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The dance of hormones in pregnancy and lactation

When it comes to pregnancy and lactation, hormone activity is a wild ride!

To understand how hormones influence lactation, we need to understand their role in pregnancy.

Human chorionic gonadotropin hormone (hCG). This hormone is only made during pregnancy. When a fertilised egg implants in the uterine lining, the cells surrounding the embryo produce hCG which maintain the corpus luteum - the region of the ovary from where the egg was released at ovulation. The corpus luteum produces progesterone and some estrogen in the second half of the menstrual cycle, and with a pregnancy, it continues its production under the control of hCG. Once the placenta is able to produce its own hormones around 12 weeks, hCG levels drop off and the corpus luteum function stops. hCG hormone levels are found in the mother's blood and urine rise a lot during the first trimester, doubling every 2-3 days, peaking around 8 weeks, then drops off consistently to the end of the pregnancy. This is the hormone that initially confirms a pregnancy and may contribute to sickness in the first trimester.

Human placental lactogen (hPL) is a hormone made by the placenta, regulating metabolism and insulin sensitivity, exerting a growth hormone like effect on the fetus. hPL also has a prolactin like effect, promoting ductal and glandular growth in the breasts especially during the third trimester of pregnancy. hPL rises as hCG drops off rises over a pregnancy.

Estrogen is normally primarily produced in the ovaries. In early pregnancy the corpus luteum will contribute some estrogen, followed by the placenta, to help maintain a healthy pregnancy including estradiol (E2) and estriol (E3) – the latter being a pregnancy specific estrogen. During pregnancy, estrogen helps develop the placenta and uterus, increases blood flow and vascularization, triggers fetal organ development, and promotes growth of milk ducts in the breast.

Progesterone is made by the ovaries initially – in the corpus luteum as described above, then by the placenta from around 12 weeks. Progesterone stimulates the thickening of the uterine lining for implantation of a fertilized egg and maintains the uterine lining over a pregnancy. Progesterone also stimulates growth of the glandular tissue in the breast and strengthens the uterine wall in preparation for labour.

Relaxin is a hormone in the same family as insulin, and levels are at their highest in the first trimester. It is frequently blamed for pelvic instability which is more of an issue in the latter half of pregnancy when estrogen is much higher – and the more likely culprit. Rather, relaxin is understood to promote implantation and the growth of the placenta by relaxing the uterine wall. Relaxin is also thought to relax the mother’s blood vessels to increase blood flow to the placenta and kidneys. Hence has more of a vascular role than a musculoskeletal one.

Prolactin is produced in the anterior pituitary gland in the brain, rising over the 2nd and 3rd trimester and is responsible for growth of glandular breast tissue and breast enlargement. Prolactin is required for mature breastmilk to be made and secreted. Prolactin levels halve a week post-partum and slowly lower to slightly elevated levels by 6 months post-partum even with breastfeeding, with small peaks associated with breastfeeds. In a non-breastfeeding mother, prolactin levels normalise after one month.

Oxytocin is produced in the posterior pituitary gland in the brain and is responsible for contraction of the uterus for birth of the baby and the placenta, the control of bleeding post birth and the return of normal uterus size by 6 weeks post-partum. Oxytocin is considered the hormone of love, aiding bonding with the baby and contraction of muscle cells in the breast to eject breastmilk. Massive amounts of oxytocin are produced giving birth – this combined with maximizing the ‘golden hour’ post birth (at least 60 minutes of uninterrupted skin to skin time between mother and baby) helps to foster bonding between mother and a baby and helps with establishing a good milk supply.

Progesterone/estrogen vs prolactin

Progesterone and estrogen levels continue to rise over a pregnancy and prevent ovulation from happening. They also suppress prolactin from producing breastmilk while pregnant. Immediately following the delivery of the placenta, estrogen and progesterone levels drop dramatically, leaving prolactin unopposed and now capable of acting on the milk making cells to produce mature breastmilk. The sudden drop of estrogen and progesterone is responsible for the baby blues 3-5 days post-partum. Thankfully it is usually short lived and recovers within 2 weeks for most mothers – if not, medical assessment should be sought.

During pregnancy the high estrogen and progesterone opposed the action of prolactin. Prolactin in turn suppresses the rise of estrogen and progesterone with return of menstruation and fertility. If a mother is exclusively breastfeeding day and night with interval no longer than 3-6 hours between feeds, baby is under 6 months old, and menstrual cycle has not returned, there is 98% contraceptive protection. This is called the Lactation Amenorrea Method (LAM).

Oxytocin vs Prolactin

Initially, milk production is under endocrine control – stimulation of the nipple and areola by the baby suckling stimulates oxytocin production, in turn this oxytocin causes contraction of the muscle cells surrounding the milk making glands (alveoli), squeezing milk into the ducts towards the nipple – this is known as the milk ejection reflex (MER) or the ‘let down reflex’. Most glandular tissue is within 3cm behind the nipple, with the MER often felt as a tightening in that location. Prolactin release is much slower, peaking 45 mins after a feed. Therefore, the prolactin produced at one feed stimulates the milk production for the next feed. Hence prolactin levels fluctuate over 24hours following breastfeeding and are higher at night and lower during the day – which is one reason why feeding over-night is important for maintaining milk supply, especially in the first 6 months.

After the first 1-2 weeks of endocrine control of milk production, there is a transition to autocrine control. This is where the concept of supply and demand comes in – whatever is removed from the breast, will be made again. The more that is removed from the breasts, the more is made and vice versa. If breasts are left full and time extended between feeds, the breasts slow down milk production via a substance called FIL (feedback inhibitor of lactation). This is why feeding on demand and at least 8-12 feeds per day in the early days are essential to establishing a great milk supply. For sleepy babies, they will need waking to feed to ensure they get enough breastmilk, and that milk supply is adequately stimulated.

Dopamine vs Prolactin

Dopamine is another antagonist to prolactin. With higher prolactin there is less dopamine – which is one of our feel-good hormones and may contribute to post-partum mood disorders. Fortunately, there is usually plenty of oxytocin being produced, prolactin is also a calming hormone, and cholecystokinin is also produced following breastfeeding, aiding sleepiness and relaxation. Knowing the effect dopamine has on prolactin, dopamine agonist drugs can be used in certain circumstances to reduce milk supply e.g., cabergoline, and dopamine antagonists can be used to increase supply, e.g., domperidone, but either need using within the first few weeks of birth to be effective – just like frequent and effective removal of breastmilk in the first month is required to establish a good milk supply.

It is a rather complex and fascinating dance of hormonal balance to become pregnant, stay pregnant, to breastfeed successfully and to sustain optimal mental health. Its little wonder things go wrong, and all the more reason women and families need great support along the way.

Kristine Miles is a Senior Physiotherapist and Lactation Consultant (IBCLC) at Balance Health. She is available for appointments Tuesday-Fridays with exclusive availability for lactation consultation on Fridays. She can assist with issues such as positioning and attachment, supply issues, recurrent mastitis and more.


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