This study aims to investigate the concentration of various growth factors and cytokines in blood, and to examine the gut microbiota of low birth weight infants fed with formulas with or without Human Milk Oligosaccharide (HMO) supplement. Eligible low birth weight infants are allocated to two groups, Investigational formula (with HMO) or Control formula (without HMO). The subjects are taking the assigned formula when they need to be supplemented with formula. After the informed consent was obtained and eligibility was confirmed, the intervention period begins, and ends at the one-month-old medical check-up with the assessment of the various blood growth factors and cytokines, and the gut microbiota.

1. Inclusion Criteria:

   1-1) For infants

   i. Low birth weight infants (defined as a birth weight ≥1,500 g and <2,500 g) during hospitalization and may require milk powder formula as supplement.

   1-2) For mothers

   i. Mothers who do not find serious viral infection

   ii. Mothers who are willing to provide consent for provision of study required information and specimen(s) from both mothers and infants

   iii. Mothers who are willing to provide consent as infant's legally acceptable representative (LAR)

2. Exclusion Criteria:

   2-1) For infants

   i. Infants who receive any formulas before Informed Consent.

   ii. Infants with serious infections.

   iii. Infants with necrotizing enteritis.

   iv. Infants with gastrointestinal perforation.

   v. Infants whom the PI determines inappropriate as a study subject (e.g. inflammatory, metabolic abnormalities and difficulty in enteral feeding).

   2-2) For mothers

   i. Mothers the PI determines inappropriate as a study subject

   ii. Mothers under 18 years old.

3. Withdrawal criteria:

A subject must be discontinued from treatment with test formula if any of the following apply:

i. If the mother, who is a subject of the study as well as infant's LAR , asks for withdrawal from the study or withdraws consent.

ii. When a serious adverse event occurs.

iii. When an adverse event occurs, and the PI or co-investigator determines that the study should be discontinued.

iv. If it is decided that there is a risk of compromising the safety of the study subject.

v. If a subject is found to be ineligible after the start of the study.

vi. If it is found that there is a serious or continued non-compliance with the study protocol by a study subject.

vii. In addition, if the PI decides to end the study.