The hormones behind your cycle

A period begins when the womb lining is shed, following a fall in oestrogen and progesterone if pregnancy has not occurred. Behind the bleeding, signals pass between the brain and ovaries throughout the cycle.

The main signals and their jobs

Hormones are chemical messengers. Follicle-stimulating hormone, or FSH, and luteinising hormone, or LH, come from the pituitary gland at the base of the brain. They help coordinate activity in the ovaries, where eggs develop and are released.

FSH supports the growth of developing eggs. Eggs develop within follicles in the ovary. As a follicle grows, oestradiol, a form of oestrogen, rises. Oestrogen helps prepare the womb lining, while the developing follicle and the brain continue exchanging hormonal signals.

LH rises sharply just before ovulation, the release of an egg. After ovulation, cells from the follicle form a temporary structure called the corpus luteum. It produces progesterone, which prepares the womb lining for a fertilised egg to attach.

If pregnancy does not happen, the corpus luteum loses function and hormone levels fall. The lining then leaves the body as a period. Oestrogen and progesterone do not stay at a single “ideal” level throughout a cycle; rising and falling levels are part of how it works.

Why ovulation timing can vary

The follicular phase runs from the start of a period until ovulation. It includes the bleeding days. The luteal phase follows ovulation. The time needed before ovulation can vary, so a calendar cannot assume that the egg is released on the same cycle day each month.

Much of the variation in cycle length comes from the follicular phase. After ovulation, timing follows the life of the corpus luteum. This helps explain why the luteal phase tends to be steadier. Steadier does not mean identical for everyone or for every cycle.

ASRM describes a typical luteal phase as relatively fixed at 12–14 days, while also giving a range of 11-17 days. These ranges describe typical timing; they cannot tell you whether there is a problem with your cycle. If the pattern has changed, bring it to a clinician instead of trying to label a hormone problem from dates.

What the evidence says about cycle syncing

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

A recent systematic review of elite and well-trained athletes found no statistically significant difference in maximal oxygen uptake between the early follicular and mid-luteal phases. The finding concerns a specific measure of aerobic capacity in those athletes. It does not rule out changes in how exercise feels.

A meta-analysis of cognitive performance also found no consistent evidence that women’s abilities changed across cycle phases. Together, these findings do not support a universal timetable for workouts or demanding work. They also do not tell you to ignore pain, tiredness or difficulty concentrating when you experience them.

Use a diary to record how you felt, without assuming a cycle phase decided your mood or ability. You might write, “Could not concentrate during the afternoon”, without deciding that a particular hormone caused it. Treat a suggested cycle schedule as a claim to question: what outcome was studied, in whom, and how certain 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 periods become irregular or you miss your period 3 times in a row. The NHS also advises assessment if a missed period comes with symptoms such as tiredness or weight changes. Knowing the hormone names cannot identify the cause of a change, but you can use them to ask your clinician questions.