Sequential Analysis of Spontaneous Abortion. II. Collaborative Study**The Collaborative Study of Cerebral Palsy, Mental Retardation, and Other Neurological and Sensory Disorders of Infancy and Childhood is supported by the National Institute of Neurological and Communicative Disorders and Stroke (NINCDS). The following institutions participate: Boston Lying-In Hospital; Brown University; Charity Hospital, New Orleans; Children’s Hospital of Buffalo; Children’s Hospital of Philadelphia; Children’s Medical Center, Boston; Columbia University; Johns Hopkins University; Medical College of Virginia; New York Medical College; Pennsylvania Hospital; University of Minnesota; University of Oregon; University of Tennessee; and the Developmental Neurology Branch, NINCDS. Data Show that Gravidity Determines a Very Substantial Rise in Risk