Hydatidiform mole is a part of generic term, Gestational Trophoblastic Disease (GTD). This term describes a spectrum of abnormal trophoblastic proliferation associated with villous enlargement (moles) or neoplasm without villi (choriocarcinoma, placental site trophoblastic tumor). At present, hydatidiform mole (HM) occurs in 1-2 per 1000 pregnancies in Europe and the United States. HM is much more frequent in some Asian countries but these data were from hospital studies and thus misleading (1–3). Approximately, 15-20% of cases of complete moles require future treatment with chemotherapy (2). Currently, there is no reliable method to determine which molar pregnancy will be cured after evacuation or in which, further treatment will be required. Fortunately, a change in Beta-HCG level gives a very accurate assessment of the level of disease activity and this forms the basis of the follow-up protocol (2, 4).

The important point about HM is that it can be changed to other forms of GTD which need chemotherapy, so early detection and treatment of HM is significant. Prevalence of HM in different countries is different and perhaps it depends to socioeconomic, genetic, nutritional, cultural and other factors. In a survey in Pakistan, the incidence of GTD was 28 per 1000 live births and 70% of them were HM (5). In a report from Tunisia, the frequency of complete mole was 1 per 1347 deliveries (6) and in Nigeria, it was 3.8 per 1000 deliveries (7). In Morocco, the incidence of HM was 4.3 per 1000 pregnancies (8). In Nepal, it was 1 per 276 births (9). In Texas, incidence of GTD was 2.06 per 1000 live births and it was more prevalent in Hispanics (1). In a report from Brazil, the incidence of HM was 8.5 per 1000 deliveries (10). In Finland, the incidence of HM was reported to be 9.84 per 1000 deliveries (11) and in Sweden, it was 1.2 per 1000 deliveries (12). In UK, HM was reported to be 1 in 591 viable conceptions (13). In Japan, the incidence of complete mole has decreased since 1991 and at present is as low as the one in Europe or in USA and is 2 in 1000 pregnancies (14, 15).

Almost all of these researches are retrospective and the statistics are derived from obstetric admissions and GTD patients referred to trophoblastic disease center and/or pathology reports and hospital ICD-9 codes. In this study, pregnant women from early pregnancy and after positive pregnancy test, until 8-10 weeks of pregnancy were evaluated to detect the proportion of HM in prenatal clinics in health care centers of IUMS.

Between January 2012 and January 2013, all women who immediately after positive pregnancy test or after retarded menstruation came to prenatal clinics in health care centers of IUMS, were included in the study. They were followed until 8-10 weeks of pregnancy and at this time abdominal sonography was used to check the well-being of their fetuses. If fetal heart rate was seen on abdominal ultrasound, the molar pregnancies could be ruled out. If the pregnant woman had abortion or bleeding before this time, sonography and the evidence of pathologic evolution were used to determine the presence or absence of HM. Therefore, the proportion of HM in all pregnant women was determined.

The data were analyzed by SPSS version 13, using mean and standard deviation and percentages. Chi squared and Fisher exact tests were used for descriptive data analysis.

In this cross sectional study, 8614 pregnant women with mentioned criteria were included. 61 cases of hydatidiform mole were diagnosed (0.7% or 7 per 1000 pregnancy). Partial moles were diagnosed in 10 cases (16.4%) of molar pregnancies. The age range of women was 17-48 years (Table 1). The mean age for non-molar group was 34.2±3.41 and mean age for molar group was 26.6±5.3 years (p<0.05). In 19.7% of molar and 0.1% of non-molar pregnancies, patients had a history of molar pregnancy (p<0.05). In 23% of molar and 3.7% of non-molar pregnancies, there was a history of using oral contraceptive pills before pregnancy (p<0.05). In 8.1% of molar and 0.4% of non-molar pregnancies, this pregnancy was due to assisted reproductive technology (p< 0.05). In 19.7% of molar and 5.4% of non-molar pregnancies, there was a history of abortion in the last pregnancy (p<0.05). The distribution of blood types was not significant between molar and non-molar pregnancies. Blood type O with frequency of 40.1% in non-molar group and 32% in molar group was the most common blood type.

In this study, the proportion of HM in pregnant women in prenatal clinics in health care centers of IUMS in 2012 was 7 in 1000 pregnancies. It appeared that the rate of HM in Asian countries is more frequent than Europe and USA; however, these data were from hospital studies and therefore misleading (3). The difference of this study from others is that the pregnant women were followed until confirmation or exclusion of a molar pregnancy, but in other studies, the statistics were derived from admission of cases with molar pregnancies in hospitals.

It seems that there are many variations in the incidence of HM in different countries and in some studies, it is reported that the incidence of HM in Asia is 5-15 times more than western countries (3, 16, 17). A number of environmental factors might contribute to the incidence of complete mole. For example, a low-carotene diet and vitamin A deficiency, age of mothers, parity, history of previous molar pregnancy, history of OCP intake, pregnancies due to ovulation induction and type A blood group may account for the incidence (4). In a Korean study, the incidence of HM was 2 per 1000 deliveries (18). In Pakistan, in a total of 1056 obstetric admissions, the frequency of GTD was 28 per 1000 live births and HM was the commonest type of GTD (70%). In this study, the frequency of GTD was higher than international studies (5). In China, it was 3.87 per 1000 live births (19).

In most studies, the incidence of HM was higher in women under 20 and over 40. In a retrospective study between 1996-2005, in Nigeria, incidence of HM was 3.8 per 1000 deliveries and 71.8% of cases were partial mole and peak age of mothers with molar pregnancy was 17.5 years (7). In this study, only 14.8% of mothers with molar pregnancy were under 20 and perhaps this is related to the small number of women under 20 years of age in this research. Only 16.3% of molar pregnancies were partial. In this study, the incidence of complete mole was 0.7% and incomplete mole was 0.11% among all pregnancies. In some studies, incidence of complete mole was more than partial (6, 20–22). In Nigeria, the ratio of complete to partial mole was reported to be 1 (23), but in another study in Nigeria, the rate of molar pregnancy was 3.8 per 1000 live births and partial mole was 71.8% of molar pregnancies (7). In a study to determine the incidence of HM, for the period between the years 2000-2009 in UK, the incidence of molar pregnancy was 1 per 591 viable pregnancies and the incidence increased from 1 per 611 in 1997 to 1 per 528 viable pregnancies in 2008. The risk of HM was higher in young teenagers and women more than 40 years old (13).

History of OCP intake and previous mole or miscarriage increases the chance for HM (2). In this study, the history of these risk factors resulted in significant difference between molar and non-molar pregnancies.

In this study, there was no significant difference for blood types between molar and non-molar pregnancies.

The frequency of HM in different countries shows a dramatic difference which depends on multiple factors that should be evaluated locally. The frequency of HM in Asia is more than European and American statistics (17), but in this study it was not so high as 5-15 times. Its frequency in European and American statistics is 2 per 1000 pregnancies but in population of this study, it was 7 in 1000 pregnancies.