Human milk (also known as breastmilk and hereafter referred to as ‘milk’) is the optimal source of nutrition for most infants.^1,2 Some infants do not feed directly at the breast for every feeding for various reasons, including medical conditions, separation from the lactating parent, or parental needs and preferences (e.g., to gauge milk supply, to avoid mental or emotional discomfort with feeding directly at the breast).^3,4 Lactating parents may choose to express milk to feed it to their infants, a common practice in the USA.^5,6 A 2005–2007 US study indicated that, among persons who breastfed their infants in the early postpartum period (N = 1564), 85% of them had expressed milk during the first year after birth.⁷ That sample was more likely to be White, more educated, employed, and have fewer children than a nationally representative group. However, more current milk expression practices are unknown among other populations, such as families with low income participating in the US Department of Agriculture’s (USDA) Special Supplemental Nutrition Program for Women, Infants, and Children (WIC).

WIC is a federal program that serves over 6 million people and provides supplemental foods, nutritional education, peer counselling, and health services referrals for eligible pregnant, postpartum, and breastfeeding persons, infants, and children aged < 5 years.^8,9 Individuals are eligible if they meet certain criteria, including having lower income (i.e., ≤ 100%–185% of federal poverty level) or participating in certain programs (e.g., Medicaid or Supplemental Nutrition Assistance Program) and being at nutritional risk.¹⁰ WIC supports breastfeeding families who practice milk expression by providing health education and breast pumps.¹¹ Understanding the characteristics, practices, and reasons of persons with low income who express milk in the first year postpartum can provide information for interventions to support families in optimal expressed milk feeding and storage practices. The present study aimed to examine the prevalence of, characteristics associated with, and reasons for milk expression in the first year postpartum among breastfeeding persons with term singletons who were enrolled in WIC.

We aimed to understand the prevalence of and characteristics associated with milk expression among a sample of WIC enrollees during the first year postpartum. We also examined the reasons for milk expression. Milk expression was common in this sample, with 70% expressing milk at some point in the first year among those who fed their infants milk in the first month postpartum. Our findings of milk expression in the first year postpartum were slightly lower (70% vs. 85%) than those from a 2005–2007 study of a non-nationally representative sample of persons who breastfed their infants in the early postpartum period (1.5–4.5 months).⁷ Among participants who fed their infants human milk at Month 1 and among those who did so at Month 3, over half at each time period expressed milk, and that proportion decreased throughout the first year. By Month 13, only 14% were expressing milk among those feeding human milk at that month.

We identified differences in sociodemographic and perinatal characteristics, such as breastfeeding history and working for pay while pregnant, among persons who did not express milk, who expressed milk ≤ 1 per day, or who expressed milk > 1 per day in the first month. As has been documented previously,⁷ persons with no breastfeeding history were more likely to express > 1 per day than those who previously breastfed. This finding may reflect a lack of comfort or confidence in the ability to breastfeed successfully among persons who are new to breastfeeding and the desire to have an emergency supply. Furthermore, persons who expressed milk at Month 1 reported working for pay while pregnant for more months than those who did not express milk. Evidence indicates that parents who worked while pregnant are likely to return to the workforce within the first year postpartum.^14,15 It is possible that these persons were expressing milk to allow others to feed their infants with expressed milk and to build an emergency milk supply in anticipation of returning to work.

Reflecting the dynamic relationship between parent and growing infant, reasons for milk expression changed over time throughout the first year postpartum in this study, similar to previous studies.^4,7 In our analysis, the most common reason varied throughout the first 7 months from ‘to increase milk supply’ and ‘to relieve engorgement’ in earlier months to ‘to get milk so that someone else can feed their infant’ and ‘to keep milk supply up when their infant could not nurse’ in later months. A previous study assessing reasons for milk expression at various time points in the first 9.5 months postpartum indicated ‘for someone else to feed infant’ as the most common reason at each time point.⁷ Our results are consistent with a previous study that identified decreasing percentages of persons reporting ‘to relieve engorgement’ and increasing percentages of persons reporting ‘to mix with cereal or other food’ over time during the first year.⁷

Clinicians, health educators, WIC staff, and others working with WIC families can be mindful of these milk expression practices and reasons as they design and implement interventions that promote optimal infant feeding practices. Extra attention and support may be particularly important in the first months postpartum when milk expression rates are high and infants are more vulnerable to illness. Recognition of sociodemographic and perinatal differences among groups by frequency of milk expression can help identify groups that may especially benefit from additional support. Education, equipment, and other support for persons who are expressing milk can be tailored for reasons of milk expression. Emphasising safe practices for milk expression, handling, storage, and feeding can help prevent illness of the caregiver and infant.^16,17 Supportive environments and cultures for milk expression, such as clean places and clear policies for milk expression and storage at the workplace, can also assist families. Additionally, people working with WIC families can provide information on responsive bottle feeding of expressed milk, which can affect infants’ nutrition, feeding habits, and healthy weight.^18,19 Of concern, some study participants reported expressing milk to mix with cereal or other food at Months 1 and 3, when the introduction of complementary foods (anything other than human milk or infant formula) is considered too early.²⁰ Education on appropriate introduction of complementary foods and the risk of choking when adding cereal or other foods to milk in a bottle can be reinforced to prevent adverse health outcomes.²

At Month 1, over 90% of persons who expressed milk primarily used an electric or manual breast pump. WIC provided breast pumps to about 17% of those persons, and over half bought, rented, or were gifted a breast pump. We collected our data around the time that the Affordable Care Act, which mandated that health insurance companies cover breast pumps and supplies at no cost to pregnant or postpartum members, went into effect.²¹ With potentially increased access to breast pumps, it is possible that more persons are currently using breast pumps than the present study estimates. Specific instruction can be given to persons using breast pumps to reinforce how to use, clean, and store breast pumps and associated parts properly.²²

The present study has several strengths and limitations. The longitudinal study design minimised recall bias with prospective data collection and allowed for unique analyses of milk expression practices in the first year postpartum. Cross-sectional analysis of these data allowed for larger sample sizes and less missing data for each survey month; however, because we calculated estimates from different samples for each survey month, we did not follow the same persons over time. These recent data provide an updated estimate on characteristics, practices, and reasons of milk expression in a sample representative of WIC enrollees who were eligible to enroll in WIC services for the first time during their current pregnancy or enroll an infant aged < 2.5 months at a large WIC site, were aged ≥ 16 years, and spoke English or Spanish. However, the results may not be generalisable to WIC enrollees at small WIC sites, such as those in rural areas. These data were collected before the COVID-19 pandemic, which affected maternity care and infant feeding practices in hospitals and access to lactation services in the community, and it is possible milk expression practices have changed since the data were collected.²³ We were unable to distinguish additional reasons for milk expression if the study participant reported another reason that was not provided by the interviewer. Data on expressed milk feeding, milk storage, and breast pump cleaning behaviors were unavailable; therefore, we were unable to analyse whether study participants were following recommended practices.

Milk expression was common among WIC enrollees in the first year postpartum. Among WIC enrollees who reported feeding human milk at Month 1, 70% expressed milk at some point in the first 13 months postpartum. Among participants who fed their infant human milk at Month 1 and among those who did so at Month 3, over half at each time period expressed milk, and that proportion decreased throughout the first year. Reasons for milk expression changed throughout the first year; increasing milk supply, relieving engorgement, and having an emergency supply of milk were common reasons in the first month, and obtaining milk so that someone else can feed their infant was common in later months. These results may guide clinicians, health educators, WIC staff, and others working with these families to support optimal expressed milk feeding and storage practices, such as responsive bottle feeding of expressed milk. Future research can explore associations between these milk expression practices and breastfeeding outcomes, such as duration and exclusivity.²⁴