the proportion of women giving birth to large infants has increased around the world [ 1 , 2 ] , most likely because of the rising rates of maternal overweight and obesity [ 37 ] . whereas consequences of low birth weight may include infant mortality and morbidity , high birth weight has been related to increased risk for caesarean section , chorioamnionitis , fourth degree perinatal lacerations , postpartum haemorrhage , shoulder dystocia [ 911 ] , and low apgar score . additionally , high birth weight has been positively associated with obesity   and type 2 diabetes   in adulthood . previous studies have reported that physical activity in pregnancy is related to foetal growth rate and birth weight [ 15 , 16 ] , and that physically active women have a reduced risk of delivering a large infant [ 17 , 18 ] , possibly by increased insulin sensitivity . however , not all studies have reported consistent inverse associations between physical activity in pregnancy and birth weight [ 1922 ] . although women who exercise regularly before pregnancy are more likely to continue to exercise during pregnancy [ 2325 ] , few studies have examined the associations between prepregnancy physical activity and birthweight , and the results have been inconsistent [ 18 , 20 , 26 , 27 ] .   in this prospective study in norway , we have utilized data on women participating in a population - based health study linked with information from the medical birth registry to study the association between maternal pre - pregnancy physical exercise and offspring birth weight . additionally , we explored the combined association of pre - pregnancy body mass index and physical exercise on birth weight . the nord - trndelag health study ( the hunt study ) is a large population - based health study conducted in the county of nord - trndelag , norway . the hunt study is a collaboration between hunt research centre ( faculty of medicine , norwegian university of science and technology ntnu ) , nord - trndelag county council , central norway health authority , and the norwegian institute of public health . it constitutes three consecutive cross - sectional waves ; the first was conducted in 19841986 ( hunt 1 ) , the second in 19951997 ( hunt 2 ) , and the third in 20062008 ( hunt 3 ) . for the purpose of the present study , we have used information from the first wave ( hunt 1 ) . in hunt 1 , 87,285 persons aged  20 years were invited to participate , and 77,216 ( 88.5% ) accepted the invitation , filled in questionnaires , and attended a clinical examination ( 37,826 men and 39,390 women ) . a more detailed description of participation , method , and procedures of the hunt study can be found elsewhere .    for the purpose of the current study , we first selected all 3,739 women aged 2039 years at participation in hunt 1 ( 19841986 ) who gave birth to at least one child during a five year period after participation . of these 3,739 women , we excluded 232 women due to pre- ( < 37 weeks ) or postterm ( > 44 weeks ) delivery , 32 women with multiple births , 10 women with gestational diabetes , 118 women with preeclampsia , 831 women without information of physical activity , body mass index , or gestational age , and 490 women who gave birth within 10 months after participation ( i.e. , possibly pregnant at the time of participation ) . information on the offspring was obtained by a linkage to the medical birth registry of norway . these data were obtained for the first child born during five years after participation in hunt . the main outcome variable was newborn birth weight , measured in grams ( g ) , first analyzed as a continuous variable and then as a dichotomized variable using 4,000  g as cutoff . the participants were asked to report how many exercise sessions ( e.g. , walking , skiing , swimming , or other sports ) they usually had during a week , with five response options ( 0 , < 1 , 1 , 2 - 3 , and 4 times ; coded 15 ) . if the participants reported exercising at least once a week , they were also asked to report the average duration ( < 15 , 1530 , 3160 , and > 60 minutes ; coded 14 ) and intensity ( light , moderate , and to exhaustion ; coded 13 ) of the activity . among participants who reported exercising at least once a week , a summary score of frequency , duration , and intensity was calculated according to the following equation : 1/5    frequency + 1/4    duration + 1/3    intensity . this procedure intended to give equal weight to each component of physical activity and resulted in a maximum score of 1.0 for each of the three components of the summary score . the median score value of 1.97 ( range , 1.23.0 ) was then used as a cut - off to classify women into two categories of score values ( median ) . this information was used to construct a variable of total physical exercise with four unique categories : ( 1 ) no activity , ( 2 ) low activity ( < 1 session per week ) , ( 3 ) medium activity ( < median score value ) , and ( 4 ) high activity ( median score value ) . body mass index ( bmi ) was computed from standardized measures as weight divided by the squared value of height ( kg / m ) and categorized into two groups : bmi < 25.0  kg / m and bmi  25.0  kg / m ( i.e. , overweight or obese ) . we used linear regression to analyze the association between measures of leisure time physical exercise and mean birth weight . we also calculated odds ratios ( or ) for having a macrosomic infant ( 4,000  g ) in different categories of leisure time physical exercise using logistic regression . precision of the estimated associations was assessed by a 95% confidence interval ( ci ) . women who reported the highest activity level were used as the reference category in all analysis . the following variables were considered as potential confounders in the analysis ; age ( 2024 , 2529 , 3034 , and 3539 years ) , smoking ( never , former , and current ) , frequency of alcohol consumption during the past 2 weeks ( none , 14 times , 5 times , abstainer , and unknown ) , education ( < 10 , 1012 , > 12 years , and unknown ) , marital status ( unmarried , married , and previously married ) , and parity ( primiparous , 1 - 2 children , and 36 children ) . covariates were removed from the model if there was no meaningful difference between adjusted and unadjusted estimates . tests for trend across categories of leisure time physical exercise were conducted by treating the categories as an ordinal variable in the regression model .   since maternal bmi could be both an effect modifier and on the causal pathway between exercise and birth weight , bmi was not included as a confounder in the primary analyses . however , additional analysis was conducted for the combined associations of prepregnancy bmi and total leisure time physical exercise in relation to birth weight , using linear and logistic regression as described earlier . we also included a product term of bmi ( < 25 versus 25  kg / m ) and exercise level ( no or low activity versus medium or high activity ) to assess possible interaction between the two variables and as well as stratified the analyses of physical exercise on the two bmi groups . all statistical analyses were performed using the statistical software spss for windows , version 17.0 . all eligible participants received a written invitation with information about the study , and all participants gave their consent by filling in and returning the first questionnaire that was mailed together with the invitation . mean baseline maternal age among the 2,026 women in the study was 26.9 years , whereas mean birth weight of their offspring was 3,620  g ( sd , 502 ) , and a total of 416 ( 20.5% ) newborns weighed 4,000  g or more ( i.e. , had macrosomia ) . table 2 presents results from linear regression showing that there was no clear association between maternal leisure time physical exercise and mean birth weight of their offspring , neither in relation to number of exercise sessions per week ( p trend , 0.49 ) nor in relation to total amount of exercise ( p trend , 0.56 ) . correspondingly , results from logistic regression presented in table 3 provide no consistent evidence for an association between maternal physical exercise and risk of a macrosomic infant . however , women who reported being inactive before pregnancy had a lower risk of giving birth to an infant with excessive birth weight ( or , 0.6 ; 95% ci , 0.40.9 ) compared to women with a high total exercise level ( table 3 ) . the combined association of pre - pregnancy bmi and leisure time physical exercise in relation to birth weight is shown in table 4 . women who were overweight ( bmi  25.0  kg / m ) before pregnancy and reported no or low leisure time physical exercise gave birth to infants with significantly higher mean birth weight ( 134  g ; 95% ci , 41.0227.3 ) and had a higher risk for a macrosomic infant ( or , 1.9 ; 95% ci , 1.22.9 ) , compared to women with bmi < 25.0  kg / m who had a medium or high exercise level . in additional analysis stratified by body mass index , overweight women ( bmi  25.0  kg / m ) who reported no or low exercise levels had significantly higher offspring birth weight ( 132  g ; 95% ci , 20.4243.7 ) and higher odds ratio for a macrosomic infant ( or , 2.0 ; 95% ci , 1.13.5 ) than women who reported a medium or high exercise level ( data not shown ) . however , there was no statistically significant interaction between bmi and total exercise level ( p = 0.08 ) . in this large prospective study of norwegian women , we found no clear association between reported leisure time physical exercise level before pregnancy and offspring birth weight . there was some evidence that inactive women had a slightly lower likelihood of giving birth to a child with excessive birth weight than more physically active women , but the small numbers of inactive women call for a cautious interpretation . analysis of the combined association of bmi and exercise on birth weight showed that women with a bmi  25.0  kg / m gave birth to infants with significantly higher birth weight than women with a bmi < 25.0  kg / m , but only if they also reported no or low levels of physical exercise . this could suggest that physical exercise to some extent reduces the effect of maternal adiposity on offspring birth weight . the suggestive evidence that women who were inactive before pregnancy had lower risk for delivering a macrosomic infant is contradictory to some previous studies . voldner et al .   reported that inactive women ( defined as < 1  h per week ) had almost a threefold higher odds ratio for fetal macrosomia than physically active women ( > 1  h per week ) . however , another norwegian study found no association between frequency of regular exercise before pregnancy and offspring with excessive birth weight ( 90th percentile ) . in the present study , those who were classified as inactive reported never engaging in physical exercise , and studies have shown that a sedentary lifestyle in pregnancy is associated with lower birth weight   and an increased risk of a very low birth weight infant . it has been observed that mothers of very low birth weight infants were less likely to be physically active during their pregnancy . it has been suggested that both excessive and insufficient physical activities in pregnancy are related to an inadequate fetal growth , although some women might be advised to be inactive and at rest to reduce the risk of adverse pregnancy outcomes .   unlike the present study , some previous studies have shown inverse associations between maternal pre - pregnancy exercise behaviors and offspring birth weight or risk of having excess weight [ 17 , 18 ] , although the results are not entirely consistent . a recent study of leisure time physical activity during pregnancy is more in agreement with the results of the present study . .   found no association with mean birth weight or the risk of giving birth to low ( < 2,500  g ) or high ( 4,500  g ) birth weight infants . the inconsistent results in these studies could be due to different measures of physical activity , in addition to the possibility for chance findings in the smaller studies . there is growing evidence that overweight or obesity before pregnancy is a risk factor for macrosomia [ 3 , 12 , 35 , 36 ] . the results from the present study suggest that the effects of maternal pre - pregnancy overweight were associated with higher birth weight only among women who reported no or low level of activity . this is contradictory to the findings by lf et al .   who showed that a high pre - pregnancy activity level did not reduce the risk of high birth weight infants in women who were overweight or gained much weight in pregnancy . nevertheless , physical activity may improve maternal weight control , both before and during pregnancy [ 37 , 38 ] . the strengths of the present study include the prospective design , the large sample size of women reporting physical activity before pregnancy , and the standardized measures of size at birth obtained from the medical birth registry of norway . however , some of the categories of physical exercise ( e.g. , inactive ) suffered from small samples size , and this could result in chance findings . moreover , as in any observational study , residual confounding due to unmeasured ( e.g. , smoking during pregnancy and gestational weight gain ) or unknown factors can not be ruled out . since the information on physical activity was self - reported , it could be subject to misclassification , although a validation study has shown acceptable agreement with objective measures . although pre - pregnancy physical activity has been associated with physical activity level during pregnancy [ 2325 ] , it has also been shown that exercise levels decline in pregnancy [ 24 , 25 , 40 , 41 ] . unfortunately , our data did not allow us to examine if pre - pregnancy physical exercise was related to the activity level during pregnancy . there was no clear association between maternal prepregnancy leisure time physical exercise and offspring birth weight . however , the result may indicate that inactive women gave birth to infants with lower birth weight and had lower risk for delivering an infant with macrosomia . the results also show that high maternal pre - pregnancy bmi was associated with higher mean birth weight and increased risk of macrosomia in offspring of physically inactive women , whereas pre - pregnancy bmi was not associated with birth weight and risk of macrosomia among more active women .
background . the objective was to examine the association between prepregnancy physical exercise and offspring birth weight and to assess the combined association of pre - pregnancy body mass index ( bmi ) and physical exercise on birth weight . 
 methods . the study included 2,026 women aged 2039 years participating in the norwegian hunt study and linked with the medical birth registry . 
 we calculated mean differences in birth weight and odds ratios ( ors ) for a macrosomic infant ( i.e. , birth weight > 4000  g ) using linear and logistic regression analysis . 
 results . 
 there was no clear association between leisure time physical exercise and mean birth weight . 
 women who reported no exercise had reduced risk of a macrosomic infant ( or , 0.6 ; 95% confidence interval ( ci ) , 0.40.9 ) compared to women with a high exercise level . 
 overweight ( bmi  25.0  kg / m2 ) was associated with an or of 1.9 ( 95% ci , 1.22.9 ) for a macrosomic infant among women who reported low exercise levels , whereas the or was 1.2 ( 95% ci , 0.81.8 ) among women with higher exercise levels . 
 conclusion . 
 there was some evidence that women who reported no exercise before pregnancy had lower risk for a macrosomic infant than women who exercised . 
 pre - pregnancy bmi was positively associated with birth weight and risk of macrosomia but only among the least active women .