Let’s Get Back to Biology with Hormones

Hormones. We learnt about them at school, probably during a dull biology lesson or perhaps during sex ed. It’s the sort of information you partially retain and have some brief awareness of, and unless you work in human science, you probably won’t have thought about it too much until you start trying to conceive (TTC). Suddenly you are sitting thinking, ‘I really wish I paid more attention in class. Why don’t I remember this stuff?!’

The Menstrual Cycle

Let’s start by looking at what actually happens in the lead-up to your period every few weeks. We know everyone’s cycle is a different length and can vary from month to month (we touched on this in a TikTok here), but we will discuss a 28-day cycle in this blog for simplicity. We start counting on Day 1, the first day of your period. 

The first 14 days are called the follicular phase. A small group of ovarian follicles, each containing an egg, develops on the ovarian surface. Later in the phase, one of these follicles is selected for ovulation. This follicle matures and grows and produces oestrogen.

This triggers hormonal reactions which in turn cause ovulation: the rupture of the mature ovarian follicle and release of the egg. The egg can be fertilised at this point, and the endometrial lining starts to build up and prepare for possible embryo implantation.

The rest of the cycle after ovulation is called the luteal phase. The ruptured follicle closes and becomes a structure called a corpus luteum. This is important as it produces progesterone. If the egg is fertilised after ovulation, the corpus luteum persists, and the progesterone it produces maintains the endometrial lining. If no fertilisation occurs, the corpus luteum degenerates and the endometrial lining is shed. This is your period.


menstrual cycle chart

So, which hormones are involved in regulating the menstrual cycle? 

Many hormones work together in harmony to regulate the menstrual cycle. You need to know four main hormones: luteinising hormone, follicle-stimulating hormone, oestrogen, and progesterone. Throughout your cycle, the fluctuations of these hormones signify the start and end of various phases. Some cause other hormones in the cycle to be produced and others stop them from being produced in mechanisms called positive and negative feedback. It can all sound quite complicated, but here is a summary of the role of each hormone.

Follicle-Stimulating Hormone (FSH)

FSH is also produced in the anterior pituitary gland. Its primary role is stimulating the ovarian follicles to grow and mature in preparation for ovulation. These follicles then release oestrogen and low levels of progesterone, which are important for the next stages of the menstrual cycle.

FSH levels are highest just before an egg is released and remain low for the luteal phase. Because of this variation in levels, it is best to get your FSH level tested on day 3 of your cycle.

Both high and low FSH levels are associated with various fertility problems. For example, a low level can be associated with a high oestrogen level or a more general hormone issue. High FSH levels may indicate ovarian failure.  

Luteinising Hormone (LH) 

Like FSH, LH is made in the anterior pituitary gland and is one of the key hormones responsible for ovulation. 

LH travels in the blood to the ovary and plays a distinct role in the two phases of the menstrual cycle. During the follicular phase (generally the first two weeks of a 28-day cycle), LH stimulates the ovarian follicles to produce oestradiol. At the midpoint of your cycle (e.g. usually day 14 of a 28-day cycle), a surge in the LH level (along with FSH) triggers ovulation, causing the ovarian follicle to rupture and release the mature egg. In the luteal phase (the two weeks following ovulation), LH stimulates the production of progesterone, which is vital to support the early stages of pregnancy. 


Oestradiol, the primary and most potent form of oestrogen, is predominantly produced by the ovaries. Oestradiol plays a crucial role in ovulation and regulates the normal functioning of the breasts, vagina, and uterus. 

As the follicles in the ovary grow during the menstrual cycle, blood oestradiol levels increase, leading to a subsequent rise in luteinising hormone (LH) levels, which in turn cause ovulation. 


Progesterone is produced by the ovaries and, during the luteal phase, the corpus luteum. Its primary role in the menstrual cycle is preparing the uterus's lining for implantation and maintaining it throughout pregnancy. If fertilisation does not occur, the corpus luteum degenerates and the production of progesterone decreases, leading to the start of a new menstrual cycle. 

Because of fluctuations in progesterone levels, it is most accurate to have your progesterone tested when it is highest. This is in the mid-luteal phase, about 7 days before your anticipated period. A normal progesterone level is a good indicator of ovulation.

Thyroid-Stimulating Hormone (TSH)

Although this hormone does not have as direct a role in the menstrual cycle as the above four hormones, we have included TSH here as it is an important hormone to consider in fertility. TSH is responsible for controlling the production of thyroid hormones and is the primary indicator used to screen for thyroid conditions and diseases. High or low TSH levels can cause problems with fertility, so getting these levels in check is important. 

So, hormones and ttc...

Knowing your hormone levels can provide valuable insight into your fertility, such as estimating the number of eggs you may have, determining whether you are approaching or have already experienced menopause, and predicting possible pregnancy outcomes. Hormone imbalances can also be the root cause of symptoms such as irregular periods, hair loss, and fatigue.

If you have been TTC for a while, it might be worth speaking with your healthcare provider about having your hormone levels checked. This information is powerful in helping you make personalised decisions and to guide you through your fertility journey. 

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