An association of Rh type and gynaecological disease with twinning.

The medical record numbers ofmothers giving birth to twins were obtained from the obstetric files of the New York Lying-In Hospital of the New York-Cornell medical Center for the period 1955-1964. This source identified all the mothers delivering twins with the birthweights of 500 g. or more. From these files a control was selected of mothers delivering only single-born. Each mother of twins was matched with a mother of a singleton by age, race, hospital service, and with delivery date as close to that of the twin birth as feasible. The medical record of each mother of twins and singleborn control was directly abstracted for the following information: birth dates, ethnic origins, and religions of the mothers and their spouses; mother's menstrual history, medical, surgical, and pregnancy experience, and blood groups (ABO and Rh). Data were recorded for all pregnancies on the records. There were 496 mothers of twins identified: 409