From 1980 to 2004, the mean birthweight decreased in Japan, specifically between 1985 and 1999. Although 2004 was 16 years ago, the birthweight trend was almost constant after the peak. Using data of years in which the BWD was rapid and precise would provide further insight into possible factors contributing to the decline in LBW through multiple regression analysis. However, based on our analyses, the decline was not fully explained by adjusting variables obtained from birth certificates.
Some studies have examined birthweight changes among subgroups, for example, first-birth singletons . As expected, the BWD was mainly associated with gestational age in the multivariable regression analysis. After adjusting for all variables used in the multiple regression analysis, the decrease in birthweight was 5.13 g/year compared to a crude reduction of 8.07 g/year. GA is not the only factor responsible for the entire birthweight decline. Indeed, Morisaki et al.  reported birthweight decreases in the USA, regardless of GA.
In this study, we used large, nationally representative data that would not change for decades and we had the necessary information on potential factors associated with trends in birthweight.
One of the limitations of this study was GA estimation. GA was entered in the birth certificate by doctors or midwives in hospitals or clinics. Some were based on the date of the last normal menstrual period, while others were based on early fetal ultrasonography. This might introduce variability and affect the validity of our GA estimation. Schonberg et al.  determined that GA calculated from the last menstrual period is reasonably accurate among term births.
Many factors affect birthweight; however, the following factors were not analyzed in this study: medical conditions in pregnancy, childbirth, and other modifiable factors, such as pregnancy diabetes, pregnancy hypertension, placental abruption and abnormal obstetric bleeding. Considering obstetrical practice, GA is affected by the mode of delivery. The induction of labor has also steadily increased in Japan . Based on the national growth survey, the rates of cesarean deliveries have increased from 19% in 2000 up to 25% in 2010 . These factors might cause a decline in GA. Data from the USA showed that a decrease in GA is affected by an increase in the number of induced labor .
Another limitation of this study is the lack of data likely associated with birthweight, such as maternal smoking status, pre-pregnancy weight, and maternal diet during pregnancy. Our data did not provide information on medical conditions such as diabetes and hypertension, which affect birthweight by causing large GA neonates and fetal growth restrictions, leading to LBW. Maternal smoking restricts fetal growth and increases obstetrical complications and the risk of preterm birth; these factors are likely to induce LBW and even stillbirths .
In Japan, 5.0%, 10.0%, and 5.0% of pregnant women smoked in 1990, 2000, and 2010, respectively . Moreover, in a recent survey on mothers and children aged 3-4 months, LBW was significantly associated with maternal smoking . A decrease in the proportion of pregnant women who smoked could be a cause of decrease in birthweight after 2000. In Japan, the Health Promotion Law was enforced in 2002, and people were encouraged to quit smoking thereafter. In Canada, intervention studies on pregnant women, including studies on health education about the adverse effects of smoking, decreased the proportion of LBW .
Pre-pregnancy BMI and GWG are factors that affect birthweight . Information on maternal pre-pregnancy weight was not included in our data. BMI distribution among women within reproductive age could be a proxy for pre-pregnant BMI. The prevalence of underweight has been increasing over the decades in Japan, contrasting with other countries . According to the National Health and Nutrition Survey , the proportion of women with a BMI of less than 18.5 kg/m2 in the age group of 20–39 years is increasing concomitantly with a decline in the mean birthweight (Figure 2). For the discussion of causality between maternal BMI and birthweight, further studies are needed.
Our data also did not include maternal diet, which might affect fetal growth. In Japan, the National Health and Nutrition Survey has revealed synchronized time trends for per capita calorie intake (Figure 3); however, the causality is still unclear.
Time-trend synchronization was suggested between birthweight and BMI in women of reproductive age and birthweight and energy intake of the whole population. The decline in energy intake of the entire population is a reflection of the decrease in macronutrient intake in pregnant women, which is likely to correlate with LBW. These factors other than GA can most likely result in LBW and their effect may be more substantial than that of GA.
Another factor suggested to increase the prevalence of LBW in Japan is the decline in adult height . An increase in maternal age  has been shown to be a factor associated with decreased birthweight in Korea. In our study, maternal age did not show a linear correlation with birthweight and could not be included as an explanatory variable. Another maternal condition that lowers birthweight is anemia and parasitic infections. In Pakistan, the relationship between iron-deficiency anemia and LBW was identified , and in Sudan , the effects of maternal malaria on LBW have been reported. Among Japanese mothers living in urban area, hemoglobin change during pregnancy was inversely associated with birthweight.
Although socioeconomic status is a well-known factor affecting birthweight , birth certificates do not contain any variable which allow analysis related to this factor. A comparison of secular trends of economic growth[32-34] and mean birthweight in Japan is shown in Figure 4. Although the timings of deterioration in economic growth and birthweight are almost the same, the synchronization is unclear. Yorifuji et al.  pointed out that socioeconomic position is related to air pollution, which influences the occurrence of LBW in Japan, suggesting the importance of socioeconomic factors.
Further, climate affects birth outcomes; in a study including 32 million US singletons , extremely high temperature was associated with preterm birth, which has a strong correlation with LBW. In the Japanese setting, urbanization has caused long-term climate changes related to a rise in temperature . Thus, such factors should also be considered.
In this study, we aimed to provide further insights on factors contributing to LBW through the analysis of variables contained in birth certificates in Japan. By analyzing neonates born from 1980 to 2004, decreases in birthweight were not fully explained by factors included in the birth certificates. A decrease in GA only partially explained the decreasing birthweight. Birth size might influence not only short-term conditions but also long-term prognosis . Factors lowering birthweight, although not analyzed fully in the present study, could cause various health problems among children as they grow up and even in their adult lives. Therefore, a follow-up study is necessary to investigate which sequalae would derive from LBW neonates.
In conclusion, our study based on data from birth certificates showed that infants’ birthweights have decreased over the decades. These findings might partially be explained by the decline in GA, considered to result from a change in the mode of delivery. Thus, further studies are needed to determine the clinical and social significance of these findings.