The hormones behind your cycle

A period starts when the womb lining is shed. This happens after oestrogen and progesterone fall, if you are not pregnant. Behind the bleeding, the brain and the ovaries send signals to each other all through the cycle.

The main signals and their jobs

Hormones are chemical messengers. Two of them come from the pituitary gland at the base of the brain: follicle-stimulating hormone (FSH) and luteinising hormone (LH). They help control the ovaries, where eggs grow and are released.

FSH helps eggs grow. Each egg develops inside a small fluid-filled sac in the ovary called a follicle. As a follicle grows, oestradiol (a form of oestrogen) rises. Oestrogen helps prepare the womb lining. Meanwhile, the growing follicle and the brain keep sending hormone signals back and forth.

LH rises sharply just before ovulation, which is when an egg is released. After that, cells from the follicle form a short-lived structure called the corpus luteum. It makes progesterone, which gets the womb lining ready for a fertilised egg to attach.

If you don’t get pregnant, the corpus luteum stops working and hormone levels fall. The lining then leaves the body as a period. Oestrogen and progesterone don’t stay at one “ideal” level through the cycle. Their rise and fall is part of how the cycle works.

Why ovulation timing can vary

The follicular phase runs from the first day of a period until ovulation, so it includes the bleeding days. The luteal phase comes after ovulation. How long it takes to reach ovulation can change from cycle to cycle. A calendar can’t assume the egg is released on the same day each month.

Most of the change in cycle length comes from the follicular phase. After ovulation, timing follows the life of the corpus luteum. That is part of why the luteal phase tends to be steadier. Steadier doesn’t mean the same for everyone, or the same every cycle.

ASRM (the American Society for Reproductive Medicine) describes a typical luteal phase as fairly fixed at 12–14 days. It also gives a range of 11–17 days. These numbers describe what is typical. They can’t tell you whether something is wrong with your cycle. If your pattern has changed, talk to a clinician rather than trying to name a hormone problem from dates.

What the evidence says about cycle syncing

You can plan your day around how you feel. But some claim that your cycle phase tells everyone when to train hard or do their best thinking. That needs more proof than a diagram of changing hormones.

A recent systematic review looked at elite and well-trained athletes. It found no statistically significant difference in maximal oxygen uptake between the early follicular and mid-luteal phases. Maximal oxygen uptake is one measure of fitness, so the finding is narrow. It doesn’t rule out changes in how exercise feels.

A meta-analysis (a study that combines the results of many studies) looked at thinking skills. It also found no consistent sign that women’s abilities changed across the cycle. Taken together, these results don’t support one timetable for workouts or hard mental work. They also don’t mean you should ignore pain, tiredness or trouble concentrating when it happens.

Use a diary to note how you felt, without assuming your cycle phase decided your mood or ability. You might write “Could not concentrate during the afternoon” without deciding which hormone caused it. When you see a cycle schedule, ask some questions about it. What outcome was studied, and in whom? How sure were the findings?

When to book an appointment

Book a GP appointment in the UK, or see your doctor in the US and elsewhere, if your periods become irregular or you miss your period 3 times in a row. The NHS also advises getting checked if a missed period comes with symptoms such as tiredness or weight changes. Knowing the hormone names can’t tell you the cause of a change. It can help you ask your clinician better questions.