Risk of recurrent molar pregnancies following complete and partial hydatidiform moles

What is the risk of further molar pregnancies for women with one or more hydatidiform moles (HM) in relation to molar subtype.Women with a complete hydatidiform mole (CM) have a 1 in 100 and 1 in 4 risk of further CM after one or two consecutive CM, respectively, while women with a partial hydatidiform mole (PM) have only a small increase in risk for further molar pregnancies.Women with a molar pregnancy have an increased risk of further HM. A small subgroup of women with recurrent HM has an autosomal recessive condition, familial recurrent hydatidiform moles (FRHM), that predisposes them to molar pregnancies.A retrospective study of subsequent pregnancies in 16 000 women registered at a centralized referral centre, with a CM or PM, between 1990 and 2009.One hundred and sixty-six women with two or more molar pregnancies were identified from electronic records and patient notes. Histopathological features of all molar tissue were reviewed in these cases and genotyping performed where diagnosis was not possible on the basis of histopathological features alone. In addition, genotyping of molar tissue was performed in all cases of women with three or more CM to establish whether the tissue was diploid and biparental or androgenetic.This study confirms an increased recurrence risk of ∼1% for a second molar pregnancy and in addition that this risk is associated with CM rather than PM. The data further indicate that the risk of a third HM is associated almost exclusively with CM and enabled an estimate that 1 in 640 women registered with a CM has the rare condition FRHM. The study also found that there was no significant difference between the risk of developing gestational trophoblastic neoplasia (GTN) for typical sporadic CM and the diploid biparental CM associated with FRHM (GTN; proportion difference 0.05, Z = 0.87, P = 0.29).While pathology was reviewed for all women with two or more molar pregnancies, not all cases registered underwent central review particularly those women registered in the early 1990s. It is therefore possible that the total number of CM and PM may differ slightly from that stated. While women were followed for a minimum of 5 years, it is possible that some women may subsequently have further molar pregnancies that will not have been included in the present study.This is the largest study to date on recurrence for molar pregnancies, and as such provides the most detailed information so far regarding the risk of further molar pregnancies for women with a PM or CM. Furthermore, the data provide new insights into the incidence of the rare autosomal recessive condition, FRHM, important information for counselling women with molar pregnancies.No competing interests declared. No funding was obtained for this study.