The two primary female sex hormones are estrogen and progesterone and you've seen in the last blog how they change across the menstrual cycle.

The follicular phase is fairly simple in that progesterone is very low and has very little effect overall with estrogen being the primary determinant of what is occurring physiologically. The surge of estrogen in the late follicular phase has a number of effects but the overall picture there is fairly simple. Things become more difficult in the luteal phase where estrogen first drops (this drop causing one set of effects) before both estrogen and progesterone increase and then fall again, causing different effects still. While I will discuss each hormone individually below, the simplest way of looking at this issue is that estrogen and progesterone have effectively opposite effects on a woman's physiology. Importantly, when progesterone is high during the luteal phase, its effects dominate as it blocks/opposes estrogen's effects.

Even here there is a further complication as estrogen sensitizes the progesterone receptor so that progesterone will have a larger impact during the luteal phase. In that sense, at least some of progesterone's overall effects are can be indirectly attributed to estrogen. Regardless, once I've looked at the effects of both estrogen and progesterone and consider the interactions (along with the spike of estrogen before ovulation), the overall structure of the menstrual cycle should make some logical sense in how it is trying to prepare a woman's body for the potentiality and eventuality of pregnancy.

Let's Look At Estrogen First

Estrogen is produced primarily by a woman's ovaries although it can be produced elsewhere, generally by the conversion of other hormones such as testosterone via aromatase.

Estrogen has its own specific receptor and there are two subtypes called estrogen receptor alpha and estrogen receptor beta. These are found in varying levels in different tissues in the body which not only explains how estrogen can have differential effects in different places but also why certain drugs that target specific receptors can be used to treat such diseases as breast cancer. In this case, it is estrogen receptor alpha that is primarily at play and drugs that specifically block that receptor allow estrogen to work in other tissues that express estrogen receptor beta while still treating the disease itself.

In the same way that testosterone is responsible for the development of male secondary sexual characteristics, estrogen has a primary effect on the development of female secondary sex characteristics. Estrogen is critically involved in the deposition of breast fat and contributes both to women's increased overall body fat levels and her lower body fat patterning. In men estrogen can have the same effect, for example and some males develop gynecomastia, the development breast tissue, under some conditions such as puberty or testosterone abuse.

High levels of estrogen can also cause water retention. Estrogen causes the growth plates of bones to close and this is part of why women are typically shorter than men; at puberty their bones fuse and stop lengthening. Critically, estrogen is a major player in increasing bone density although it is not the only factor here. Estrogen also plays a role in cognition and

mental function.

Estrogen dominates during the follicular phase (first half of the cycle), starting at a low level and gradually increa