Department of Obstetrics and Gynecology, Chiba University School of Medicine, 18-1 Inohana, Chuo-ku, Chiba 2608670, Japan 1 To whom correspondence should be addressed. e-mail: hmatsui{at}med.m.chiba-u.ac.jp
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Abstract |
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Key words: hydatidiform mole/incidence/population-based study
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Introduction |
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The incidence of molar pregnancy demonstrated marked geographic and ethnic differences, ranging from the highest incidence of 1 in 120400 pregnancies in Asian countries such as Taiwan, Philippines and Japan, to the lowest incidence of 1 in 1000 to 2000 in Europe and the USA (Bracken et al., 1984). These 1020-fold variations in the incidence of molar pregnancy might be overestimated by reporting biases, such as population-based and hospital-based data. Recently, the incidence of molar pregnancy in Japan was reported to be only two or three-fold higher than that in the USA on the population-based study (Hayashi et al., 1982
; Takamizawa et al., 1987
; Ishizuka and Tomoda, 1990
).
The population-based registration system for molar pregnancy was established in 1974 in Chiba Prefecture together with 12 other prefectures in Japan (Takamizawa et al., 1987; Ishizuka and Tomoda, 1990
). This registration system has been introduced in more regions, and now this population-based registration system is adopted in 22 of the 46 prefectures in Japan. We have analysed trends in the incidence for hydatidiform mole in Chiba Prefecture and 22 prefectures of Japan over a period of 27 years.
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Materials and methods |
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Since 1985, we have established a new system to register CM and PM, separately. The diagnostic criteria of CM and PM were based on the morphological findings and/or pathological findings at evacuation. Complete mole was diagnosed as the presence of diffuse hydropic vesicles and the absence of identifiable fetal parts. In contrast, PM was diagnosed as the presence of both normal and hydropic villi with or without fetal parts. The morphological diagnostic criteria for hydatidiform mole (hydropic vesicles >2 mm in diameter) have not changed for the 27 year study period.
A similar population-based registration system of hydatidiform mole has been carried out in 22 prefectures in Japan. The outcomes of this registration system in Japan (from 1976 to 1997) were compared with those in Chiba Prefecture.
Statistical analysis
Statistical analyses were performed by Welchs t-test, 2-test for heterogeneity and the
2-test for trend (Fisher and Van Belle, 1993
).
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Results |
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Discussion |
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In this study, the incidence of molar pregnancy in Chiba Prefecture has been constant from 1974 to 1990, as observed in our previous study (Takamizawa et al., 1987), while a significant decline has been demonstrated since 1991. A similar decreasing tendency has also been observed in Japan as a whole. Changes in the incidence of molar pregnancy seem to result from the decline of complete mole, and now the incidence of molar pregnancy in Chiba Prefecture has become as low as that in Europe or the USA.
It has also been reported (Martin and Kim, 1998) that the median incidence of molar pregnancy per 1000 live births in Korea had decreased over the past four decades from 4.4 in the 1960s to 1.6 in the 1990s. They considered that the decreasing incidence was due to changing social conditions, the decreased percentage of older women and those of higher parity in the obstetric population in Korea. Maternal ages >40 years old were generally accepted as being a high risk factor for developing complete mole. On the contrary, the percentage of deliveries at an older maternal age (>40 years) in Japan has slightly increased (0.45% in 1980, 1.0% in 1990, and 1.1% in 1998) (Ministry of Health and Welfare, 2000).
Other possible causes for the recent decrease in the incidence of molar pregnancy may be either missed or underestimated complete mole, which may be diagnosed as partial mole or non-molar spontaneous abortion based on macroscopic examinations (Soto-Wright et al., 1995; Mosher et al., 1998
; Lindholm and Flam, 1999
). Recent advances of high sensitivity pelvic ultrasonography may have led to early termination of abnormal pregnancies, before we can detect classical symptoms or identify hydatidiform vesicles macroscopically. At our hospital, the mean gestational ages at evacuation of patients with CM and PM were 11.0 ± 3.0 weeks (n = 285) and 11.0 ± 3.1 weeks (n = 55) in the former period (19851990), and 9.9 ± 2.2 weeks (n = 176) and 9.5 ± 2.0 weeks (n = 133) in the latter period (19912000) respectively. The mean gestational ages at evacuation of patients with CM and PM were significantly shortened in the latter period (P < 0.05; Welchs t-test).
It has been previously reported (Jacobs et al., 1982) that Japanese people in Hawaii who had acclimatized to an American lifestyle for a long period appeared to lose the increased risk of molar pregnancy. Although the reason for this change has not been entirely clear, an improvement of several environmental factors such as vitamin A deficiency, decreasing levels of consumption of dietary carotene and animal fat, and low socio-economic status have been reported to contribute to the decrease of molar pregnancy.
In this study, the ethnic background of the Japanese is relatively homogeneous and stable, and the diagnostic criteria for CM and PM have remained unchanged for these two decades. Moreover, the percentage of deliveries at an older maternal age, with a high risk factor for developing complete mole, is increasing in Japan. However, the incidence of complete mole per 1 000 live births has demonstrated a significant decrease in 1991 compared with that in the previous period (P = 0.0008), and it has been further decreasing from 1.09 in 1991 to 0.49 in 2000 (P = 0.0001). In contrast, the incidence of PM has stayed constant during these16 years. Since the genesis of complete hydatidiform mole results from fertilization of an empty oocyte by a haploid sperm or by two sperm (Kajii and Ohama, 1977; Ohama et al., 1981
), it could be speculated that Japanese women of reproductive age may have lost, over the last decade, the increased risk of ovulating empty oocytes.
In conclusion, the incidence of hydatidiform mole and complete mole has significantly decreased in Chiba Prefecture during the last decade. Some environmental or socio-economic factors have been proposed to contribute to the decrease in CM rates. Although we cannot clarify these aetiologic factors at the moment, a prolonged period of acclimatization to a Western lifestyle could be a factor responsible for a change in female reproductive physiology.
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References |
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Submitted on May 16, 2002; Accepted on September 9, 2002