Day 13 of the menstrual cycle
In short: what happens on day 13?
Day 13 of the menstrual cycle is when, in a 28-day cycle, LH surges and the chance of pregnancy reaches its highest level. Peak oestrogen triggers the brain to release LH; about 24–36 hours after the LH surge begins, the mature follicle ruptures and releases an egg.
Day 13 of the menstrual cycle – animated explainer
Day 13: 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
Follicular phase
Chance of pregnancy
Highest
Uterine lining
10.5 mm
Follicle
20 mm
FSH
50
LH
59
Oestrogen
100
Basal body temperature
36.25°C
What is happening in your body today?
The dominant follicle reaches about 20 mm. The uterine lining is around 10 mm.
Hormones
Oestrogen is at its peak; LH and FSH rise rapidly.
How you might feel
Discharge is at its most slippery. You may feel a mild pulling sensation in your lower abdomen.
Chance of pregnancy
The chance of pregnancy is at its highest.
Tips for today
- If you are trying to conceive, sex today and tomorrow is most effective
- Once you get a positive ovulation test, you can stop testing
- If you track basal body temperature, measure it at the same time every morning
When to see a doctor
See your doctor if you have severe pelvic pain that does not go away or you feel faint.
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
How long after an LH surge do you ovulate?
Ovulation usually happens 24–36 hours after the LH surge begins.
Can you get pregnant on day 13 of your cycle?
Yes. In a 28-day cycle, day 13 is the day before ovulation and one of the days with the highest chance of pregnancy.
When should you have sex after a positive ovulation test?
Having sex on the day of the positive test and over the following 1–2 days gives you the best chance of pregnancy.
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