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the prevalence of low birth weight ( lbw ) is different from 5 to 7 percent in developed countries and 19 percent in developing countries .
a systematic review in iran showed the prevalence of 7% and its increasing during 1991 - 2010 .
after the establishment of health care networks in iran and the approximately 98 percent access to health care services , over the past three decades , the life expectancy has improved and infant mortality has decreased in this country , the birth of infants weighing less than 2500 g , however , is still considerable .
decrease in life expectancy , increase in infectious and respiratory diseases , anemia , hypothermia , chromosomal abnormalities , and nutritional and health care problems are among the low birth weight consequences .
lbw plays a direct or indirect role in more than 70% of infant mortalities . due to the importance of this subject and the paucity of studies investigating lbw - related risk factors in iran ; the present study was carried out in shahroud to determine the prevalence of lbw and its related risk factors .
this study was conducted 2011 in fatima hospital , the main maternity unit in shahroud , northeast iran .
the first stage of research was a cross sectional study for determination of lbw prevalence .
sample size calculation was estimated to be about 1000 subjects based on classic formula assuming of 10% of prevalence for lbw ( p=10% ) , precision of 2% ( d=2% ) , 4500 annual births ( n=4500 ) and =5% .
therefore , we entered 1000 neonates in the study to measure their birth weight ( approximately 25% of the total number of births during a year ) . in the second stage , we compared the normal birth weight ( nbw ) group with birth weight 2500 g ( control group ) to lbw group with birth weight < 2500 g ( case group ) .
a range of factors influence fetal growth , although they can be grouped into several general categories , we categorized them in two groups . first , maternal risk factor group consisting of high risk pregnancy , type of delivery , maternal age , mothers level of education , pregnancy interval , parity and mother 's smoking .
high risk pregnancy included history of chronic diseases ( e.g. gestational hypertension , diabetes ) , history of abortion , preeclampsia , multigravity and multiple pregnancies .
among 1000 subjects , 48% ( n=480 ) were females and 52% ( n=520 ) males .
seventy - two ( 7.2% ) of newborns had birth weight < 2500 g , 6.1% of them were premature with less than 37 weeks of gestational age and 46.2% of mothers had a normal vaginal delivery .
only 3 subjects ( 2 in case and 1 in control group ) had smoked during pregnancy . in case group 44 ( 55.6% ) subjects and in control group 366 ( 39.4% ) lived in villages , a significant relationship between mothers place of residence and lbw was not evident .
comparison between variables of maternal age , education and occupation , inter - pregnancy interval , infant 's gender , delivery type , gestational age , parity and high risk pregnancy with the responding variable of infant 's lbw , the results showed significant relationships between four variables of delivery type ( or=2.05 , 95%ci : 1.22 - 3.44 ) , pregnancy interval ( or=2.35 , 95%ci:1.18 - 4.68 ) , gestational age ( or=43.97 , 95%ci : 23.64 - 81.81 ) , pregnancy status ( or=3.71 , 95%ci : 2.18 - 6.32 ) ( high risk vs low risk ) and low birth weight . there was significant statistical association between lbw and prematurity ( p<0.001 ) .
forward stepwise logistic regression method was used for multivariate analysis and after adjustment of prematurity effect as a confounder , multivariate analysis showed significant associations for high - risk pregnancy ( p=0.002 ) and maternal age ( p=0.01 ) ( table 1 ) .
odds ratio for prematurity was 42.82 ( 95%ci ; 21.94 - 83.57 ) , for high risk pregnancy was 2.76 ( 95%ci ; 1.47 - 5.19 ) and for maternal age group more than 35 in comparison to 19- 35 age group was 0.2 ( 0.95%ci ; 0.05 - 0.71 ) . 
 
results of multiple logistic regression method for risk factors of low birth weight * prematurity is adjusted as a confounding factor ; or : odds ratio ; ci : confidence interval
the prevalence of lbw and prematurity in the studied population was 7.2% and 6.1% , respectively . in the final model ,
only three variables of gestational age , pregnancy status and maternal age showed significant association with birth weight .
improving the health care during pregnancy , the income level and the density of the sub - population can be a reason for different results . in this study , there were significant relationships between the prematurity and lbw , this finding is close to other studies .
the baby 's low weight at birth is the result of preterm birth ( before 37 weeks of gestation ) .
the odds ratio of giving birth to infants weighing less than 2500 g in mothers > 35 years old was 0.2 compared to mothers with an age of 19 - 35 years .
this is inconsistent with the results of the study by zahed pasha and colleagues which indicated a higher odds ratio ( or=2.3 ) of low - birth weight infants in mothers with > 35 years of age .
a large number of epidemiological studies have reported that the incidence of lbw increases in the extremes of women 's reproductive life ; that is , between 15 and 19 years and between 35 and 40 years of age .
it is widely believed that women older than 35 years have a higher incidence of lbw , although there is some debate about the latter .
some authors suggest that the risk is related , not with age itself , but rather with complications of other diseases such as the larger number of chronic diseases ( hypertension , diabetes ) .
no significant relationship was observed between maternal level of education and birth weight which is consistent with the results of studies by , zeyghami et al and delaram .
inconsistent with our finding , some studies suggest the possibility of an important relation between maternal educational level and lbw so , with an increased risk of lbw being associated with decreasing educational level of the mother .
there was significant association between lbw infants and high risk pregnancy ( or=2.76 ) , according to extremes of pregnancy interval , medical history of disease , preeclampsia , history of abortion and multigravity .
chronic hypertension and renal disease , as well as some maternal diseases , may provoke alterations in fetal growth , perhaps as a consequence of reduced uteroplacental fluid .
it is well known that second and third children weigh more than the first . beginning with the fourth pregnancy ,
this increased birth weight is inverted , so that the risk of lbw again increases with the fourth and subsequent children . in one study carried out in women with a history of previous miscarriages , it was observed that a previous miscarriage doubled the risk of a preterm birth and of lbw .
there was a relationship between delivery type and low birth weight so that the probability of birth weight < 2500 g in neonates born through c - section was 2.05 times greater than that in neonates born through natural method which is not consistent with the findings of delaram . after adjusting for gestational age , maternal age , parity , pregnancy interval and other variables , the relationship between delivery type and lbw was not significant .
as cesarean section occurred frequently in most very low birth weight groups and complicated pregnancies .
a significant relationship was observed between lbw and inter - pregnancy interval so that with the decrease in pregnancy interval , the probability of lbw increased .
short birth intervals , varying from 3 to 6 months in developing countries and from 1 to 2 years in the developed countries , may lead to an increased tendency toward low birth weight and prematurity in subsequent pregnancies .
there was also a significant relationship between gestational age and lbw so that there was a greater risk of lbw in preterm neonates than the term ones .
based on this study findings ; prematurity and high risk pregnancy are the most important risk factors for lbw .
there was also a reverse association between maternal age and lbw . despite the health care services provided via network system , the mean national prevalence of low birth weight is still high .
prevention of preterm delivery , educational interventions for high risk groups especially those with maternal age less than 18 years , and setting appropriate inter - pregnancy intervals , proper nutrition during pregnancy and reducing high risk pregnancies can play a major role in preventing and reducing the birth of low weight neonates .
objectivethis study was conducted to determine the prevalence of low birth weight ( lbw ) and its related risk factors in an appropriate sample of neonates in shahroud , northeast iran.methodsat this study , a random sample of 1000 neonates were selected of which 72 neonates were lbws . 
 we used univariate and multivariate logistic regression methods to evaluate the lbw risk factors in lbws compared to normal weight infants.findings7.2% of neonates were lbws and 6.1% born before 37 weeks of gestation . 
 prematurity , high - risk pregnancy and maternal age have significant statistical association with lbw . 
 odds ratio ( or ) for prematurity was 42.82 ( 95%ci ; 21.93 - 83.57 ) , for high risk pregnancy 2.76 ( 95%ci ; 1.47 - 5.19 ) and for maternal age group more than 35 years in comparison to 19 - 35 years age group 0.2 ( 95%ci ; 0.05 - 0.71).conclusionbased on this study ; prematurity and high risk pregnancy were the most important risk factors for lbw . 
 there was also a reverse association between maternal age and lbw .