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Menstruation, gestation and lactation

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We've seen that negative and positive feedback loops are fairly straightforward concepts. That's because we were thinking of just one variable which either gets kept constant or gets increased/decreased continually. What happens when you have a complex interplay between 4 different variables?

The Mammalian Oestrus is Controlled by 4 Hormones: FSH, LH (pituitary gland), Progesterone and Oestrogen (ovaries)

FSH is follicle-stimulating hormone (it stimulated follicles to develop which are immature eggs surrounded by other cells). LH is luteinising hormone (it stimulates the corpus luteum to develop which literally means yellow body and literally is a yellow body which secretes progesterone and forms from the "ashes"/remains of the follicle following ovulation). 

Here's an overview of the entire cycle (it's arbitrarily decided that day 1 is the first day of menstruation just because it's easily spotted... no pun intended!):

1. Ovulation: a) high levels of FSH stimulate the ovaries to produce oestrogen
                       b) the follicle in the ovary gets stimulated by FSH to develop
                       c) increasing oestrogen levels inhibit FSH, but very high levels of oestrogen stop inhibiting FSH
                       d) oestrogen also stimulates LH secretion by the pituitary gland
                       e) FSH and LH peak, an egg is released

2. Luteal Phase: a) the corpus luteum formed from the remains of the follicle following ovulation secretes progesterone
                            b) increasing progesterone levels inhibits LH secretion
                            c) oestrogen stimulates, and progesterone maintains, the uterus lining in case of pregnancy
                            d) progesterone levels drop, and the cycle starts again

Bottom Line

The actual gory details aren't necessarily something you must learn entirely. The purpose of this topic altogether is to exemplify negative and positive feedback. So let's just focus on that at least, shall we? There are 3 feedback loops to take note of:

1. Negative feedback from oestrogen to FSH - hey FSH, I'm oestrogen and I know that you stimulated me in the first place, but how about you lay low now that I'm around. Until I get a bit more abundant! My presence means your absence for the time being, so tell that pituitary gland to stfu.

2. Positive feedback from oestrogen to LH - hey LH, I'm oestrogen and I want more of you. See how abundant I am? You want to be here with me, don't you? Come on, come closer.

3. Negative feedback from progesterone to LH - hey LH, I'm progesterone and you're making too much noise partying like that with oestrogen, so here's the deal. Go away. Thank you in advance.


If pregnancy occurs, a different pattern of hormone regulation will take place. As the embryo implants in the uterus, menstruation must cease, which is signalled by the human chorionic gonadotropin hormone secreted by the placenta.

Estrogen secretion can reach up to 30 times the level of ovulation (non-pregnant) estrogen during pregnancy. Its function is the support of growing breasts and the womb, as well as increasing blood supply in the final trimester of pregnancy via vasodilation.

Progesterone is also produced to help embryo implantation and stimulated the production of nutritious substances for its development. Crucially, it also inhibits uterus contractions which would compromise the pregnancy.

The placenta serves to protect the foetus from the parental immune system, as well as to nourish it. This is in addition to its already-mentioned endocrine functions of releasing hormones for various functions.

Birth is induced as uterine contractions begin, oxytocin is released, and progesterone decreases. Labour is split into three phases: the first phase involves contractions and cervix dilation, the second phase involves the baby being delivered, and the final phase involves the expelling of the placenta.


Lactation occurs post-birth as a result of a drop in hormones including estrogen, progesterone and prolactin. Prolactin has a key role in the production of milk, and it starts developing the breast milk ducts from the second trimester of pregnancy.

After birth, oxytocin helps contract the smooth muscle cells surrounding the alveoli to get milk into the ducts, as well as cause ejection of milk.