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Seasonality in early loss of one fetus among twin pregnancies.

Ultrasonographic examinations were performed on 519 pregnant women in the first trimester at three hospitals in Tokyo from August 1985 to March 1987. Among 519 women, two fetal heart movements (FHMs) were confirmed in six cases. In two of these six cases, one of two FHMs disappeared two weeks later, and these were considered the "vanishing twin". In eight cases among those with ultrasonographic image of echo-free-space (EFS) in the uterus, the observed EFS was considered to be a probable empty gestational sac (GS-like-EFS-image). Seven of eight GS-like-EFS-images were observed in October-December (p less than 0.01) and two "vanishing twin" cases were also observed in the same season. Spontaneous abortion occurred in 11 cases and these were also more frequently observed in October-November (p less than 0.05). Some abortive factor is supposed to have prevailed in October-December. All six women having twin fetuses (including the "vanishing twin" cases) were born in January-May (p less than 0.05) and those having GS-like-EFS-images or terminating their pregnancy in spontaneous abortion were also born more frequently in the same season. Twin-prone and abortion-prone characteristics of the women born in this season are considered.