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The author made use ot parochial archives for a genetic research. All parochial archives keep the registers of the newborn, the dead, the marriages, the confirmands, the amounting of the population in a parish. According to the canon 1813 (§ 1), these registers are public and ecclesiastical documents, and they form a mine of biological and genetic information too.
In this work the author, by making use of parochial archives in Busnago (Milan), — starting in 1634 — has controlled the proceeding of multiple pregnancies. These appear to have a percentage of 1,53% — superior to that of the Italian population (1, 13%) and to that of the human population officially numbered (1, 25% according to Caullery).
During the three centuries considered by the author, the proceeding of these multiple pregnancies shows a certain periodicity, but, from 1764 to 1804, there is an abrupt fall occuring at the same time of an endemic pellagrous disease. A further inquiry on 23 twin-pairs selected during the last forty years, and a research during the three centuries considered did not show any relationship between consanguinity and multiple pregnancies. On the contrary, this research showed a periodical proceeding of the frequency of multiple pregnancies on the total of the newborn with cusps, every quarter of a century, which seems to support the hypothesis of the existence of a dominant hereditary factor. At last, the greatest number of percentage of multiple pregnancies on the mother's side, as compared with the father's one, makes us think of a hereditary factor on the mother's side.