Participants, recruitment, and randomization
Considering alpha 0.05 and statistical power of 80%, based on the below formula and a similar study [Participants were enrolled in the study by continuous non-probability sampling method of eligible samples according to the chronological order of neonates’ admission to the wards. The patients were divided into two groups. Allocation of patients to the groups was done using block randomization. Block randomization was used in order to balance the number of samples allocated to each of the studied groups. The block size was 4. In order to homogeneity the distribution of the confounders of gender (boy/girl), type of delivery (normal vaginal/cesarean section), birth weight (< 2000 g/ ≥ 2000–2500 g), and the newborns’ type of nutrition (mother’s breastfeeding/breast milk by other routes) based on these variables stratified block randomization was created and then samples randomly were placed the groups in a balanced way. The random sequence was created by the epidemiologist consultant using the block randomization package in the R software. For allocation concealment, sealed envelopes were used, which were kept by the secretary of the departments who were not a member of the research team. In order to preserve the random sequence, the outer surface of the envelopes was numbered in order. The lids of the letter envelopes were glued and placed in a box respectively. At the time of sampling, when a new patient was eligible to take part in the study, the researcher asked the secretary of the departments which group to assign the new participant. Subsequently, they were divided into two groups, including a group that received mother KC (control) and another group with maternal grandmother KC (intervention). The sampling period lasted from 15–1-2021 to 30–6-2021.