PREGNANCY AND BREAST

During the first half of pregnancy, extension and branch-
ing of the ductal system occurs, along with intensified
lobular–alveolar growth (mammogenesis). Growth of the
mammary gland is influenced by a number of hormones,
including oestrogen, progesterone, prolactin, growth hor-
mone, epidermal growth factor, fibroblast growth factor,
insulin-like growth factor,
28,29
and parathyroid hormone–
related protein.
30
Growth of the glandular tissue is be-
lieved to occur by invasion of the adipose tissue.
4
By
mid-pregnancy, there is some secretory development,
with colostrum present in the alveoli and milk ducts. In
the last trimester, there is a further increase in lobular
size.
While these changes typically lead to a marked in-
crease in breast size during pregnancy, the proportion of
growth varies greatly between women, ranging from
little or no increase to a considerable increase in sizE While the major increase in breast size is usually com-
pleted by week 22 of pregnancy, significant breast
growth occurs during the last trimester of pregnancy in
some women, and some women undergo significant
breast growth postpartum. At the end of pregnancy, the
volume of breast tissue had increased by 145
19 ml
(mean
standard error of the mean; n
13; range,
12–227 ml), with a further increase to 211
16 ml (n
12; range, 129–320 ml) by 1 month of lactation. The rate
of growth of the mother’s breast during pregnancy is
correlated with the increase in the concentration of
human placental lactogen in the mother’s blood, which
suggests that this hormone stimulates breast growth in
women.
31
During pregnancy, mammary blood flow approxi-
mately doubles in volume. This increased blood flow is
concomitant with both the increased metabolic activity
and temperature of the breast. This elevation in blood
flow persists during lactation and appears to decline to
prepregnancy levels about 2 weeks after weaning.
32
GROSS ANATOMY OF THE LACTATING BREAST
The breast reaches its full functional capacity at lactation,
and as a result, several internal and external changes
occur. During pregnancy, the areola darkens in colour,
and the Montgomery glands, which are a combination of
sebaceous glands and mammary milk glands, increase in
size. The secretions of these glands, which number
between 1 and 15,
33
are thought to provide maternal
protection from both the mechanical stress of sucking
and pathogenic invasion. In addition, it is also suspected
the secretion may act as a means of communication with
the infant via odor. In this connection, a recent study
demonstrated that increased numbers of Montgomery
glands is associated with increased infant weight gain in
the first 3 days after birth, infant breastfeeding behaviour
(increased latching speed and sucking activity), and
decreased time to onset of lactation in primiparous
mothers,
33
suggesting that there is indeed a functional
role of the Montgomery glands during lactation.
The standard descriptions of the human breast are
based on Cooper’s
16
magnificent cadaver dissections of
the breasts of women who were lactating at the time of
death. Although imaging modalities have become more
sophisticated, research has focused extensively on abnor-
malities of the non-lactating breast. Mammography of
the lactating breast is limited because of the increase in
glandular tissue and the secretion of breast milk, which
causes an increase in radiodensity, making images of the
breast difficult to interpret.
34
Galactography (the injection of radio-opaque contrast
media into the duct orifice at the nipple and subsequent
radiography) has illustrated only a portion of the ductal
system, and few studies have examined lactating women.
Of those that have looked at lactating breasts, some have described the milk ducts as being significantly larger
compared to those of the non-lactating breast. In contrast,
Cardenosa and Eklund
35
have reported that the ducts do
not enlarge during lactation.
To date, both computed tomography and magnetic
resonance imaging have had little to offer in elucidating
mammary anatomy. However, in two recent studies that
used magnetic resonance imaging to image the breast,
one study was able to identify some central ducts in the
breasts of lactating women,
36
and another attempted to
quantify fatty and glandular tissue volumes in the breasts
of non-lactating women.
37
These findings suggest that
this modality may offer some new insights into the
anatomy of the breast in future.
Our laboratory has recently reinvestigated the anatomy
of the lactating breast using high-resolution ultrasound.
38
Ultrasound is non-invasive and allows the structures of
the breast to be examined without distortion. Compared
with the quoted 15 to 25 ducts of conventional texts,
4,5
fewer ducts were imaged with ultrasound (mean, 9;
range, 4–8), which concurs with both Love and Bar-
sky’s
21
observations of lactating women expressing milk
with a breast pump (mean, 5; range, 1–17) and Going and
Moffatt’s
39
dissection of a nipple (4 patent ducts) from a
woman who was lactating. Interestingly, these are in
agreement with Cooper,
16
who found 7 to 12 patent ducts
in cadaver dissections of breast from a woman who was
lactating before death, although he could cannulate up to
22 ducts.
Ultrasound imaging has also elucidated other charac-
teristics of the milk ducts, in that they are small (mean, 2
mm), superficial, and easily compressed. In addition,
they do not display the typical sac like appearance of the
“lactiferous sinus” originally thought to exist (Figure 3).
Instead, branches drain glandular tissue located directly
beneath the nipple and often merge into the main
collecting duct very close to the nipple
38
(Figure 3).
Furthermore, the milk ducts increase in diameter at milk
ejection,
40
leading to the conclusion that it is likely that
the main function of the ducts is the transport rather than
storage of milk. In addition, the actual course of the ducts
from the nipple into the breast is erratic, and they are
intertwined much like the roots of a tree
38
(Figure 1),
making them difficult to separate surgically.
5
It is widely believed that the lactating breast is
predominantly composed of glandular tissue during lac-
tation. Using a semi-quantitative ultrasound measure-
ment of the glandular and adipose tissues in the breast,
we have found there to be approximately twice as much
glandular tissue as adipose tissue in the lactating breast.
However, there is great variability, and in some women,
up to half of the breast is comprised of adipose tissue. In
addition, the amount of fat situated between the glandular
tissues is highly variable. At this stage, no relationship
between the amount of glandular tissue in the breast