatal mortality are important public health issues in many developing countries. Low birth weight is one of the major determinants of neonatal survival as well as postnatal morbidity.    Low birth weight neonates are subgrouped according to the degree of smallness at the first weight determination after birth:
Low birth weight (LBW): < 2500 g
Moderately low birth weight (MLBW): between 1500 and 2499g Very low birth weight (VLBW): < 1500 g
Extremely low birth weight (ELBW) : <1000 g
     LBW is caused by preterm birth, IUGR, or both factors. The predominant cause of LBW in the United State is preterm birth, whereas in developing countries, the cause is more often IUGR. Over the past 2 decades, LBW rate has increased primarily because of an increase in preterm births (1). The percentage of LBW has also increased in the United States, from 6.7% to 8.2% of all live births between 1984 and 2005, respectively, and the VLBW has increased from 1.2% to 1.5% between 1980 and 2005 (2). Both premature birth rate and LBW rate vary among different states of America, ranging from 8.3 to 17.9, and 6 to 11.6 per 1000 live births, respectively, in 2005 (2). “Low birth weight and prematurity are the second leading cause of infant mortality after congenital anomalies, but contribute disproportionately to the infant mortality rate (deaths in the first year after birth) ”  (2).  Very low birth weight rate is an accurate predictor of the infant mortality rate. Infants with a LBW are 40 times more likely to die than infants with normal birth weight (NBW) and infants with a VLBW are 200 times more likely to die (1). “ Infants with LBW are at a much higher risk of being born with cerebral palsy, mental retardation, and other sensory and cognitive impairments, compared with infants of NBW ” (1).
      Neonatal mortality rates in different countries and cities have to be identified and compared so that correct actions and interventions can be devised for each setting.  Despite the apparent importance of LBW as an indicator, there have been few studies of outcome for LBW infants in developing countries. The main objective of the present study was to determine neonatal mortality rate (NMR) in LBW infants in Yazd, Iran.
 
Materials and methods
 
     In a prospective - cohort study, during 2004, all births taking place in all maternity hospitals of Yazd- Iran, were evaluated and followed  for one month .
     The objective was to quantify mortality in LBW population over the course of the first month of extrauterine life. Variables such as age of mother, maternal educational level, type of delivery, neonate sex; gestational age and vital condition of newborn in delivery time were carefully recorded via medical records of mother and neonate. If an infant died before discharge from the hospital, an immediate verbal autopsy was sought and the diagnosis confirmed with attending doctors.
     Postnatal mortality surveillance included regular visits to hospitals, cemeteries, register offices and local health Departments. For deaths occurred out of the hospital, information was obtained from death certificates and complemented with information from home visits. Gestational age was calculated using the first day of the last normal menstrual period (LMP), estimated by obstetric sonography and the Dubowitz Scale.  
    “Births of less than 37 weeks were classified as preterm and those more than 42 weeks, as post term. Neonate birth weight of less than 2500gr was classified as LBW   and that of more than 4000gr as LGA (large for gestational age). Neonatal mortality rate (NMR) is defined as deaths occurring from birth to 28 days of age per 1000 live births that include early NMR (ENMR, deaths occurring in the first week of life) and late NMR (LMNR, deaths occurring from eighth to 28th day of life). ” (2) 
     This study has been approved by the Ethic Committee of Shahid Sadoughi University of Medical Sciences, Yazd, Iran. 
 
Statistical analysis
     The data were analyzed using SPSS.11 statistical software. X² analysis and Fisher exact test were used for data analysis of qualitative variables. Differences considered as significant at p-values < 0.05.
         
Results
 
     During one year, 8.4 %( 507 of 6016 births) of all newborns were LBW and 0.7 % (46 of 6016) of them were VLBW.  In LBW neonates, 7%, 9% and 84% were ELBW, VLBW and MLBW respectively.
     Table I illustrates NMR based on birth weight. 18.7% (95/507) of all LBW neonates, 0.8% of NBW (2500- 4000 g) newborns (46/5369) and 0.7% (1/140) of LGA died. In other words, two third (95 /143) of all neonatal deaths occurred in LBW. NMR in LBW was 23 times more than that of normal weight newborns (p-value=0.0001). Nearly 65% (61/95) of all LBW neonatal deaths occurred in first 24 hours after birth, 93% (N= 60) in   early and 7% (N=5) in late neonatal periods. Overall NMR, ENMR and LNMR   in LBW were 187, 118 and 9.8 in 1000 live births, respectively. Type of delivery was cesarean section in 41% of LBW births (N=207) and 43% (N=41) of neonatal deaths. Frequency of neonatal death was 18% in vaginal deliveries and 20% in cesarean section, and neonatal death was not different in both types of delivery (p-value =0.42).  48 % (N=243) of all LBW births were girls and 52% (N=264) were boys. Neonatal deaths in boys and girls were 52 and 43 neonates respectively (p-value = 0.16). 31 % (161/507) of all LBW were preterm and 69% of them were term newborns.
     Table II demonstrates neonatal mortality based on maternal age. Neonatal mortality was higher in mothers younger than 20 years old (p-value=0. 04).
     Table III shows neonatal deaths based on maternal educational level that  indicates NMR in illiterate mothers  was 4.3 times more than that of  highly educate mothers (p-value=0.0001). The main causes of mortality among LBW in order of prevalence were (RDS) respiratory distress syndrome (59%), asphyxia (20%), septicemia (12%) and congenital malformation (9%).





Discussion
 
     Neonatal mortalities are important public health issues in many developing countries. “Although infant mortality rates in developing countries have declined significantly in the past two to three decades, NMR has remained relatively constant”  (3). LBW and especially VLBW are major predictors of infant morbidity and mortality. The risk of neonatal death for LBW in this study was 23 times more than that of NBW. In other studies, this rate varies between 25- 40 times (4). 
     In the present study, the risk of neonatal death in MLBW, VLBW and ELBW were respectively 11.5, 62.5 and 117 times higher than that of normal weight newborns. In other studies, the risk of early death for infants born at MLBW (1500–2499g) is 5 times higher than that of heavier infants and the risk for VLBW infants is >100 times that of infants born at 2500g (5) and also VLBW infants account for 50% of neonatal death and have a 200-fold higher risk of neonatal death (1). In this study, 23% and 66 % of neonatal deaths occurred in ELBW and LBW. In the United States, ELBW infants account for <1% of all births but approximately one third of the total infant mortality rate (6) and in Karachi 77% of neonatal deaths were in LBW (7).
     In the present study, 0.8% of live births were VLBW which was similar to this rate in Italy (8). In our study, 18.7% of LBW, 50% of VLBW and 94 % of ELBW died. In Thailand , survival rate of VLBW infants and ELBW infants were 81% and 52%   (9)  and in the United Arab Emirates,   mortality rates among  LBW, VLBW and  ELBW were 5.7% , 20% and 50% respectively. (10)  In Italy the mortality rate in VLBW was 19.6% (8). In Sao Paulo, Brazil, according to birth weights of 500-749, 750-999, 1000-1249 and 1250-1499 g, survival rates were 15, 71, 93 and 96% in 2000 (11). In New Delhi, India, neonatal mortality rate till discharge was 15.7% in VLBW group and 33.3% in ELBW group (12).
     The most obvious factor in the present study which influenced the NMR was LBW which is similar to other studies (13-15). In this study, neonatal mortality rate was higher in males than in females which are similar to other studies (16, 17). Our data showed that neonatal mortality was higher in mothers younger than 20 years old. “Adolescent childbearing causes many adverse outcomes including unwanted, repeated childbearing and increased risks of having low birth weight, preterm births and neonatal mortality” (18). In this study, low maternal educational level associated with increase in NMR. In a study in San Bernardino County, the most significant contributors to adverse birth outcomes among Black women were length of gestation and maternal education, whereas the most significant predictor of infant mortality was birth weight (19).
     Finaly, in the present study, 66% of neonatal deaths occurred in LBW, while in the similar studies in Iran, this rate was 82% in Valiasr Hospital of Tehran and 50% in Fars province (20, 21). However in Fars, 18.46% of dead infants had no records.

Conclusion
 
     Neonatal mortality rate in Yazd is high, and LBW is accounted for two-third of neonatal deaths. Therefore, effort should be intensified to implement effective strategies for the reduction of LBW births and improving the care of these vulnerable neonates.