A survey of X chromatin in the newborn.

Buccal smears were analyzed for X chromatin in 1,000 newborn infants. The mean percentage of X chromatin in the 506 female infants was 39.5 (range 19-61) and in the 494 males 0.13 (range 0-3.0). Evaluation of the data comparing the percentage of X chromatin with caucasian and noncaucasian infants, the weight of the newborn, and the gestation time, indicated no significant differences within the male or female populations. A longitudinal study on 25 female infants for the first 4 days postpartum indicated a significant increase in the percentage of X chromatin in females.