What is ovulation?
Ovulation occurs when a mature egg is released from a follicle in one of the ovaries. The egg then travels along the fallopian tube toward the uterus. The egg itself is viable for fertilisation for roughly 12 to 24 hours after release. Because sperm can survive in the reproductive tract for up to five days, the combined fertile window extends over approximately six days: the five days leading up to ovulation plus the day of ovulation itself.
You can estimate your likely fertile window using our ovulation calculator, though tracking your own signs gives the most personalised picture.
The fertile window
The fertile window refers to the days in each cycle when pregnancy is biologically possible. Research suggests the probability of conception is highest in the two to three days immediately before ovulation and on the day itself, with probability declining sharply after the egg has been released.
Because ovulation timing can shift from cycle to cycle — even for people with regular cycles — the fertile window does not fall on the same calendar dates each month. This is why tracking ovulation signs, rather than relying solely on cycle-day averages, tends to be more informative.
Sign 1: Changes in cervical mucus
Cervical mucus changes in texture and appearance throughout the cycle in response to oestrogen. In the days following a period, mucus is typically scant or absent. As ovulation approaches, it tends to become more abundant, progressively clearer, and increasingly stretchy and slippery — often described as resembling raw egg whites. This texture is sometimes called "fertile-quality" mucus because it creates an environment that supports sperm survival and movement.
After ovulation, mucus typically becomes thicker, stickier, or drier again under the influence of progesterone. Tracking daily changes over multiple cycles helps establish your own pattern.
Sign 2: Basal body temperature (BBT) shift
Basal body temperature is the body's resting temperature, measured first thing in the morning before any activity. After ovulation, progesterone causes a small but measurable rise in BBT — typically around 0.2 to 0.5 degrees Celsius (0.4 to 1.0 degrees Fahrenheit). This shift usually persists for the remainder of the luteal phase.
Because the temperature rise confirms ovulation has already occurred rather than predicting it, BBT charting is most useful over multiple cycles to identify your typical ovulation timing. A consistent thermal shift, when combined with cervical mucus observations, helps confirm ovulation has taken place.
Sign 3: LH surge and ovulation tests
Luteinising hormone (LH) surges approximately 24 to 36 hours before ovulation, triggering the release of the egg. Over-the-counter ovulation predictor kits (OPKs) detect this LH surge in urine. A positive result typically indicates that ovulation is likely within the next one to two days, making it one of the most practical advance indicators available.
LH tests are straightforward to use but work best when tested at consistent times of day (mid-morning is often recommended) and on consecutive days around the expected fertile window. Some medical conditions and medications can affect LH readings, which is worth noting when interpreting results.
Sign 4: Ovulation pain (mittelschmerz)
Some people experience a mild, one-sided twinge or ache around the time of ovulation, on the side of the ovary releasing the egg. This sensation, sometimes called mittelschmerz (German for "middle pain"), can range from a brief flutter to a cramp lasting a few hours. It is not experienced by everyone, and its absence does not indicate ovulation has not occurred.
Other possible signs
Some people also notice light spotting, breast tenderness, a heightened sense of smell, or a subtle increase in libido around the time of ovulation. These signs are variable and less consistent than the indicators above, but some individuals find them useful as additional data points when tracked alongside other methods.
Timing varies cycle to cycle
It is worth emphasising that ovulation timing — and therefore the fertile window — can shift meaningfully from cycle to cycle. Stress, illness, significant changes in sleep or weight, and many other factors can influence when ovulation occurs in any given month. This variability is one reason healthcare providers generally recommend tracking signs across multiple cycles before drawing conclusions about your typical pattern.
For a fuller picture of the menstrual cycle and how each phase fits together, see our guide to understanding your menstrual cycle.
Disclaimer
This guide is for informational and educational purposes only and is not medical advice, a diagnosis, or a substitute for professional care. Menstrual cycles, fertility, and pregnancy vary significantly between individuals. If you have concerns about your cycle, fertility, or pregnancy, consult a gynaecologist or healthcare provider.
Track this in OptiAI
Use OptiAI to log cycle signs, track your fertile window, and view personalised trends over time — helping you understand your unique cycle patterns.