Recent trend of increase in proportion of low birthweight infants in Japan

Hiroki Ohmia, Kenzou Hirookab, Akira Hatab and Yoshikatsu Mochizukic

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: 1500–2499 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.2–4 We have analysed the trend of birthweight and related factors in Japan using vital statistics and national nutritional survey data (1950–1998) provided by Japan Ministry of Health and Welfare, and smoking prevalence survey data (1965–1998) provided by Japan Tobacco Inc. From the 1950s to the 1970s, the proportion of LBW babies (LBW: 1500–2499 g) decreased steadily, on the other hand, that of very low birthweight babies (VLBW: <1500 g) increased (Figure 1AGo). 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 1BGo), but the reasons for this have not been analysed yet. Mean maternal height in Japan has increased—based 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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Figure 1 Trends in proportion of low birthweight (LBW) and very low birthweight (VLBW) and related factors in Japan

A: Proportion of LBW and VLBW among singleton livebirths

B: Proportion of LBW for strata of gestational age among singleton livebirths. Values prior to 1979 were not recorded in vital statistics

C: Smoking prevalence among women in their twenties and thirties

D: Mean body mass index of women in their twenties and thirties

E: Numbers of livebirths to mothers in their thirties and to mothers in their twenties, and ratio: (No. livebirths to mothers in their thirties/No. of livebirths to mothers in their twenties). Because of Japanese superstition that women born in ‘Hinoeuma's years (1966) would lead unhappy lives, parents tended to register the birth of their female infants in the previous or the next year

 
What then, are the major factors for the recent trend of increased LBW since the 1970s? The two known factors causing LBW (i.e. maternal smoking, and pre-pregnancy weight) were analysed using the statistics data. First, unlike other developed countries, smoking prevalence in young women increased rapidly in Japan (Figure 1CGo). The increase in women in their thirties started in the 1970s, but that in women in their twenties started in the 1960s. Second, although the prevalence of obesity is increasing and becoming a major risk factor for common diseases for most Japanese people, body mass index has been decreasing only in subgroups of young women, presumably due to excessive dieting (Figure 1DGo). In this area, the decrease in women in their thirties also began in the 1970s, while that of women in their twenties began in the 1960s. In order to detect major factors for increasing IUGR since the 1970s, the points to which special attention should be paid are the health and behaviour of mothers in their thirties and for both smoking and dieting the increase began in the 1970s. Moreover, since the late 1970s the ratio of deliveries to mothers in their thirties has increased by more than three times compared to mothers in their twenties (Figure 1EGo). In addition, increased Chlamydia trachomatis infection among young women, which is considered to be a risk factor for low birthweight, was also described recently. The prevalence among pregnant women in their twenties and thirties was reported to be over 7.0%.5

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 1970–1998, corresponding to 0.2–0.5% 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

  • Since the 1970s a consistent increase in the proportion of low birthweight babies (1500–2499 g) has been observed in Japan. This follows a decrease from 1950 to the 1970s.
  • Increased prevalence of smoking and decreased body mass index in young women, especially those in their 30s, since the 1970s appeared to be the major factors for this phenomenon.
  • The educational approach to young women aimed at preventing smoking and excessive dieting, and monitoring of factors related to low birthweight should be strengthened.

 

References

1 Kramer MS. Determinants of low birth weight: methodological assessment and meta-analysis. Bull World Health Organ 1987;65:663–737.[ISI][Medline]

2 Evans S, Alberman E. International collaborative effort (ICE) on birthweight; plurality; and perinatal and infant mortality—II: Comparison between birthweight distribution of births in member countries from 1970 to 1984. Acta Obstet Gynecol Scand 1989;68:11–17.[ISI][Medline]

3 Alberman E, Evans SJW. The epidemiology of prematurity: aetiology, prevalence and outcome. Annales Nestlé 1989;47:69–88.

4 Matsumoto T, Takasaki Y, Takagi S et al. A downward trend in the birthweight of infants in Fukuoka City over the past decade. J Japan Pediatr Soc 2000;104:1012–17.

5 Nishimura M, Kumamoto Y, Koroku M et al. Epidemiological study on Chlamydia trachomatis infection in pregnant housewives and investigation on its influence on outcome of pregnancy and on their newborn. Kansenshogaku Zasshi 1990;64:179–87.[Medline]

6 Zubrick SR, Kurinczuk JJ, McDermott BM, McKelvey RS, Silburn SR, Davies LC. Fetal growth and subsequent mental health problems in children aged 4 to 13 years. Dev Med Child Neurol 2000;42:14–20.[ISI][Medline]

7 Barker DJ. Fetal origins of cardiovascular disease. Ann Med 1999;Suppl.1:3–6.





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