Day 21 of the menstrual cycle
In short: what happens on day 21?
Day 21 of the menstrual cycle is when progesterone peaks in a 28-day cycle. That is why the 'day 21 progesterone test', used to check whether ovulation has happened, is taken on this day. A high progesterone level shows that you ovulated and that the corpus luteum is working.
Day 21 of the menstrual cycle – animated explainer
Day 21: cycle dial, uterus and ovaries, hormone levels (illustrative animation).
What the animation shows
- FSH
- Follicle-stimulating hormone: starts the growth of follicles in the ovary
- LH
- Luteinising hormone: its sudden surge triggers ovulation
- E2
- Oestrogen (oestradiol): thickens the lining of the uterus
- P4
- Progesterone: prepares the uterus for a possible pregnancy
- BBT
- Thermometer and dots below: basal body temperature
- RING
- Coloured ring: the 28-day cycle and its phases; the green arc marks the fertile days
- MM
- The measurement in the uterus shows lining thickness; in the ovary it shows follicle or corpus luteum size
Phase
Luteal phase
Chance of pregnancy
Very low
Uterine lining
14 mm
Corpus luteum
23 mm
FSH
19
LH
19
Oestrogen
64
Basal body temperature
36.74°C
What is happening in your body today?
The corpus luteum is at its most active. The uterine lining is at its thickest and most nourishing.
Hormones
Progesterone is at its peak; oestrogen reaches its second (smaller) peak.
How you might feel
Bloating, breast tenderness and sleepiness may increase.
Chance of pregnancy
The chance of pregnancy is very low.
Tips for today
- If your cycle is not 28 days long, agree the day of your progesterone test with your doctor
- A balanced diet and regular sleep may ease PMS symptoms
- Keep up moderate exercise
When to see a doctor
See your doctor if you have bleeding outside your period, severe pain or a fever.
Menstrual cycle day by day: table
| Day | What happens | Fertile window |
|---|---|---|
| Day 1 | First Day of Your Period | — |
| Day 2 | Heaviest Bleeding Day | — |
| Day 3 | FSH Rises, Follicles Grow | — |
| Day 4 | Bleeding Eases, Oestrogen Rises | — |
| Day 5 | Last Day of Your Period | — |
| Day 6 | Follicular Phase Begins | — |
| Day 7 | Dominant Follicle Is Selected | — |
| Day 8 | Oestrogen Rises, Fertile Window Nears | Low |
| Day 9 | Fertile Window Begins | Moderate |
| Day 10 | Fertile Window, Higher Pregnancy Odds | Moderate |
| Day 11 | 3 Days Before Ovulation | High |
| Day 12 | Oestrogen Nears Its Peak | High |
| Day 13 | LH Surge: Ovulation Is Near | Highest |
| Day 14 | Ovulation Day | Highest |
| Day 15 | Luteal Phase Begins | Moderate |
| Day 16 | Progesterone Rises | Low |
| Day 17 | Luteal Phase and Increased Appetite | — |
| Day 18 | Breast Tenderness May Start | — |
| Day 19 | Implantation Window Opens | — |
| Day 20 | Implantation Bleeding Possible | — |
| Day 21 | Progesterone Peak (Day 21 Test) | — |
| Day 22 | Mid-Luteal Phase | — |
| Day 23 | PMS Symptoms May Start | — |
| Day 24 | Hormones Start to Fall | — |
| Day 25 | Late Luteal Phase and PMS | — |
| Day 26 | Progesterone Drop and Cramps | — |
| Day 27 | 1 Day Before Your Period | — |
| Day 28 | Last Day of Your Cycle | — |
Frequently asked questions
What is a day 21 progesterone test?
It is a blood test taken in the middle of the luteal phase to confirm that ovulation has happened. In a 28-day cycle it is done on day 21; in other cycles, about 7 days before your next period.
What should progesterone be on day 21?
In general, a level above 10 ng/mL (about 30 nmol/L) suggests that ovulation has occurred. Discuss your result with your doctor.
Can you get pregnant on day 21 of your cycle?
In a regular 28-day cycle, day 21 is outside the fertile window and the chance is very low.
Gynaecology check-ups and follow-up at SoftCare Clinic
Book an appointment with our specialist doctors for irregular periods, painful periods or pregnancy planning.
The information on this page is general guidance based on a typical 28-day cycle and does not replace a medical examination. It should not be used as a method of contraception.
Medical content editor: Op. Dr. Şükrü Sanus