Progesterone: what it is, what it tells you and when it’s tested

Progesterone is a hormone made mainly by the ovary after ovulation. It prepares the womb for a pregnancy. If you do not ovulate, progesterone stays low and your periods often become irregular.

When it’s tested

Doctors mostly order it when you are trying for a pregnancy and they want to know whether you ovulate. They also order it when periods are irregular. In pregnancy it is measured only in particular cases, and your gynaecologist decides.

It is often called the day 21 progesterone because, in a 28-day cycle, day 21 is a week before your period. If your cycle is longer or shorter, the day changes: what counts is the week before your expected period.

It is not a test for people who feel well, which is why Rosa does not test it. Instead it looks at what often weighs on your cycle and its effects: TSH for the thyroid, and ferritin and haemoglobin, which can fall with heavy periods.

What it tells you, and what it does not

A progesterone above 3 ng/mL, taken a week before your period, usually confirms that you ovulated.

It does not say how good the phase after ovulation is. Progesterone can change several-fold within a few hours, so a higher or lower value within the normal range means little.

A low value often just means the test was on the wrong day, or that you did not ovulate in that cycle. That can happen now and then.

That is why it is read together with other tests:

Normal progesterone levels

There is no single normal value. Before ovulation progesterone is low. After it, it rises, then falls again before your period. After menopause it stays low. In pregnancy it rises a lot.

That is why every lab prints different intervals per phase: follicular, luteal, postmenopausal and often pregnancy. Read your value against the interval for the right phase, on your own report.

Check the unit: some labs use nmol/L, which gives bigger numbers.

If your progesterone is low or high

Why progesterone can be low

When it is low in the week before your period, you usually did not ovulate. The most common causes:

Progesterone in perimenopause and menopause

In the years before menopause, ovulation becomes less regular. In those cycles progesterone stays low, and periods can become irregular, sometimes heavier.

After menopause progesterone is low for everyone. Measuring it is usually not needed.

If you take hormone therapy with oestrogen and still have your womb, your doctor adds a progestogen, because oestrogen alone raises the risk of endometrial cancer.

What can skew the result

A few things change the value without saying much about ovulation. It helps to know them before your blood test.

When to worry

Talk to your doctor or gynaecologist if:

Frequently asked questions

When is the day 21 progesterone test done? About seven days before your expected period. In a 28-day cycle that is day 21; in a 35-day cycle, roughly day 28. With very irregular cycles, your doctor tells you what to do.

What are normal progesterone levels? They depend on the phase of the cycle, and every lab has its own intervals. A week before your period, above 3 ng/mL usually means you ovulated.

What are the symptoms of low progesterone? There are no reliable symptoms. Irregular periods or spotting before your period can go with cycles without ovulation, but they have other causes too. A list of symptoms is not enough: talk to your doctor.

Can I get pregnant with low progesterone? A low value in one cycle does not mean you cannot. Often it is the wrong day, or a cycle without ovulation. If you have been trying for a while, talk to your gynaecologist.

Is progesterone worth measuring after menopause? Usually not. After menopause it is low for everyone. Over 45, menopause is recognised from your symptoms and periods, without tests.

Does Rosa test progesterone? No. Rosa measures TSH, ferritin and haemoglobin, which are useful when your cycle changes. If your doctor wants progesterone, they order it separately.

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