Association of Milk Secretion Volume with Nipple Pain: A Prospective Hypothesis Verification Study

Abstract

Background:

Many mothers stop breastfeeding due to nipple pain, which is experienced when the baby sucks the mother's nipples during breastfeeding.

Objective:

To verify how nipple pain during breastfeeding is associated with nipple perfusion and volume of milk secretion.

Study Design:

Prospective hypothesis verification study.

Methods:

Fifty-nine lactating women who delivered at term were enrolled. The CS Probe S and laser Doppler ALF21 were used to measure perfusion, and PowerLab 2/26 and LabChart 8 were used to analyze perfusion. The difference in the infant's weight (g) before and after breastfeeding was used to measure milk secretion.

Results:

The participants' mean age (± standard deviation) was 30.88 ± 3.77 years. Although 80–90% of participants had proper breastfeeding posture and infant attachment, 34 (57.6%) and 30 (42.4%) mothers experienced nipple pain on postpartum days 1 and 4, respectively. For postpartum days 1 and 4, the group with nipple pain exhibited significantly less nipple perfusion than the group without pain. Milk secretion was lower in the group with nipple pain on postpartum day 4 than in the group without pain, although this was not statistically significant. The reduction in nipple perfusion did not differ significantly between the high (75%) and low secretion groups (25%) and between the nipple trauma and nontrauma groups. However, the low secretion group and nipple trauma group had larger reductions in nipple perfusion.

Conclusion:

Assessing the amount of milk secretion can provide a new perspective on preventive care for nipple pain and trauma.

Introduction

Nipple pain is generally caused by the baby sucking the mother's nipples during breastfeeding and is reported to be experienced by 96% of mothers by the first postpartum week,¹ and 22% of mothers who complain of nipple pain persisting for up to 8 weeks postpartum exhibit symptoms of nipple vasospasm.² The United Nations Children's Fund (UNICEF) and World Health Organization (WHO) recommend exclusive breastfeeding until an infant is 6 months old. However, many mothers stop breastfeeding due to nipple pain.³ Although it has multiple causes, many nipple care methods typically performed by midwives have not been effective for mothers suffering from this. Nipple pain has been associated with decreased breastfeeding duration, introduction of artificial infant milk, increased levels of stress, and inhibitory effects on the release of oxytocin.³ Therefore, nipple pain in mothers during breastfeeding is a problem that needs to be carefully addressed by the staff involved in breastfeeding.

Although there have been many observational studies on nipple care in the field of midwifery, there are no physiological studies on the association between milk secretion changes over time and the development of nipple pain and nipple perfusion. This study aimed to verify how nipple pain during breastfeeding is associated with nipple perfusion and milk secretion volume in the early postpartum period. We hypothesized that (1) milk secretion volume and nipple perfusion would be related and (2) with low milk secretion, the incidence of reduction in nipple perfusion would be increased, thus causing nipple pain and trauma.

Materials and Methods

Ethical considerations

This study was approved by the Ethics Committee of Shiga University of Medical Science (approval no.: R2019-111). We confirmed that the participants were willing to breastfeed after childbirth. It was explained to the participants that their safety was ensured, as we had confirmed with the medical device and equipment manufacturer (Advance) that the laser emitted by the CS probe used for measuring nipple perfusion would not directly come into contact with the infant and that the probe and cord were integrated such that there would be no aspiration from the infant's sucking pressure. We also explained that the investigator would always be present during the measurements to carefully observe and respond to any discomfort or feeling of constraint from having the CS probe attached to the nipple. The outline, objectives, methods, and ethical considerations of the study were explained, and written informed consent was obtained from the participants.

Study design and population

This prospective hypothesis verification study was conducted from June 2019 to July 2020 at a private obstetrics and gynecology clinic in the Shiga Prefecture, Japan. The study was explained to pregnant women in their third trimester (26 weeks or later) who were scheduled for vaginal delivery or Cesarean section and wanted to breastfeed during the postpartum period. There were 59 primiparous (n = 20) and multiparous (n = 39) women who had full-term deliveries from 37 weeks 0 days to 41 weeks 6 days of gestation. Women with retracted or flat nipples and infants incapable of direct attachment to the nipple and those using a breast pump were excluded. The newborns were full-term born infants. Preterm infants, low-birth-weight infants, infants with postnatal respiratory disorders or hypoglycemia, and infants with external deformities related to sucking functions, such as cleft lip and palate, and ankyloglossia, were also excluded.

Survey methods

The following information on the participants' baseline attributes was obtained from their hospital records: age, height, weight, body mass index, medical history, complications, delivery date and time, weeks of pregnancy and gestational age, the time required for delivery, delivery method, total bleeding volume during delivery, nipple and breast type, infant birth weight, APGAR score, and presence or absence of direct nursing immediately after delivery.

All participants received the same breastfeeding guidance, with the same investigator explaining proper breastfeeding posture and nipple attachment methods before initiating breastfeeding. During breastfeeding, the posture and infant's attachment were observed and recorded using a breastfeeding observation form that was created based on the “Direct Breastfeeding Observation Form” from UNICEF/WHO's “Ten Steps to Successful Breastfeeding”.⁴

Measurement of nipple perfusion

CS Probe S (Advanced Manufacturing Corp, Tokyo, Japan) and laser Doppler ALF21 (Advanced Manufacturing Corp) were used to measure perfusion, while PowerLab 2/26 (Bioresearch Center Corp, Nagoya, Japan) and LabChart 8 (Japanese version; Bioresearch Center Corp) were used to analyze perfusion. The measurements were taken during breastfeeding on postpartum days 1 and 4. The maximum and minimum perfusion at the highest point and the lowest point, respectively, were interpreted by an investigator and two research instructors from the mountain-shaped perfusion waveform at the start of sucking to a pause (1 burst). The reduction in perfusion from attachment to the nipple during breastfeeding was calculated as follows: (maximum perfusion-minimum perfusion = reduction in perfusion).

Measurement methods

The CS probe was fixed near the nipple with a double-sided tape where the infant's upper jaw would be. The cord of the CS probe ran over the mother's shoulder and was fixed in place with tape, such that it did not interfere with infant attachment. Once the stability of nipple perfusion was confirmed from the waveform before the infant began sucking, the start button on the rheometer was pressed, and the infant began to suck. Perfusion waveforms and the infant's nursing status were measured and observed for approximately 5 minutes. The presence of nipple pain was determined subjectively by the participants, who were asked to raise their hands if they were aware of pain. No infant refused to use the props during breastfeeding.

Measurement of milk secretion

Milk secretion was measured as the difference in the infant's weight (g) before and after breastfeeding.

Statistical analyses

A previous study that investigated the relationship between nipple injury and breastfeeding during postpartum hospitalization using observation tables reported a statistically significant difference at n = 60. The required sample size was set at n = 60 based on this study. Basic statistics were calculated, and normality was confirmed using the Shapiro–Wilk test, after which the Mann–Whitney U or unpaired t-test was performed. SPSS Statistics version 27 for Windows (IBM Corp, NY) was used to perform all statistical analyses, and p < 0.05 was considered significant.

Results

A total of 63 postpartum mothers provided consent to participate in the study, of which 59 were enrolled after excluding those who could not undergo perfusion measurement due to their postpartum physical condition or conditions pertaining to their newborns. This study included 20 primiparous and 39 multiparous women, with a mean age (±standard deviation [SD]) of 30.88 ± 3.77 years. All 59 women had vaginal deliveries. The mean infant birth weight was 3201.1 ± 279.9 g.

The nipple perfusion waveforms were evaluated based on assessments by multiple perfusion researchers and an investigator who was present during breastfeeding (Fig. 1). The perfusion waveform of sucking showed a mountain shape with points indicating increasing or decreasing perfusion. Increased perfusion was observed in the sucking waveform when the infant was attached to the nipple and areola, whereas decreased perfusion was observed when the infant swallowed. The descending waveform of the mountain shape indicated perfusion during the sucking motion. When nipple pain was present, the baseline of the point when perfusion decreases gradually declined with each sucking waveform. When there was no nipple pain, the waveform baseline remained stable. The results of these waveforms have been confirmed by experts who developed Advance's perfusion meters, and the reliability of the results has been assured.⁵

FIG. 1.

Assessment of nipple perfusion waveforms (A), waveform with nipple pain (B), waveform without nipple pain (C), and measurement of nipple perfusion (D).

Nipple perfusion (mL/min/100 g) and the number of sucks

After confirming a stable baseline waveform in the rheometer, perfusion volume was measured for 5 minutes while the infant sucked (Table 1). On postpartum days 1 and 4, the maximum nipple perfusion during 5 minutes of breastfeeding was 56.1 (38.9–71.02) and 50.81 (39.03–72.8) mL/min/100 g, minimum perfusion was 11.5 (8.65–14.83) and 11.42 (8.4–15.31) mL/min/100 g, reduction in perfusion was 42.6 (29.34–56.95) and 40.4 (28.84–57.8) mL/min/100 g, and the number of sucks was 15.67 (11.31–21.5) and 14.7 (9.63–21.0), respectively. These data show that newborn sucking behavior reduced perfusion in the breast and nipple of the 59 participants on postpartum days 1 and 4.

Table 1.

Reduction in Nipple Perfusion (mL/min/100 g) and Number of Sucks on Postpartum Days 1 and 4

	Median	25th percentile	75th percentile
Postpartum day 1
Maximum perfusion (mL/min/100 g)	56.10	38.90	71.03
Minimum perfusion (mL/min/100 g)	11.50	8.65	14.80
Reduction in perfusion (mL/min/100 g)	42.60	29.34	56.95
No. of sucks (times)	15.67	11.31	21.50
Postpartum day 4
Maximum perfusion (mL/min/100 g)	50.81	39.09	72.80
Minimum perfusion (mL/min/100 g)	11.42	8.40	15.31
Reduction in perfusion (mL/min/100 g)	40.40	28.84	57.80
No. of sucks (times)	14.70	9.63	21.00

Relationship between nipple pain and reduction in nipple perfusion

We examined the relationship between nipple pain and the reduction in nipple perfusion on postpartum days 1 and 4 (Fig. 2). On postpartum day 1, the reductions in nipple perfusion in the nipple pain and nonpain groups were 56.58 (44.44–68.49) mL/min/100 g and 30.1 (23.97–36.7) mL/min/100 g, respectively. On postpartum day 4, the reductions in nipple perfusion in the nipple pain and nonpain groups were 52.05 (44.34–65.95) mL/min/100 g and 29.23 (24.63–34.74) mL/min/100 g, respectively. On postpartum days 1 and 4, the reduction in nipple perfusion was significantly higher in the nipple pain group than in the nonpain group (p < 0.05). The nipple pain group experienced more severe reductions in nipple perfusion during breastfeeding than the nonpain group (Mann–Whitney U test [p < 0.001]).

FIG. 2.

Relationship between nipple pain and reduction in nipple perfusion (N = 42) on (A) day 1 postpartum and (B) day 4 postpartum.

Relationship between nipple pain and milk secretion

On postpartum day 1, milk secretion did not differ significantly depending on the presence of nipple pain (Mann–Whitney U test [p = 0.625]). Although there was no significant difference on postpartum day 4, the mean milk secretions (±SD) in the nonpain and nipple pain groups were 32 ± 16.65 g and 24.42 ± 15.87 g, respectively. This implied that milk secretion was higher in the nonpain group (unpaired t-test [p = 0.141]) (Fig. 3).

FIG. 3.

Relationship between nipple pain and milk secretion (N = 42). N.S., not significant.

Relationship between milk secretion and reduction in nipple perfusion

On postpartum day 4, the reduction in nipple perfusion was not significantly different between the group with high milk secretion (75%) and low secretion (25%) (Mann–Whitney U test [p = 0.163]) (Fig. 4). However, the reductions in nipple perfusion in the high and low secretion groups were 31.45 (26.70–65.42) and 44.61 (32.12–57.93), respectively, which show that the decrease in nipple perfusion tended to be larger in the group with low milk secretion.

FIG. 4.

Relationship between milk secretion and reduction in nipple perfusion (N = 33). N.S., not significant.

Relationship between nipple trauma and reduction in nipple perfusion

Nipple trauma can occur due to several causes (e.g., infant latching and breast pump use, among others). Women with erythema, eschar, fissuring, and blistering of the nipples were assigned to the nipple trauma group.

On postpartum day 4, the reduction in nipple perfusion did not differ significantly between the nipple trauma and nontrauma groups (Mann–Whitney U test [p = 0.294]) (Fig. 5). However, the reductions in nipple perfusion in the trauma and nontrauma groups were 47.82 (29.38–59.34) mL/min/100 g and 35.16 (27.5–55.1) mL/min/100 g, respectively, showing that the decrease in perfusion tended to be larger in the trauma group.

FIG. 5.

Relationship between nipple trauma and reduction in nipple perfusion (N = 59).

Timing of nipple pain onset

Nipple pain was recognized on the first suck 85 times (53.4%), second suck 24 times (15.1%), third suck 12 times (7.5%), and fourth suck or later 38 times (23.9%). Mothers were strongly aware of nipple pain when the infant began to suck.

Discussion

Awareness of nipple pain in the early postpartum period

Reportedly, 96% of mothers experience nipple pain during breastfeeding in the first postpartum week.¹ The most commonly attributed cause of nipple pain is incorrect positioning and attachment.⁶ The current study showed that 57.6% and 50.8% of mothers experienced nipple pain during breastfeeding on postpartum days 1 and 4, respectively. These proportions are lower than those previously reported. Among the current study's participants, 97% and 98% exhibited appropriate breastfeeding posture, while >80% and >98% of the infants were attached properly on postpartum days 1 and 4, respectively. However, even with proper breastfeeding posture and infant attachment, we found that most mothers still experienced nipple pain.

Nipple pain from sucking and nipple perfusion

Nipple vasospasm has been described in case studies of breastfeeding women as a reduced flow of blood through the capillaries caused by constriction in the peripheral circulation.^2,7 Mothers who complained of nipple pain during breastfeeding had more severe levels of decreased perfusion. The nipple perfusion measurements showed that perfusion reduced by 42.6 (29.34–56.95) mL/min/100 g on postpartum day 1 and by 40.4 (28.84–57.8) mL/min/100 g on postpartum day 4. Reduced nipple perfusion has been shown to be associated with nipple pain. The compression and consequent pressure changes on dermal tissues can compromise perfusion. When this inhibits the supply of necessary nutrients and oxygen to the cells that make up dermal tissue, cellular necrosis can occur.⁸ Therefore, a reduction in nipple perfusion from attachment by the infant may inhibit the supply of nutrients and oxygen to tissues, leading to fragility that causes nipple pain trauma and pain.

Low milk secretion and decreased perfusion

During breastfeeding, the act of sucking the nipple into the infant's mouth stretches it from the hard palate to the soft palate. When the baby latches to the breast deeply, the nipple is not compressed between the tongue and hard palate. It is recognized that a intraoral vacuum is integral to successful breastfeeding, with a baseline vacuum of −64 ± 45 mmHg required to sustain attachment and a peak vacuum of −145 ± 58 mmHg applied during milk removal.⁹

Stimulation of nipple sucking is necessary for milk secretion, and prolactin levels have been shown to increase 1 to 4 minutes after sucking begins.¹⁰ In this study, mothers most frequently began to experience nipple pain when the infant began to breastfeed. The suction pressure on the nipple from the infant's sucking is stronger at the commencement of sucking and when there is no milk ejection; this is thought to decrease nipple perfusion and cause nipple pain. It is important to note that prolonged breastfeeding immediately after delivery when milk production is low can cause nipple pain and nipple damage.

Our hypotheses 1 and 2 can be rejected based on our study findings. However, during periods of low milk secretion, there is a greater reduction in nipple perfusion, which may cause nipple pain. It has been reported that less milk passing through the teat causes more negative pressure on the teat.¹¹ Further research is needed on the relationship between milk production, nipple perfusion, and nipple pain.

In this study, many mothers complained of nipple pain even when proper positioning and attachment were observed. It is possible that even though infants may appear to be properly latched to the breast on the outside, the baby may exert sucking pressures directly on the nipple in the oral cavity. The staff involved in breastfeeding need to be aware that when they provide nursing assistance, mothers can experience nipple pain at the beginning of sucking.

Numerous clinical interventions are available for nipple pain, including education on positioning and attachment, application of breast milk to the nipple, and pharmacological therapy (e.g., vasospastic drugs such as nifedipine and analgesics such as ibuprofen).^3,12 Since some mothers are unwilling to take oral medication while breastfeeding, a pain management plan that considers their preferences needs to be formulated. The results of this study indicate that promoting the milk ejection reflex by gentle massage of the nipple before feeding may effectively relieve pain.

Study limitations

The limitations of this study include the small sample size, inability to generalize the findings to different populations, and possible selection bias. The influence of other factors (anxiety, depression, and morphological characteristics, such as nipple length and size)⁴ related to mothers' perception of nipple pain has not been verified.

Conclusions

Even with conventional methods such as proper breastfeeding posture and infant attachment, many mothers still complain of nipple pain. A novel finding of this study is that nipple pain may be associated with the volume of milk secretion. Assessing the amount of milk secretion can provide a new perspective on preventive care for nipple pain and trauma.

Footnotes

Acknowledgments

The authors thank the mothers who participated in this study during the early postpartum period. The authors also thank Editage for English language editing.

Authors' Contribution

All authors listed meet the qualifications for authorship. All authors confirm that the data collection and analyses were conducted fairly and appropriately. They have also read the article and agree with its contents.

Data Sharing Statement

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Disclosure Statement

No competing financial interests exist.

Funding Information

No funding was received for this article.

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