The "luteal" phase of the menstrual cycle is the phase after ovulation and before your period starts.
Progesterone is an important hormone in the luteal phase of the menstrual cycle. Progesterone is responsible for maintaining the uterus lining and the successful implantation of a fertilized egg.
As you may know, the cells surrounding the egg ("follicles") mature within the ovary in preparation for ovulation. At the end of folliculogenesis, the mature follicle ruptures to release a mature egg into the uterus for fertilisation. The ruptured follicle is called a "corpus luteum".
The corpus luteum secretes progesterone which plays a role in thickening the uterus in preparation for the implantation of a fertilized egg. Progesterone also can travel to the brain to inhibit LH release from a specific area called the anterior pituitary. This process is referred to as "negative feedback". (To help understanding, have a look at the image below)
"So why are progesterone levels low?"
In PCOS, some women do not have regular menstrual cycles and do not ovulate due to impaired folliculogenesis. This prevents the production of the "corpus luteum" (ruptured follicle) responsible for the secretion of progesterone.
"What are the symptoms of low progesterone levels?"
High anxiety and low mood
Low progesterone can have a negative impact on your mental health. Progesterone acts as an anti-depressant. Therefore, reduced progesterone levels can lead to periods of low mood.
Weight gain
Brain fog
Fluid retention
Low progesterone can increase fluid retention, in turn leading to symptoms such as a swollen stomach or sore breasts.
Fertility or menstrual problems
Thyroid issues
Sagging skin
Intense hot flushes
Oestrogen dominance
Oestrogen and progesterone work in synergy to maintain the uterus lining. Progesterone acts on the uterus tissue to decrease the number of oestrogen receptors to decrease its sensitivity to oestrogen.
Oestrogen thickens the uterus membrane and progesterone inhibits oestrogen to prevent extreme thickening of the uterus membrane.
Simply, progesterone prevents oestrogen from thickening the uterus membrane too much.
Therefore, low progesterone levels can not correctly control oestrogen properly. This increases the risk of endometriosis and heavy bleeding.
"How does low progesterone affect fertility?"
Progesterone is responsible for the successful implantation of a fertilized egg by maintaining the uterus lining. Therefore, low progesterone levels can cause early pregnancy loss.
Additionally, low progesterone in the luteal phase promotes a viscous cycle of hyperandrogenism and ovulatory dysfunction. This is because progesterone is meant to "feedback" to the brain to reduce LH secretion.
As there is no negative feedback, the brain still secretes high levels of LH which acts on the ovarian theca cells to increase the production of testosterone ("hyperandrogenism") and inhibition of early folliculogenesis (ovulation).
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