PHYSIOLOGY OF LACTATION

The physiology of lactation

All women are not alike regarding their capacity
for lactation. Some possess a much higher
potential than others.
In the same women, second and later lactations
tend to be more successful than the first,
indicating that "trial runs" are necessary before
optimal performance is achieved.
Form the physiological point of view, lactation
performance is related to nutrition, endocrine
and psychological factors in the mother
 Nutrition
During pregnancy maternal metabolism changes
so that a woman lays down body stores of
energy in the form of fat, which is deposited in
the subcutaneous tissue of the trunk and on the
legs.
In a well-nourished woman, the increase in body
fat amounts to about 4 kg, which is equivalent
to a store of 35000 kcal. This is enough to
provide for lactation for 4 months at the rate of
nearly 300kcal/day.
Thus the average woman enters the final weeks
of pregnancy with a considerable store of food
energy to act as a buffer against sudden
deprivation of food.
The stimulus to accumulate this fat store is
through a change in the control of the energy
balance caused by progesterone and other
hormones, as well as a slowing down of energy
expenditure as pregnancy advances.
Mothers who do not breast-feed their babies
will carry these extra stores of fat on their
bodies unless they resort to dieting.
In general, in a well-nourished community,
mothers who breast-feed are able to regain their
figures more easily than those who do not.
As lactation proceeds, the accumulated body fat
is converted into energy in the milk.

A study showed that during the period of

lactation, weight was lost at an average of 0.28
kg/week, even though the women were eating
an average of 590 cal more than women in a
control group who were not lactating.
A factor which contributes to larger milk outputs
initially is the size of the baby.
There is a good correlation between the weight
of the baby at birth and the amount of milk
produced.
Another factor is high frequency of feeding as
happens with true demand feeding in all
societies.
When the diet during pregnancy is poor, the
mother will gain little weight. Most of this
weight consists of the weight of the baby and
other products of conception, so that the true
weight gained by the mother is very little.
Such a mother will commence lactation with
inadequate body stores. In spite of the loss of
weight, during lactation, the secretion of milk
was adequate to support the growth of the
infants.
 Endocrine factors
The development, growth and secretory
functions of the mammary gland are dependent
upon stimulation by appropriate hormones. In
the adolescent girls, the breast develops and
grows to adult size under the influence of the
sex hormones.
In pregnancy, there is further development of
the secretory apparatus of the gland under the
influence of high levels of circulating estrogens
and progesterone.
Parturition triggers the secretion of prolactin
from the anterior lobe of the pituitary, and under
its influence, cells of the mammary gland
synthesize and secrete milk.
In the human, prolactin and oxytocin can be
considered the key lactogenic hormones in both
initiating and maintaining milk secretion.
Prolactin released from the anterior pituitary
stimulates the synthesis of milk.
Oxytocin secretion from the posterior pituitary
causes the milk to flow into the lactiferous ducts
which lie behind the nipple, from where it can be
obtained by the infant during suckling.
During pregnancy, the level of prolactin in the
maternal bloodstream rises steadily, but milk
production cannot begin until the inhibitory
effect of placental estrogen and progesterone is
removed. After the delivery of the placenta this
inhibition declines, and milk flow commences
usually within 48-96 hours.
In rare instances, retained fragments of the
placenta can continue to inhibition of milk
secretion. Putting the baby to the breast soon
after the delivery has many advantages, one of
them being the evacuation of the uterus which
contracts in response to oxytocin released.
During lactation, oxytocin release takes place in a
pulsatile manner. It usually commences a few
minutes before lactation in response to
preparation for or expectation of a feed.
On the other hand prolactin is not released until
after suckling has started, and is always in
response to tactile stimulation of the nipple
during suckling.
In the non-lactating individual, the secretion of
prolactin is inhibited by a hypothalamic factor
termed "prolactin-release inhibiting hormone"
 The let-down reflex
Once the acinar cells of the breast begin to secret
milk, its continuing secretion and flow along the
lactiferous ducts is maintained by a neuro-
endocrinologic mechanism commonly known as the
"let-down reflex".
During lactation, The tactile stimulation at the nipple
during suckling results in afferent nerve impulses
which travel to the hypothalamus. In turn the
hypothalamus activates the anterior and the
posterior lobes of the pituitary gland.
Prolactin is secreted from the anterior lobe and
under its effect the secretory activity of the
acinar cells of the mammary gland is stimulated
and maintained.

At the same time oxytocin is secreted from the

posterior lobe of the pituitary. It causes the
contraction of the myoepithelial cells in the
mammary gland, thereby propelling the milk
along the duct.
It is a common experience of many mothers that
when the baby is put to the breast on one side,
some milk may drip from the breast on the
other side, and hence the term let-down or
milk-ejection reflex.
Later, when lactation has been well established
and the reflex mechanism has been reinforced
several times over, many mother experience a
tingling sensation or heaviness in the breast as
feeding time approaches, or even on hearing the
cry of the baby in the next room.
The let-down reflex is the most crucial
physiological mechanism in successful lactation.
Any factor interfering with suckling at the breast
by the infant will interfere with this mechanism
and affect milk secretion, eventually causing the
breasts to dry up.
On the other hand, regular and repeated
emptying of the breast by suckling will stimulate
milk secretion and flow. In order to help
establish lactation, the baby should be put to
thebreast as soon after delivery as possible.
After this, the breast should be offered on
demand in order to establish a flexible regime of
feeding.

The let-down reflex is highly variable. In some

cases it can be vigorous, causing a sharp pain in
the breast, and milk may spurt out in small jets.
Other mothers may experience a tingling
sensation and milk may drip from the breast.
 Psychological aspects
The mother´s attitude is very much dependent
upon the social and cultural milieu in which she
has been brought up.

Infants on an unrestricted feeding schedule are

known to gain weight and grow faster than
those on a rigid schedule.
The mother´s desire to feed her infants is
aroused if there is close physical contact.
Those who are nursed alongside the mother are
fed more frequently than if they are kept in a
separate nursery.
A lethargic baby suckles very little and thus does
not stimulate milk production. In case of mothers
who are heavily sedated with barbiturates during
labour, the infant may be drowsy for as long as 5-6
days after birth and are not capable of effective
suckling.
Mothers with positive attitude who react to their
babies with joy and delight are on the whole more
successful at breast-feeding.
Other psychological factors that may contribute to
the success or failure of lactation may include:
 Physical difficulties and problems
 Emotional support in post-partum period
 Family relationships
 Mothers childhood experience
 Successful lactation in previous pregnancies

At each episode of feeding the close body contact,

the eye-to-eye contact, and the pleasurable feeling
of satiety in the infant reinforce the growing bond
between the pair.
 Development of the mother’s milk

I- Colostrum.

At 0 to 3 days after parturition the mother

produces colostrum.
It is thicker than mature milk
higher in protein and antibodies, and acts as a
laxative for the baby.
 II- Transitional milk.

At 4 to 7 days after parturition, the mother

produces transitional milk
It contains high levels of fat and lactose
helps the baby regain weight lost after birth.
 III- Mature milk.
 At 7 to 10 days after parturition, the mother
starts producing mature milk.
 Consisting largely of water.
 mature milk is necessary to maintain hydration
for the infant.
 The milk also includes carbohydrates, proteins,
and fats that are necessary for both growth and
energy.
Mature milk is further classified into foremilk and
hindmilk.
A- Foremilk is found during the beginning of a
feeding session and contains more water,
vitamins, and protein.

B- Hindmilk occurs after the initial release of

milk and contains higher levels of fat and aids in
the weight gain of infants. The different stages
of breast-feeding can affect the amount transfer
of lipid-soluble drugs into breast milk.
 Factors that determine the safety of
the drugs used during breastfeeding
1.Factors related to breastmilk

A. Milk composition (lipid and protein concentrations)

Breastmilk is subject to significant changes in lipid and protein
concentrations, which depend upon the stage of lactation (colostrum versus
mature milk) or even upon the stage of a breastfeed (foremilk versus
hindmilk).
Such changes influence how much of a drug is transferred
from plasma to milk, causing variations in drug concentration in the
breastmilk

B. pH.
Because breast milk is more acidic than plasma, drugs
with a high pH may concentrate more in breast milk than plasma.
2. Factors related to the mother
A. Renal and hepatic excretion
B. Dose and duration of treatment
C. Route of administration
3. Factors related to the infant
A. Age
B. Drug absorption
C. Renal and hepatic excretion
D. Volume of milk intake
E. Safety of the drug for the infant
4. Factors related to the drug

A. pKa (dissociation constant)

B. Solubility in water and lipids
C. Molecular size
D. Oral bioavailability
E. Toxicity
F. Suppressive effect on milk production
G. Long-acting drugs & short-acting drugs
5. Pharmacokinetic factors
A. volume of distribution (Vd)
Drugs with high Vd may enter different
compartments of the body, therefore resulting in a
lower concentration in the blood and may produce
lower milk levels. Drugs that have a Vd between 1
and 20 L/kg are generally compatible for breast-
feeding.
B. the percentage of maternal protein bindingPB
PBshows the extent to which a drug is bound to
the plasma albumin and other proteins. Therefore,
drugs that have high PB would generally reduce
the infant’s exposure to the medication. Drugs
that have a PB greater than 90% are usually
compatible for breast-feeding
C. Tmax.
The amount of a drug in the breast milk and in
the plasma usually are in equilibrium, When a
higher drug concentration is in plasma, the drug
concentration in breast milk is generally higher
as well.
D. t½.
Infant exposure to drugs in breast milk can be
minimized by using drugs with shorter half-lives
and thus shorter peak intervals. Mmvgf
There is no doubt that breast-feeding is
one of the essential biologic functions of
humans.

Any decision to limit a mother's breast-

feeding must be justified by the fact that
the risk to her baby clearly outweighs
the benefits conferred by nursing.
 Assessing the risk of drug-induced
toxicity among breast-fed infants
The analysis of the risk of an infant's exposure to a
drug excreted in milk needs to take into account the
answers to two key questions:

1- How much of the drug is excreted in milk?

2- at this level, what is the risk of adverse effects?
Thank You