|Budget Amount *help
¥2,900,000 (Direct Cost : ¥2,900,000)
Fiscal Year 1998 : ¥700,000 (Direct Cost : ¥700,000)
Fiscal Year 1997 : ¥2,200,000 (Direct Cost : ¥2,200,000)
In order to clarify whether twin pregnancy is associated with reduced fecundity, we carried out the following epidemiological research. (1) In two natural fertility populations, the Hutterites and the historical French Canadians, birth interval was compared between singleton and twin deliveries, considering maternal age and birth order. Only at a relatively higher age, the birth interval tended to be longer before the twin than before the singleton deliveries. The difference was mostly due to the unlike-sexed twins. This suggests that multiovulatory twin pregnancies at higher maternal ages are linked to reduced fecundity. (2) On the contemporary population in Japan, we sent a questionnaire to members of a twin mother's club to compare reproductive characteristics between the mothers of like-sexed twins (mostly monozygotic) and those of unlike-sexed twins (definitely dizygotic). The mothers of unlike-sexed twins had a significantly younger age at menarch, as well as a higher proportion of those with a shorter menstrual cycle. This is in contrast with the results from the natural fertility populations : contemporary dizygotic mothers seemed more fertile than monozygotic mothers. The relationship between subfecundity and twinning may be, however, hidden in the present population because most of the contemporary couples bear children at a relatively younger age. The other women who reported to have experienced any medical infertility treatment showed much more irregularity in their menstruation and experienced severer nausea and threatened abortions more frequently. The infertile group had delivered twins with a lower birth weight, but this weight reduction was not significant after being adjusted for birth order, sex and sex combination of the twin babies, and the length of gestation period. The twin babies born through the infertility treatment were thus comparable in birth weight with the other naturally conceived babies.