From the Department of Obstetrics and Gynecology, Yale University School of Medicine, New Haven, Conn.

The author is Markle Scholar in Medical Science, Yale University School of Medicine.

This work was aided by grants from the National Institutes of Health, United States Public Health Service, H-2272 (C) and the Hilles Fund of Yale University.

This study has been made possible by the patient and persevering work of the research nurses and the secretarial and electronic assistants. I wish to thank them and the obstetrical, nursing, and resident staffs of the Grace-New Haven Community Hospital for their help and cooperation.