Postnatal body weight changes were assessed in 385 surviving infants with birth weights of less than 2,500 g. Body weight was measured daily between birth and 45 days of age. Infants were grouped according to 100-g birth weight categories, and mean body weight changes for each group were compared. Initial postnatal weight loss occurred in each group and ranged between 7.9% and 14.6% of birth weight. Mean postnatal weight loss was greater in the lowest birth weight groups, but considerable variability was observed among individual infants. Duration of postnatal weight loss was similar among all birth weight groups. Weight gain usually began between four and six days of age, and the rate of weight gain expressed as grams per kilogram per day was similar in all birth weight groups.
Inasmuch as spinal taps in preterm infants are frequently accompanied by clinical deterioration, the optimal position for this procedure was investigated. Three positions were each randomly assigned for five minutes to 17 healthy preterm infants without a spinal tap actually being performed: (1) lateral recumbent with full flexion (flexed position), (2) lateral recumbent with partial neck extension (extended position), and (3) sitting with head support and spine flexion (upright position). Transcutaneous Po2 and Pco2 were monitored in all infants, minute ventilation (V1) in seven, and heart rate and blood pressure in ten infants. Mean transcutaneous Po2 decreased in each of the three positions. This decrease was significantly greater in the flexed (28 ± 8 mm Hg) as compared with the extended (18 ± 8 mm Hg, P < .001) and upright (15 ± 11 mm Hg, P < .001) positions. Mean transcutaneous Pco2 increased only in the flexed position (3 ± 4 mm Hg, P < .005) and levels were still elevated five minutes after that position had been discontinued. The consistent decrease in transcutaneous Po2 was accompanied by a variable effect of positioning on V1 and there were no episodes of airway obstruction or apnea >10 seconds. Heart rate increased in each position whereas blood pressure remained unchanged. These data suggest that although hypoventilation may contribute to the observed decrease in transcutaneous Po2, ventilation/perfusion imbalance appears to be the major mechanism. As spinal taps performed in the widely accepted flexed position carry the greatest risk of potential morbidity, it is recommended that this position be modified with neck extension or that spinal taps be performed in the upright position.