Day 1 of the menstrual cycle
In short: what happens on day 1?
Day 1 of the menstrual cycle is the day your period starts. Because no pregnancy occurred, progesterone and oestrogen have fallen and the lining of the uterus begins to shed. At the same time, the brain increases FSH release and small follicles in the ovaries start to wake up for a new cycle.
Day 1 of the menstrual cycle – animated explainer
Day 1: 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
Menstrual phase (period)
Chance of pregnancy
Very low
Uterine lining
4 mm
Follicle
5 mm
FSH
42
LH
18
Oestrogen
14
Basal body temperature
36.35°C
What is happening in your body today?
The muscles of the uterus contract to push out the lining; these contractions are caused by substances called prostaglandins. In the ovaries, small follicles (antral follicles) that are candidates for the next ovulation begin to grow.
Hormones
Oestrogen and progesterone are at their lowest point in the cycle. This drop signals the pituitary gland in the brain, and FSH starts to rise.
How you might feel
Lower abdominal and back pain, cramps, tiredness, bloating and headaches are common. Bleeding is usually heaviest in the first 2 days.
Chance of pregnancy
The chance of pregnancy is very low. However, unprotected sex during your period is not completely risk-free; sperm can survive for several days, especially if you have short cycles.
Tips for today
- Place a warm compress or hot water bottle on your tummy to relax the muscles
- Gentle walking and stretching can ease cramps
- Drink plenty of water and cut down on salt and caffeine
When to see a doctor
See your doctor if bleeding is heavy enough to soak a pad or tampon every hour, if you feel faint, if severe pain does not respond to painkillers, or if heavy bleeding follows a late period.
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
Is the first day of your period day 1 of your cycle?
Yes. Your cycle is counted from the day proper bleeding starts. Light spotting before your period is usually not counted as day 1.
Why is the first day of your period the most painful?
In the first days the uterus contracts more strongly to shed its lining. Prostaglandins, which drive these contractions, are at their highest level, so cramps are more noticeable.
Can you get pregnant on day 1 of your period?
The chance is very low but not zero. With short cycles or long periods, sperm can survive until an early 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