The goal of this clinical trial is to evaluate the effect of breastfeeding on the infant's test weight and physiological characteristics (oxygen saturation and heart rate) in preterm infants in the Neonatal Intensive Care Unit during the transition to oral feeding.

The main question it aims to answer are:

• Is there a difference in test weight and physiological parameters between the infants in whom the first oral feeding was performed by the mother and the infants in whom the first oral feeding was performed by the intensive care unit nurse with a bottle? Researchers will compare the breastfed group with the bottle-fed group to see if there are differences in test weight and physiological parameters.

In the pre-feeding stage of the research, the parents of the preterms included in the sample will be met and informed about the research. Written and verbal consent will be obtained from parents who agreed to participate in the study. "Preterm Infants Information Form" will be filled by the researcher before feeding and "Feeding Monitoring Form" will be filled during feeding. By randomization, it will be determined which of the experimental and control groups the preterm infant will be assigned to. It will be ensured that the interventions that will affect the vital signs of the preterms in both groups before feeding are not applied. Oxygen saturation and heart rate will be monitored for 30 minutes before, during and after feeding for all groups. Preterms in both groups will be dressed in clean and dry diapers by cleaning their bottoms before feeding, and will be weighed naked with a digital baby scale by removing their clothes. Weighing the preterms with an electronic precision balance before and after feeding - the test weighing - will be used to measure the amount of milk the preterm is expressing from the breast or bottle. The preterm will then be fed loosely wrapped in a cotton blanket. If the oxygen saturation level of all preterm infants falls below 90% during feeding, it will be considered desaturated and the feeding will be stopped. The preterm infants will continue to be fed when the oxygen saturation is ≥ 90% and the heart rate is 120-160/min. Feeding will be limited to a total of 30 minutes for infants in both groups, including rest intervals, and will not take longer. When the feeding is complete, preterms in both groups will be weighed naked without changing the diaper under the feeding process. It will be ensured that interventions that may affect the vital signs of all preterms included in the study are not applied when feeding is completed. The oxygen saturation and heart rate of the preterms in both groups will be monitored for 30 minutes after the feeding is completed. When feeding is complete, preterms in both groups will be placed on the same mother's breast in the right lateral position to facilitate gastric emptying.

Inclusion Criteria:

• Gestation week at birth determined according to the mother's last menstrual date is 26-31+6 weeks,
• Postmenstrual week at the time of the study was 32-36+6 weeks,
• Having a body weight of 1500 grams or more at the time of inclusion in the study,
• Suggested by the physician to switch to oral nutrition and switched from enteral nutrition to oral nutrition for the first time,
• Breastfed,
• Preterm infants whose parents' consent was obtained for inclusion in the study and whose parents signed the informed consent form

Exclusion Criteria:

• Receiving oxygen,
• Craniofacial anomalies such as cleft palate, cleft lip, facial muscle paralysis,
• Preterm infants with any gastrointestinal, neurological and genetic disease (necrotizing enterocolitis, third and fourth level intracranial hemorrhage, periventricular leukomalacia, hydrocephalus, down syndrome, omphalocele, non-gastrodeia, short bowel syndrome and other diseases)