Before you look at what’s different in the menstrual cycle with PCOS, please review the biology of the typical menstrual cycle. (link here to Normal Menstrual Cycle)
For women with PCOS, there are complex biochemical imbalances within the ovary.
The underlying cause is Insulin Resistance.
The ovaries have more androgenic (male type) hormones, such as DHEA, DHEAS and Testosterone, so can not respond in the same way to FSH, Follicle stimulating hormone.
The ovary has many (poly) small follicles, that aren’t responding to FSH in the same way and so there is not a dominant follicle that produces a peak of Estrogen.
In this ultrasound photo that I took, you can see many small follicles, and none that are dominant.
There is a lot of circulating estrogen causing stimulation of the uterine lining, but no peak.
Without the peak of Estrogen, there is no signal to produce a peak of LH, Luteinizing hormone. Instead the body secretes a lot of LH all the time, which further confuses the cycle.
Without a peak of LH, there is no ovulation and no corpus luteum to make adequate Progesterone, which means things are out of balance and can lead to heavy, irregular and prolonged periods.
Meanwhile, other hormones are being produced
- In the ovary and elsewhere in the body, some of the excess Estrogen is converted to more androgenic (male-type) hormones such as DHEA, DHEAS, and Testosterone
- This conversion of hormones into androgenic ones leads to more hair growth and acne