Pregnancy is possible on about six days of each cycle: the five days before ovulation and the day of ovulation itself. Sex every two to three days covers this window without needing exact timing, and ovulation kits and apps can help but have limits.
The cycle in brief
A menstrual cycle runs from the first day of one period to the day before the next. The first day of bleeding is day 1.
In adults, cycles commonly last somewhere between about 21 and 35 days. A cycle that varies by a few days from month to month is normal. What matters most for conceiving is whether an egg is released, which is called ovulation.
Very roughly, a cycle has two halves:
| Phase | What happens |
|---|---|
| Before ovulation (follicular phase) | A follicle in the ovary grows and the egg inside it matures. Rising oestrogen thickens the lining of the uterus. |
| Ovulation | A surge of luteinising hormone (LH) triggers the release of the egg. |
| After ovulation (luteal phase) | The hormone progesterone prepares the lining for a possible pregnancy. If there is no pregnancy, the lining is shed as a period. |
The phase after ovulation is fairly steady in length, often around two weeks. The phase before ovulation varies more. That is why ovulation is usually about two weeks before the next period, not always on day 14.
What the fertile window is
An egg survives for only about a day after it is released. Sperm, though, can survive in the reproductive tract for up to about five days.
So the fertile window is about six days long: the five days before ovulation, plus the day of ovulation. The chance of pregnancy is highest on the two or three days before ovulation and on the day itself.
Good to know. You do not need to find the exact day. Sex every two to three days through the cycle means there will usually be sperm waiting when the egg is released.
Signs that ovulation is near
Some people notice changes in their body around ovulation:
- Cervical mucus. Vaginal discharge often becomes clearer, wetter and more stretchy, like raw egg white, in the days before ovulation.
- Mild one-sided pain. Some people feel a twinge low on one side of the abdomen.
- Body temperature. Resting body temperature rises slightly after ovulation. This confirms ovulation only after it has happened, so it is less useful for timing.
Not everyone notices these signs, and that is fine.
Ovulation (LH) test kits
Ovulation predictor kits are urine tests sold at chemists. They detect the LH surge, which usually happens a day or so before ovulation. A positive result suggests the next day or two are likely fertile.
They have limits:
- Some people, including some with polycystic ovary syndrome (PCOS), can have raised LH levels at other times, which can give confusing results.
- A positive test shows the surge; it does not prove that an egg was released.
- Testing every day can become stressful. Use them only if they help you.
What period-tracking apps can and can't do
Apps are good for keeping a record of your period dates, which is useful for any doctor. But most predict the fertile window from past cycle lengths. They are estimates, not measurements. If your cycles vary, the prediction can be off by several days.
Our fertile window tool gives a similar rough estimate from your cycle length. Treat it, and any app, as a guide rather than a precise answer.
Keep it simple
It is easy for timing to take over. Many couples find that sex on schedule becomes a source of pressure for both partners, especially for the male partner. Guidelines suggest regular sex every two to three days rather than focusing only on the fertile days.
A few other points:
- Some lubricants can affect sperm. If you use one, you can ask a chemist for a sperm-friendly product.
- There is no good evidence that particular positions, or lying down after sex, make a difference.
- Take folic acid (400 micrograms a day) if you might become pregnant.
When to see a doctor
- Your periods are very irregular, come less often than about every 35 days, or have stopped. This can mean ovulation is not happening regularly.
- Your periods are very heavy or very painful.
- Ovulation kits never show a positive result over several cycles.
- You have been trying for 12 months (under 35), 6 months (35 to 39), or you are 40 or over.
Our guide on when to see a fertility doctor explains these timings. Severe pain, very heavy bleeding or fainting needs urgent care: call 112 or go to the nearest hospital.
Sources
This is general information, not medical advice. It can't account for your history. Please talk to a doctor about your own situation. Medical disclaimer
Severe pain, heavy bleeding or fainting? Call 112 or go to the nearest hospital.