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in the institute of medicine ( iom ) guideline in 2009 , optimal range of weight gain during pregnancy has been recommended based on the body mass index ( bmi ; kg / m ) of women during pre - pregnancy such as underweight ( bmi < 18.5 ) , normal ( bmi : 18.5 - 24.9 ) , overweight ( bmi : 25.0 - 29.9 ) , and obese ( bmi  30 ) . in a recent study in japan by enomoto et al , the bmi classification by the iom guideline seemed to be valid in pregnant japanese women associated with the different incidences of adverse pregnancy outcomes . in their observation ,
optimal weight gain recommended by the iom guideline also seemed to be associated with a better outcome in all pre - pregnancy bmi category groups . for example , excess gestational weight gain was associated with a higher frequency of large for gestational age and macrosomia , while poor weight gain was correlated with a higher frequency of small for gestational age . to date , the recommended weight gain during pregnancy for japanese women has been smaller than that in the iom guideline in 2009 [ 3 - 5 ] , although the bmi classification during pre - pregnancy seemed to be the same between the iom and japanese guidelines [ 1 , 3 - 5 ] . in the iom guideline , optimal ranges of weight gain during pregnancy in underweight , normal , overweight and obese women
have been reported to be 12.7 - 18.1 , 11.3 - 15.9 , 6.8 - 11.3 and 5.0 - 9.1 kg , respectively . however , in the japanese ministry of health , labour and welfare ( jmhlw ) guideline , optimal ranges of weight gain during pregnancy in underweight and normal women have been recommended to be 9 - 12 and 7 - 12 kg , respectively .
in addition , in the japan society for the study of obesity ( jasso ) guideline , optimal ranges of weight gain during pregnancy in overweight and obese women have been recommended to be  7 and  5 kg , respectively .
the average physique of japanese women is certainly short and slender in comparison to that of the western woman ; however , the studies about the recommended value of weight gain during pregnancy in compliance with the japanese women have not been well documented [ 1 , 6 ] . therefore , in the current study , we examined the perinatal outcomes in japanese women with the optimal range of weight gain during pregnancy in the jmhlw and jasso guidelines compared with those in the iom guideline .
the protocol for this study was approved by the ethics committee of the japanese red cross katsushika maternity hospital .
we reviewed the obstetric records of singleton pregnant japanese women who delivered at our institute at  22 weeks gestation from april 2012 through march 2016 . in order to examine the optimal weight gain during pregnancy in japanese women , we compared the obstetric outcomes in two groups of gestational weight gain within the optimal range based on the iom and jmhlw guidelines in women of pre - pregnancy bmi categories of underweight and normal weight , and in two groups of gestational weight gain within the optimal range based on the iom and jasso guidelines in women of pre - pregnancy bmi categories of overweight and obese .
as characteristics of patients , we examined maternal age , primiparous rate , height , body weight and bmi during pre - pregnancy , body weight at delivery and total weight gain during pregnancy .
the main obstetric outcomes were hypertensive disorders , gestational diabetes mellitus ( gdm ) , macrosomia , low birth weight , preterm delivery , cesarean delivery and postpartum hemorrhage  1,000 ml .
gdm was diagnosed when at least one of the following was found : fasting blood glucose level of  92 mg / dl , blood glucose level at 1 h of  180 mg / dl , and blood glucose level at 2 h  153 mg / dl by 75-g oral glucose tolerance test .
macrosomia was defined as a neonatal birth weight of  4,000 g , while low birth weight was defined as a neonatal birth weight of < 2,500 g. gestational age was calculated based on the ultrasonographic findings at 9 - 11 weeks gestation .
data were expressed as mean  standard deviation ( sd ) or number ( percentages ) .
cases and controls were compared by means of student s t - test for continuous variables , and the  or fisher s exact test for categorical variables .
odds ratios ( ors ) and 95% confidence intervals ( cis ) were also calculated . differences with p
there were 5,351 women who delivered singleton neonates at  22 weeks gestation at our institute during the study period .
of these , 515 ( 9.6% ) , 4,158 ( 77.7% ) , 555 ( 10.4% ) and 123 ( 2.3% ) women were defined as underweight , normal weight , overweight and obese based on the bmi during pre - pregnancy , respectively . of the 515 underweight women , the weight gain during pregnancy was 9 - 12 kg ( recommended weight gain in the jmhlw guideline ) in 204 women ( 39.6% ) , while it was 12.7 - 18.1 kg ( recommended weight gain in the iom guideline ) in 127 women ( 24.7% ) . of the 4,158 normal - weight women , the weight gain during pregnancy was 7 - 12 kg ( recommended weight gain in the jmhlw guideline ) in 2,136 women ( 51.4% ) , while it was 11.3 - 15.9 kg ( recommended weight gain in the iom guideline ) in 1,348 women ( 32.4% ) . of the 565 overweight women , the weight gain during pregnancy was < 7 kg ( recommended weight gain in the jsso guideline ) in 141 women ( 25.4% ) , while it was 6.8 - 11.3 kg ( recommended weight gain in the iom guideline ) in 161 women ( 29.0% ) . of the 123 obese women , the weight gain during pregnancy was < 5 kg ( recommended weight gain in the jsso guideline ) in 61 women ( 49.6% ) , while it was 5.0 - 9.1 kg ( recommended weight gain in the iom guideline ) in 41 women ( 33.3% ) . 

table 1 shows the characteristics of the underweight women with the optimal weight gain recommended in the jmhlw and iom guidelines ( jmhlw and iom groups ) .
the ages of the women in the jmhlw group were lower than those in the iom group .
table 2 shows the perinatal outcomes of the underweight women with the optimal weight gain recommended in the jmhlw and iom guidelines ( jmhlw and iom groups ) .
the incidence of gdm in the jmhlw group was significantly lower than that in the iom group , while the incidences of preterm delivery and low - birth - weight infant in the jmhlw group were significantly higher than those in the iom group .
jmhlw : japanese ministry of health , labour and welfare ; iom : institute of medicine .
data are presented as number ( percentages ) . * p < 0.05 vs. iom group .
jmhlw : japanese ministry of health , labour and welfare ; iom : institute of medicine . 

table 3 shows the characteristics of the normal - weight women with the optimal weight gain recommended in the jmhlw and iom guidelines ( jmhlw and iom groups ) .
the rate of women of  40 years old in the jmhlw group was lower than that in the iom group .
table 4 shows the perinatal outcomes of the normal - weight women with the optimal weight gain recommended in the jmhlw and iom guidelines ( jmhlw and iom groups ) .
the incidences of hypertensive disorders , macrosomia and postpartum hemorrhage  1,000 ml in the jmhlw group were significantly lower than those in the iom group , while the incidences of preterm delivery and low - birth - weight infant in the jmhlw group were significantly higher than those in the iom group .
jmhlw : japanese ministry of health , labour and welfare ; iom : institute of medicine .
data are presented as number ( percentages ) . * p < 0.05 vs. iom group .
jmhlw : japanese ministry of health , labour and welfare ; iom : institute of medicine . 

table 5 shows the characteristics of the overweight women with the optimal weight gain recommended in the jsso and iom guidelines .
table 6 shows the perinatal outcomes of the overweight women with the optimal weight gain recommended in the jsso and iom guidelines ( jsso and iom groups ) .
the incidences of preterm delivery and low - birth - weight infant in the jsso group were significantly higher than those in the iom group .
jasso : japan society for the study of obesity ; iom : institute of medicine .
jasso : japan society for the study of obesity ; iom : institute of medicine . 

table 7 shows the characteristics of the obese women with the optimal weight gain recommended in the jsso and iom guidelines . there were no significant differences in these valuables between the two groups .
table 8 shows the perinatal outcomes of the obese women with the optimal weight gain recommended in the jsso and iom guidelines .
jasso : japan society for the study of obesity ; iom : institute of medicine .
jasso : japan society for the study of obesity ; iom : institute of medicine .
recently , there have been many underweight women who abhor a large weight gain during pregnancy in japan associated with their slimming desire [ 4 , 6 ] .
in particular , young japanese women s strong desire to be thin has been pointed out as the underlying cause . in conjunction with these trends ,
twenty to thirty years ago , the average birth weight in japan was about 3,300 g ; however , it has now decreased to be about 3,000 g and the proportion of low - birth - weight infants in japan has been increased to about 10% [ 4 , 6 ] . recently , in addition , some birth cohort studies in japan have also been believed to be suitable for epidemiological studies to demonstrate the  developmental origins of health and disease ( dohad )  indicating the developmental plasticity and the mismatch concept [ 6 - 8 ] . based on these backgrounds , the importance of nutrition and weight gain during pre - pregnancy and pregnancy
therefore , examinations concerning the optimal weight gain during pregnancy in japanese women are very important . in the current study
, we examined which guidelines indicate the optimal range of weight gain during pregnancy in pregnant japanese women based on their bmi during pre - pregnancy .
although some bias seemed to be presented in the current study , we could not find whether or not the jmhlw guideline is more appropriate for the underweight and normal - weight pregnant japanese women compared with the iom guideline .
otherwise , the median range of the two recommended weight gains by the jmhlw and iom guidelines may be suitable for the underweight and normal - weight japanese women during pregnancy . in this study , because we could not assess weekly weight gain , it is unknown whether or not the less weight gain in the jmhlw group is the cause of the increased incidence of premature delivery and/or low - birth - weight infants . in the jmhlw group , the premature delivery associated with the increased low - birth - weight infants might increase before maternal weight gaining by other reasons .
we understand that this may be one of some serious limitations in this study . in order to assess the optimal range of weight gain for underweight and/or normal - weight women during pregnancy , a larger prospective study will be required . on the other hand
, the iom guideline seemed to be more appropriate for the overweight pregnant japanese women compared with the jsso guideline . in this study
, the neonatal outcomes in these women seemed to be improved according to the optimal weight gain in the iom guideline . to date , in japan severe limit of weight gain has been seemed to be instructed for obese women for preventing dystocia [ 4 , 9 ] ; however , the optimal weight gain recommended by the iom guideline seemed to be more suitable for the overweight japanese women at least during pregnancy . in this study , in addition , we could not well examine about the appropriate weight gain for obese pregnant japanese women .
one of some possible reasons leading to this observation may be the small sample size in the current study .
otherwise , in the obese women , the pre - pregnancy obesity itself might affect the perinatal outcomes more than the range of weight gain during pregnancy as previously reported [ 1 , 2 ] .
therefore , a larger prospective study will be also required to assess the optimal range of weight gain for obese women during pregnancy .
we should be more tolerant for the weight gain during pregnancy in japanese woman than ever , especially in overweight women .
we should be more tolerant for the weight gain during pregnancy in japanese woman than ever , especially in overweight women . a larger study may be needed .
this study was supported by the japan health , labour and welfare scientific research subsidies : 
study on the support of nutritional management of pregnant women , mothers and infants  .
backgroundin order to examine the optimal weight gain during pregnancy in japanese women , we analyzed the perinatal outcomes in japanese women with the optimal range of weight gain during pregnancy according to the japanese ( the japanese ministry of health , labour and welfare : jmhlw and the japan society for the study of obesity : jasso ) guidelines compared with those according to the usa ( the institute of medicine : iom ) guideline.methodswe compared the obstetric outcomes in two groups of gestational weight gain within the optimal range based on the iom and japanese guidelines in women of pre - pregnancy body mass index ( bmi ) categories of underweight , normal , overweight and obese.resultsin the underweight and normal - weight women , the incidences of preterm delivery and low - birth - weight infant in the jmhlw group were significantly higher than those in the iom group ; however , the incidence of some other perinatal complications in the jmhlw group was significantly lower than that in the iom group . in the overweight women , 
 the incidences of preterm delivery and low - birth - weight infant in the jsso group were significantly higher than those in the iom group ; however , there were no significant differences in the obstetric outcomes between the obese women in the jsso and iom groups.conclusionbased on the current results , we should be more tolerant for the weight gain during pregnancy in japanese woman than ever , especially in overweight women .