# Fertile Days Calculator: Complete Guide to Your Cycle and Ovulation
5 key facts about the fertile window
Ovulation occurs approximately 14 days before the start of the next period.
The fertile window includes the 5 days before ovulation plus the day of ovulation itself.
Sperm can survive up to 5 days in the female reproductive tract.
Irregular cycles reduce the reliability of calendar-based calculations.
Combining this tool with basal body temperature or LH tests increases accuracy.
# How the fertile window calculation works
The method is based on the Ogino-Knaus rule and the standard ovulation model. With a 28-day cycle, ovulation typically occurs around day 14. The calculator identifies this point and marks the days of highest fertility around it.
14 days Before the next period
5 days Fertile window duration
12-24 h Egg lifespan
# Phases of the menstrual cycle
Follicular phase
Begins with menstruation and lasts until ovulation.
- Ovarian follicles mature under the influence of FSH.
- Lasts between 10 and 14 days in regular cycles.
- Oestrogen levels rise progressively.
Luteal phase
Begins after ovulation and ends with menstruation.
- The corpus luteum produces progesterone to prepare the endometrium.
- Lasts approximately 14 days in normal cycles.
- If fertilisation does not occur, the corpus luteum degenerates.
Although the calculator uses a standard model, menstrual cycles can vary from month to month. For greater accuracy, combine this tool with basal body temperature tracking or ovulation tests.
# Biological signs of ovulation
- Changes in cervical mucus: during fertile days it becomes clear and stretchy, similar to raw egg white.
- Slight rise in basal body temperature (0.2–0.4 °C) immediately after ovulation.
- Mild pain or discomfort on the side of the active ovary (Mittelschmerz) during ovulation.
- Slight increase in libido near ovulation, driven by hormonal changes.
- Mild bloating or feeling of abdominal fullness during fertile days.
# Lifestyle and fertility
- Chronic stress can disrupt ovulation and lengthen or shorten the menstrual cycle.
- Body weight has a direct impact: both underweight and overweight can affect ovulation.
- Moderate exercise supports cycle regularity; extreme exercise can suppress it.
- Smoking reduces ovarian reserve and brings forward menopause by several years.
- A diet rich in folic acid, iron and antioxidants supports reproductive health.
# Cycle monitoring methods
| Method | Reliability | Cost |
|---|---|---|
| Calendar calculator | Medium | Free |
| Basal body temperature | High with practice | Low |
| Urine LH test | High | Moderate |
| Digital fertility monitor | Very high | High |
| Follicular ultrasound | Maximum | Clinic |
- Ovulation
- Release of the mature egg from the ovarian follicle, triggered by a surge in luteinising hormone (LH). It occurs once per cycle.
- Fertile window
- The period of highest likelihood of conception, covering the 5 days before ovulation plus the day of ovulation.
- Cervical mucus
- Secretion from the cervix whose texture varies throughout the cycle. During fertile days it becomes clear, stretchy and slippery, facilitating sperm passage.
- Basal body temperature
- Body temperature at complete rest, measured upon waking before any activity. It rises slightly (0.2–0.4 °C) after ovulation due to the effect of progesterone.