a Health and Disease Prevention Division, Department of Health and Welfare, Hokkaido Government, N3-W6, Chuo-ku, Sapporo, Hokkaido 060-8588, Japan.
b Department of Public Health, School of Medicine, Asahikawa Medical College, Asahikawa, Hokkaido, Japan.
c Department of Community Health Nursing, School of Nursing, Asahikawa Medical College, Asahikawa, Hokkaido, Japan.
Hiroki Ohmi. E-mail: hiroki.oomi{at}pref.hokkaido.jp
Abstract
Background The proportion of low birthweight babies (LBW: 15002499 g) in Japan decreased steadily from 1950 to the 1970s. However, since then it has started to increase consistently, the reason for which has not been discussed in detail.
Methods Trends of birthweight and the two known factors for low birthweight (maternal smoking, and pre-pregnancy weight) were analysed with vital statistics, national nutritional and smoking prevalence survey data.
Results Increase in term LBW due to intrauterine growth retardation, is the major risk factor for the overall increase since the 1970s. The increase of smoking prevalence in women in their thirties started in the 1970s, while that for women in their twenties started in the 1960s. The decrease of body mass index for women in their thirties also began in the 1970s, while that for women in their twenties began in the 1960s. The ratio of delivery to mothers in their thirties to mothers in their twenties has increased more than threefold compared to the late 1970s.
Conclusions Since the 1970s increase in smoking prevalence and decrease in body mass index in young women, especially those in their thirties, appeared to be the major factors involved in the increase in LBW babies.
Keywords Low birthweight, intrauterine growth retardation, maternal smoking, pre-pregnancy weight
Accepted 19 January 2001
Low birthweight is a major public health problem. There are two categories: intrauterine growth retardation (IUGR), and preterm birth. In developed countries, IUGR is associated with three major risk factors: cigarette smoking during pregnancy, low maternal weight gain, and low pre-pregnancy weight. These three factors account for nearly two-thirds of all IUGR cases.1 Cigarette smoking is the most important risk factor for preterm birth.1 Other risk factors for low birthweight include maternal age, race, maternal illness and so on.1
A recent trend of increase in the proportion of low birth-weight (LBW) infants in Japan has been reported.24 We have analysed the trend of birthweight and related factors in Japan using vital statistics and national nutritional survey data (19501998) provided by Japan Ministry of Health and Welfare, and smoking prevalence survey data (19651998) provided by Japan Tobacco Inc. From the 1950s to the 1970s, the proportion of LBW babies (LBW: 15002499 g) decreased steadily, on the other hand, that of very low birthweight babies (VLBW: <1500 g) increased (Figure 1A). General improvements in obstetric care, socioeconomic status, education and nutrition during pregnancy could explain the former, while decreased spontaneous abortion and stillbirth are considered to be reasons for the increase in VLBW. However, since the 1970s, a consistently increased proportion of both weight groups has been observed. The increase in VLBW could be explained by the same reasons as previously. On the other hand, term LBW, or IUGR, has contributed most to the increase in LBW since the 1970s (Figure 1B
), but the reasons for this have not been analysed yet. Mean maternal height in Japan has increasedbased on national statistics data for senior high school students, the mean height of girls aged 17 or 18 years old has been reported to have increased from 155.6 cm (1970) to 158.1 cm (1998). Maternal height was reported to inversely correlate with the risk of IUGR.1 Thus, the rise of term LBW is paradoxical. An analysis of the factors causing this rise is important.
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Three other possibilities have to be examined. First, paediatricians are beginning to register preterm infants as livebirths: these would not have been registered in earlier years because most of them died. This factor might be a possible cause given the increasing survival of LBW babies. However, the number of livebirths before 32 weeks gestation has consistently decreased until now, indicating this possibility is unlikely. Second, the legal gestational age limit for abortion on request in Japan has been determined by fetal viability: 32 weeks gestation until 1975, 28 weeks until 1978, 24 weeks until 1990, and it is now 22 weeks gestation. If terminated babies were selected because of their apparent small size, changes in the legal gestational age limits for abortion and the abandonment of this practice might be associated with an increase in LBW. However, the number of terminations after 20 weeks of gestation ranged between 2527 and 7362 per year over the period 19701998, corresponding to 0.20.5% of singleton livebirths. On the other hand, the proportion of low LBW among singleton livebirths ranged between 4.6% and 7.5%, thus the influence of terminations, because of small size or other problems, on the proportion of LBW is considered to be negligible. Finally, increased multiple births due to the use of stimulated ovulation for infertility treatment could be one factor but the actual increase resulting from this was assumed to be minor.
Low birthweight and, in particular VLBW, predispose to a variety of neurodevelopmental disorders and later suboptimal health.6 Moderately reduced birthweight as well as VLBW are hypothesized to be relevant to cardiovascular disease and non-insulin dependent diabetes in adult life.7 To reduce morbidity and mortality related to LBW, and to offer long-term benefits to their offspring, the education programme directed at girls and young women to prevent smoking, excessive dieting and sexually transmitted disease should be strengthened. Monitoring of factors relating to LBW should be continued in Japan.
KEY MESSAGES
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References
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