Developmental changes in pain expression in premature, full-term, two- and four-month-old infants.

The purpose of this study was to examine the behavioural responses of infants to pain stimuli across different developmental ages. Eighty infants were included in this cross-sectional design. Four subsamples of 20 infants each included: (1) premature infants between 32 and 34 weeks gestational age undergoing heel-stick procedure; (2) full-term infants receiving intramuscular vitamin K injection; (3) 2-month-old infants receiving subcutaneous injection for immunisation against DPT; and (4) 4-month-old infants receiving subcutaneous injection for immunisation against DPT.