Strengths and limitations of the study
Breastfeeding is an emotional interaction between mother and child. One strength of our study is that no invasive or interfering measurement method was used to assess duct orifices immediately before breastfeeding. Moreover, the used Marmet technique is widely known by lactation consultants and is easy to use by mothers. Love and Barsky [
11] showed that one observer looking through a breast pump is able to discriminate up to 17 ductal orifices in one nipple. Video documentation and/or direct observation by two observers improves this measurement.
However, a manual technique may bias the results because it does not guarantee the full potential of milk ejection. Ultrasound imaging and time measurements between feedings could standardize milk levels and flow rate. Yet they do not address the emotional readiness for interaction of mother and child.
As in any interview, random errors can occur in their documentation. This concern was partially mitigated by handing out the documentation sheets to the mothers, who reviewed their answers immediately after the interview.
Studies on breast carcinoma have focused on hormone levels and ductal lobular units [
11,
12]. A literature review shows that in vitro studies with surgically obtained specimens identified more ducts and suggested more orifices than in vivo observations [
13]. Analyzing histologic sections of the base of the papilla mammaria, Going and Moffat identified 27 ducts. Of these 27 ducts only 7 exhibited a patent lumen on the surface and, therefore, were classified as lactiferous ducts [
13]. Love and Barsky [
11] as well as Taneri et al. [
7] pointed out that discrepancies of the number of identified intramammary ducts and surface orifices may be due to morphological similarities between these ducts and sebaceous and sweat ducts. This may result in mistaking sebaceous or sweat ducts for milk ducts. They also pointed out the possibility for ducts to anastomose [
7,
11]. Ramsay et al. [
5] identified a mean number of 9 milk ducts (range 4–18) in each lactating breast using ultrasound imaging. They described the course of the ducts under the areola and inside the nipple as being diverse and complicated rather than systematically arranged, with anastomosis ducts coursing beneath and across one another. They, however, did not state how many of the imaged ducts ran to the nipple surface and exhibited an orifice [
5]. There is a consensus in the literature to the effect that the number of milk ducts inside the lactating breast identified by ultrasound imaging or histologic sectioning is higher than the number of orifices on the nipple surfaces [
7,
13‐
16].
There is hardly any published in vivo study that addresses the number of milk duct orifices under functional aspects and analyzes their association with variables in the infant and the mother. The only comparable investigation available is by Love and Barsky [
11], who found 5 orifices on average in every nipple. The small difference between their and our results may be explained by the different methods used. Love and Barsky used breast pumps instead of the Marmet technique. Due to the compressibility of milk ducts [
17] manual methods involve a risk of blocking ducts, the consequence being that milk is not expressed from all ducts holding milk.
When comparing this to other, previously mentioned methods, this difference of results, however, is fairly small. A potential explanation for this may be the similarity of the approach, privileging a functional method of milk duct activation. We also found that the use of a breast pump in addition to breastfeeding did not influence the number of ductal orifices.
The question may be raised as to whether the number of ducts carrying milk during lactation affects breastfeeding parameters in the mother and the child and vice versa. In vivo studies about duct orifices are not available in the literature for comparison, but similarities could be found in studies that addressed the productivity of the breast [
18,
19]. In our study we used no methods to measure the productivity of the breast. We examined immediately before breastfeeding, when mothers felt they had enough milk to feed her babies. Therefore, causal relationships between the number of duct orifices and milk productivity could not be advanced, but this needs discussion.
In this study primipara were significantly younger than multipara. We found that multipara had more ductal orifices than primipara. Looking at the women’s age did not reveal a correlation with the number of ductal orifices. Kent et al. [
19] did not find an association of a mother’s age or parity with her produced milk volume. However, our results support the assumption that parity, rather than age, affects the number of ductal orifices.
Yet another outcome of this study was that mothers with male infants exhibited more orifices than mothers with female infants, but only in right nipple. At a mean 4.2 ductal orifices in the right nipple, mothers of boys exhibited statistically significantly more orifices than mothers of girls, at a mean 3.5. Although the difference in number of orifices between left and right nipples has not been explained in the literature, in 2007 Kent [
18] described that in 70% of reviewed cases the right breast was more productive than the left one. This fact did not correspond to left- or right-handedness of the mother, and mothers did not tend to offer the more productive breast to the infant either [
18]. Mitoulas et al. [
20,
21], when considering infants’ 24-hour milk intake, found that right breasts produced significantly more milk than left breasts. At this point, the question may be raised as to whether the number of activated ductal orifices that bring milk to the surface of the nipple is associated with the milk volume the breast produces. We found that birth height and birth weight did neither activate more lactiferous ducts nor resulted in more ductal orifices expressing on the nipple surface. On the contrary male’s birth height is correlated negatively with duct orifice number in the left nipple.
A further explanation why male infants may induce more orifices is that male infants were fed significantly longer per meal than female infants. All infants in this study were breastfed 8.0 times on average in 24 hours. Breastfeeding sessions with boys, at an average duration of 25.9 minutes, lasted longer than those with girls, at 19.6 minutes. Kent et al. [
19], who analyzed a study population of mothers and infants comparable to ours, found a frequency of 7.9 times in 24 hours. In their study they concluded that during breastfeeding sessions boys had a greater maximum milk intake than girls. This was associated with a higher milk production in general in mothers of boys compared to that in mothers of girls. They did not find a correlation between produced milk volume and frequency of breastfeeding in 24 hours [
19].
It has to be discussed if the criteria “male” and “right breast” have the power to explain the higher number of ductal orifices. We found that lower birth height in boys correlated with more orifices in left nipples, which would oppose the assumption of a body side dependency. Looking at the male children, we found that primipara had less ductal orifices in their nipples than multipara, even though the number of male infants in primipara was higher than in multipara (Figure
4).
Women’s fluid intake in 24 hours correlated with more ductal orifices. This could be interpreted as conflicting with Dusdieker et al. who rejected a significant linear correlation between mothers’ fluid intake and produced milk volume [
22].
The number of lactiferous ducts that expressed as milk pores on the surface of the nipple during our observation did not differ between mothers whose infants were restless and mothers whose infants were calm during breastfeeding sessions. Therefore, we conclude that their number does not affect the child’s behavior.
McClellan et al. [
23] found that the majority of mothers feeling pain during nursing did not produce less milk than mothers without problems. We determined that breast sensitivity and pain was not linked to a lower number of milk ducts bringing milk into the infant’s mouth during breastfeeding. Pain did not result in longer or more frequent breastfeeding sessions either.
As previously mentioned, it is known that psychological aspects can affect successful breastfeeding and influence milk ejection [
10,
24]. Therefore, we included two psychological aspects in order to investigate if mothers’ personal breastfeeding intentions and attitudes affected the number of orifices. It is obvious that a longer maternity leave enables the mother to breastfeed her infant longer. Relying on that insight we expected that mothers planning to breastfeed their child for longer and being on a longer maternity leave would have more peace of mind, be more patient, feel less under pressure and stress when it comes to nursing, than mothers who deliberately step away from that breastfeeding intention. However, psychosocial aspects, e.g. duration of maternity leave or the intended total breastfeeding period, did not have any effect.
In summary, our results support the concept that the number of ductal orifices is a function of lactation. In this respect, Gooding et al. [
25] as well as Going and Moffat [
13] pointed out that regardless of the infant’s demand not all lobular structures and lactiferous ducts are activated during lactation because the overall milk-producing capacity of the breast exceeds the amount an infant needs.