Final answer:
The information should be recorded under the heading of 'Past history' in the nurse's interview notes.
Step-by-step explanation:
The information provided by the mother about her difficult delivery and preterm birth should be recorded under the heading of Past history in the nurse's interview notes. The past history section typically includes information about the patient's medical background, including previous illnesses, surgeries, and relevant obstetric history. In this case, the difficult delivery and preterm birth are important details that should be documented for the infant's medical records.