# Unequal this normal milk output between breasts.

Mothers of a preterm infant who is hospitalized in the neonatal intensive care unit (NICU) due to immaturity and perhaps other medical problems are separated from their baby after birth. These mothers must use an electric breast pump to initiate and maintain their milk supply because their baby is not able to suckle at breast. Often these mothers experience more milk coming out of one breast than the other when they express their milk via a breast pump. Some wonder, "Is this normal?"

This important question asked by pump-dependent mothers of a non-nursing preterm infant was the impetus to re-examine our data from our last study. We asked if there was a difference in milk output between breasts as measured as volume when pumping both breasts simultaneously over a period of six weeks after delivery.

The Study

In our study, mothers were all English speaking, 18 years or older, had access to a telephone, delivered a preterm infant < 32 weeks gestation that weighed [less than or equal to] 1500 grams (3.3 pounds), and planned to lactate a minimum of 12 weeks. None of the mothers smoked nor had an endocrine disorder such as diabetes mellitus or thyroid disease.

Mothers were provided a special electric breast pump and instructed to record daily in their logbook at each pumping session the number of minutes they pumped and the amount of milk expressed separately from the right and left breast into a special collection bottle. The recording of this information occurred the first 42 days after delivery.

Because mothers were recruited and entered the study at varying times during the first few days following birth, study day six after birth was chosen as the first day to report as milk output data were available for all 95 subjects. For the 37 study days (days six through 42 after birth) there were a total of 3488 days, and a total of 570 weeks for the 95 mothers who completed the study.

Right vs. Left Breast Milk Output

Out of the 3488 total study days, the left breast was dominant 1800 (51.6%) days; the right breast was dominant 1598 (45.8%) days. For 90 of the 3488 (2.6%) study days there was no breast dominance. When the right breast was dominant, the mean daily output from that breast was 308.8 ml/d, compared to the left breast output of 249.9 ml/d. When the left breast was dominant, the mean daily output from that breast was 293.7 ml/d, compared to the right breast output of 241.5 ml/d. Expressed milk output was different between breasts regardless if the right or left breast was dominant.

Similarly, for the 570 study weeks, the left breast had a greater milk output 53.3% (n = 304 weeks) of the time; the right breast had a greater output 46.7% (n = 266 weeks). In total, over the course of the 37 study days, milk expressed from the left breast accounted for 52.6% of the total milk volume, while 47.4% of the total milk volume was expressed from the right breast.

Conclusions

For mothers who are pump-dependent and delivered a preterm infant, milk output differences between breasts are a common occurrence. From our data, breast dominance--or the breast that had a greater output--was consistent over time. For the most part, mothers had either a dominant right or left breast throughout the entire six-week study period. This information can be used to alleviate any concerns that mothers may have when they see the amount of milk that is expressed into a bottle providing there are no breast pathologies.

Pamela D. Hill, PhD, RN, CBE, FAAN, is professor in Maternal Child Nursing, College of Nursing, at the University of Illinois at Chicago. For the past 22 years Dr. Hill has focused her program of research on mothers who choose to provide mother's milk to their infant. Her latest work involves pump-dependent mothers who deliver a very preterm infant. Her work suggests that this population is at high risk for producing an inadequate milk supply.
```Breastfed Babies

Percentage of U.S. babies born in 2004 who have
ever been breastfed:

Washington    88.4%    Vermont       85.2%
Oregon        88.3%    Arkansas      59.2%
Montana       87.7%    Mississippi   50.2%