I. The plan is described of a prospective study of the outcome of 5,964 pregnancies, with special emphasis on the relationship of factors in fetal environment to the incidence of abortions, stillbirths, neonatal mortality and congenital malformations. II. Data are presented on the overall incidence of congenital malformations according to sex, race, weight at delivery, maternal age and order of birth. A. The incidence of congenital malformations among 5,739 products of conception weighing over 500 grams was 7.5%. The rate was 7.0% among infants born alive and surviving the neonatal period, 13.6% among antepartum deaths, 23.3% among intrapartum deaths, 29.6% among neonatal deaths and 70.6% among deaths occurring between the ages of one and 12 months. B. The following relationships in incidence rates among live-born infants surviving the neonatal period were found. 1. The rate among males (8.4%) was half again as high as among females (5.5%). 2. Non-white infants had a higher rate (7.8%) than white infants (6.3%). 3. Infants weighing 2,500 grams or less had a higher proportion with defects (9.7%) than did those weighing over 2,500 grams (6.7%), although this difference occurred entirely among females. 4. Maternal age had no effect on the incidence of congenital malformations. 5. White infants of lower birth order had a lower rate (5.3%) than did those of higher birth orders (8.2%). No difference according to birth order was observed among non-white infants. III. A system of classification of malformations is described and discussed. Incidence rates by organ system groups are presented. A. Less than one-half of the malformations found among live-born infants were suspected or noted at birth. B. Malformations of musculo-skeletal system and skin were more frequent than malformations of other systems. Incidence rates by organ system groups are directly related to problems of diagnosis. C. Of the malformed live-born infants, 14.8% had more than one malformation and in 9.1% more than one system was involved. Of the malformed stillbirths and neonatal deaths, 66.0% had more than one malformation and in 51.1% more than one system was involved.

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