Day 18 of the menstrual cycle
In short: what happens on day 18?
Day 18 of the menstrual cycle is when the effects of progesterone become more noticeable. The lining of the uterus becomes nourishing enough for a fertilised egg to attach. Under the influence of progesterone and oestrogen, some people start to notice breast tenderness and bloating around this time.
Day 18 of the menstrual cycle – animated explainer
Day 18: 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
13 mm
Corpus luteum
23 mm
FSH
20
LH
21
Oestrogen
51
Basal body temperature
36.72°C
What is happening in your body today?
If fertilisation has occurred, the embryo reaches the uterus and gets ready to implant. The corpus luteum approaches its most active stage.
Hormones
Progesterone rises quickly; oestrogen rises for a second time.
How you might feel
You may notice fuller breasts, mild bloating and constipation.
Chance of pregnancy
The chance of pregnancy is very low.
Tips for today
- Wear a supportive, comfortable bra
- Fibre-rich foods and water help prevent constipation
- Cutting down on caffeine may ease breast tenderness
When to see a doctor
See your doctor if you notice a breast lump, nipple discharge or persistent pain on one side.
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
Why do your breasts hurt before your period?
In the luteal phase, progesterone and oestrogen stimulate the milk ducts and glands, which causes fullness and tenderness.
Is breast tenderness a sign of pregnancy?
It can occur both before a period and in early pregnancy; on its own it cannot tell the two apart.
When does the embryo reach the uterus?
The embryo reaches the uterus about 3–4 days after fertilisation; implantation usually happens 6–10 days after ovulation.
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