According to a study published in the journal Current Biology, preterm newborns do not habituate to pain in the same manner that full-term infants, children, and adults do. The researchers claimed that medical operations during the first few weeks of life have an adverse impact on the development of a preterm infant if they have not yet acquired the mechanism that allows them to become accustomed to mild pain.

Lead author Dr. Lorenzo Fabrizi of UCL Neuroscience, Physiology, and Pharmacology stated that the ability to get used to repeated pain develops during the third trimester of pregnancy, and babies who are born prematurely are still developing this ability in comparison to full-term babies, who have this ability right from birth.

The study involved 20 infants at University College London Hospitals (UCLH). In the study, the researchers measured the infants` responses to a painful but clinically required heel lance, which was conducted twice for each infant. The researchers recorded the infants` brain activity with EEG, heart rates using ECG (electrocardiography), facial expressions, and reflexes in withdrawing the leg.

The findings of the study showed that the brain activity was not as strong after the second heel lance in comparison to the first, suggesting a habituation response. The researchers also found a similar pattern for heart rate and facial expressions, as preterm infants reacted just as strongly to both heel lances, while the full-term infants appeared to habituate to the pain.

The researchers explained that this habituation response was caused by the full-term infants anticipating the pain when they receive a second heel lance, making their reaction less severe. They also added that this can also be linked to their brains, which are adjusting their reflexive survival responses. 

(Source: https://theprint.in/health/preterm-babies-do-not-habituate-to-repeated-pain-research-2/1452550/ )