Milk in Life Conditions (MiLC): Bacterial Composition of Human Milk Pumped and Stored in "Real-Life" Conditions

Participants will donate 1 oz of human milk during each of two consecutive pumping sessions, for a total volume of 2 oz donated on one day. To collect this human milk, participants will be asked to fully express one breast during each pumping session. Participants will pump once with the participant's own pump set-up and once with the sterile pump set-up (provided by the research team). Women will be randomized to which pump is used first.

Randomization will be done using stratified randomization as follows: participants were stratified by how their infants were fed, namely whether infants were fed human milk only vs. human milk and complementary foods). Then, randomization was conducted within each strata. The researchers aim to have a minimum of 25 participants in each stratum.

All human milk collections will occur at participants' homes between 0700 and 1100 hours. The second pumping session must begin 3 hours (+/-30 minutes) after the beginning of the first pumping session (e.g. the first pumping session at 7:30 am and the second at 10:30 am). Participants will elect from which breast to donate human milk and that breast will be used for both pumping sessions. Participants will be asked not to nurse from or pump that breast during the 2 hours before the first pumping session and not until after the second pumping session (a total of ~5.5 hours).

From the milk produced during each pumping session (which could be up to ~6 oz), researchers will collect 1 oz using a sterile, plastic syringe. Participants will keep the remaining volume of milk. Each ounce of milk collected will be separated into 5 sterile containers (provided). Participants will store donated milk at home until it is picked up by a researcher 2, 4, and 30 days after pumping.

Inclusion criteria

• Lactating women over the age of 18 years
• Self-reported as healthy women and infants
• Use an electric breast pump
• Confident of ability to donate 1 oz of milk from one breast during each of two consecutive pumping sessions where pumping sessions are 3 hours (+/- 30 minutes) apart and between 0700-1100 hours.
• Able to store donated milk at home for 30 days
• Have infants who do not consume formula or only consume formula episodically as long as the most recent formula-feeding occurred > 2 weeks before the day milk is pumped for this study.

Exclusion criteria

• Not confident of ability to donate 1 oz of milk from one breast during each of two consecutive pumping sessions, 3 hours (+/- 30 minutes) apart and between the hours of 0700-1100 hours.
• Infant consumption of formula in the past 2 weeks
• Current indication of breast infection (e.g., breast pain, discomfort, lumps, mastitis with fever, red streaks, or hard red portions of the breast)
• Breast pain that the woman does not consider "normal" for lactation/breastfeeding
• Signs/symptoms of acute illness in woman or infant in past 7 days including fever (rectal or temporal temperature ≥99.5 F), dark green nasal discharge, diarrhea (abrupt onset of 3 or more excessively "loose" stools in one day), vomiting (where infant vomiting is not associated with feeding), or severe cough.