Ovulation typically occurs 14 days before your next expected period. For a 28-day cycle this is day 14; for a 30-day cycle it is day 16; for a 35-day cycle it is day 21.

Fertile Window Calculator | Best Days to Conceive

Q: What are the signs of ovulation?

Common ovulation signs include a change in cervical mucus (becoming clear, slippery and stretchy, like raw egg white), a slight rise in basal body temperature, mild one-sided pelvic pain (mittelschmerz), increased libido, and a positive LH surge on an ovulation test.

Q: How long does it usually take to get pregnant?

For healthy couples having regular unprotected sex, about 84% will conceive within a year. For women under 35, the average is 3–6 cycles. Fertility declines with age, particularly after 35.

Q: Can I get pregnant with irregular periods?

Yes, but it is harder to predict ovulation. Conditions like PCOS can cause irregular cycles and anovulation (cycles without ovulation). Using ovulation predictor kits (OPKs) can help identify your LH surge regardless of cycle length.

Q: Does stress affect ovulation?

Yes. High levels of cortisol (the stress hormone) can suppress GnRH and LH, delaying or preventing ovulation. This is why many women notice cycle changes during periods of illness, travel, or significant life stress.

Enter the first day of your last period and your average cycle length to find your fertile window and best days to conceive.

Calculate Your Fertile Window

First day of your last period Average cycle length (days) Luteal phase length (days) — default 14

Your Fertile Window

Cycle Calendar

Period

Fertile window

Ovulation day

Best days to try: The 2 days before ovulation and the day of ovulation itself give the highest probability of conception in any given cycle.

Understanding Your Fertile Window

The fertile window refers to the days in your menstrual cycle when pregnancy is possible. This window is determined by two biological facts: an egg can only be fertilised for 12–24 hours after ovulation, and sperm can survive in the female reproductive tract for up to 3–5 days.

This means pregnancy is possible from intercourse on any of the 5 days before ovulation and on the day of ovulation itself — a total of 6 days. Intercourse on the two days before ovulation and on ovulation day itself gives the highest probability of conception.

How ovulation is timed: Ovulation occurs approximately 14 days before the start of your next period (the luteal phase). This means it is not fixed to cycle day 14 unless you have a 28-day cycle. For longer cycles, ovulation occurs later in the cycle.

Examples by Cycle Length

Cycle Length	Ovulation Day	Fertile Window
21 days	Day 7	Days 2–7
24 days	Day 10	Days 5–10
28 days	Day 14	Days 9–14
30 days	Day 16	Days 11–16
35 days	Day 21	Days 16–21

Signs of Ovulation

Your body gives several physical signals around the time of ovulation. Learning to recognise them can help you identify your fertile window even without technology.

Cervical Mucus Changes

The most reliable physical sign. Around ovulation, cervical mucus becomes clear, slippery and stretchy — often described as resembling raw egg white. This consistency helps sperm travel and survive.

Basal Body Temperature (BBT)

After ovulation, progesterone causes a small rise in resting body temperature (0.2–0.4°C). BBT charting helps confirm ovulation occurred — but only after the fact.

LH Surge (Ovulation Tests)

Ovulation predictor kits (OPKs) detect the luteinising hormone (LH) surge that triggers ovulation. A positive result usually means ovulation within 12–36 hours.

Mittelschmerz (Ovulation Pain)

Some women feel a mild, one-sided ache or twinge in the lower abdomen around ovulation time. This is called mittelschmerz and is caused by the follicle rupturing to release the egg.

Ovulation Tests Explained

Ovulation predictor kits (OPKs) are widely available at UK pharmacies and online. They work by detecting the LH (luteinising hormone) surge in urine. The LH surge occurs approximately 24–36 hours before ovulation.

Standard LH tests: Use a test line vs. control line — a positive is when the test line is as dark or darker than the control line.
Digital OPKs: Give a clear smiley face result, easier to interpret for beginners.
Cycle monitoring monitors: Track both LH and oestrogen, identifying up to 6 fertile days.
Best time to test: Between 10am–8pm, not first thing in the morning (unlike pregnancy tests).
PCOS consideration: Women with PCOS may get multiple positive OPK results due to elevated LH levels, making them less reliable.

Natural Family Planning (NFP)

Natural Family Planning (NFP) is a method of tracking fertility signs to either achieve or avoid pregnancy. In the UK, the Fertility UK charity trains qualified NFP teachers, and the Billings Method and Symptothermal Method are the most common approaches.

NFP involves charting three fertility indicators: cervical mucus, basal body temperature, and cycle length. When used correctly and consistently, NFP can be up to 99% effective at avoiding pregnancy, though it requires significant commitment and is less suited to highly irregular cycles.

Factors That Affect Ovulation

Stress: Elevated cortisol can suppress the hormones that trigger ovulation, causing delayed or missed cycles.
Illness: Even a common cold can delay ovulation in that cycle.
Significant weight changes: Both rapid weight loss and obesity can disrupt the hypothalamic-pituitary-ovarian axis.
Thyroid conditions: Both hypothyroidism and hyperthyroidism can cause irregular ovulation.
Polycystic Ovary Syndrome (PCOS): The most common cause of irregular or absent ovulation in women of reproductive age.
Approaching menopause: Perimenopause (typically starting mid-40s) causes irregular ovulation before periods cease.
Recent hormonal contraception: Fertility typically returns within 1–3 cycles after stopping the pill, though some women experience a longer delay.

PCOS and Irregular Cycles

Polycystic ovary syndrome (PCOS) affects approximately 1 in 10 women in the UK and is a leading cause of fertility issues. Women with PCOS produce higher levels of androgens (male hormones) and often have elevated LH levels, resulting in irregular or absent periods and anovulatory cycles (cycles without ovulation).

Options for women with PCOS who want to conceive include lifestyle changes (weight loss of even 5–10% can restore ovulation in overweight women with PCOS), clomifene citrate (Clomid) prescribed by a GP or fertility clinic, and in some cases letrozole or gonadotrophin injections.

If you have irregular cycles, have been trying to conceive for more than 12 months (or 6 months if you are over 35), speak to your GP about a fertility referral on the NHS.

How Long Does It Take to Conceive?

The chances of conception in any single cycle, even with perfectly timed intercourse, are only 15–25% for healthy couples. This is because not every embryo implants successfully. However, over time the cumulative probability is high:

After 3 months: approximately 50% of couples
After 6 months: approximately 75% of couples
After 12 months: approximately 84–85% of couples
After 24 months: approximately 92% of couples

If you have been trying for more than 12 months without success (or 6 months if aged 35 or over), the NHS recommends seeing your GP for initial fertility investigations, including a semen analysis and blood tests for ovarian reserve.

Frequently Asked Questions

What is the fertile window?

The fertile window is the 5 days before ovulation plus the day of ovulation itself — a total of 6 days. This window exists because sperm can survive in the female reproductive tract for up to 5 days, while the egg is only viable for 12–24 hours after release. The 2 days before ovulation and ovulation day itself are the most fertile days.

When do I ovulate?

Ovulation typically occurs 14 days before your next expected period (the luteal phase length). For a standard 28-day cycle this is day 14; for a 30-day cycle it is day 16; for a 35-day cycle it is day 21. If your cycle varies month to month, your ovulation date also shifts accordingly.

What are the signs of ovulation?

Common ovulation signs include: cervical mucus becoming clear, slippery and stretchy (like raw egg white); a slight rise in basal body temperature after ovulation; mild one-sided pelvic pain (mittelschmerz); increased libido; and a positive LH surge on an ovulation predictor kit. Not all women experience all signs.

How long does it usually take to get pregnant?

For healthy couples under 35 having regular unprotected sex, about 84% will conceive within a year. The average is 3–6 cycles. Fertility naturally declines with age, particularly after 35. Factors such as timing of intercourse, health, and cycle regularity all influence how quickly conception occurs.

Can I get pregnant with irregular periods?

Yes, but it is harder to predict ovulation. Conditions such as PCOS can cause irregular cycles and anovulatory cycles (no ovulation). Ovulation predictor kits can help detect the LH surge regardless of cycle length. If your periods are very irregular, a GP can investigate the underlying cause and discuss options.

Does stress affect ovulation?

Yes. High levels of the stress hormone cortisol can suppress GnRH (gonadotrophin-releasing hormone) and LH, delaying or preventing ovulation. Many women notice cycle changes during stressful periods — this is a well-documented physiological response and not a character failing.

How accurate are ovulation calculators?

Ovulation calculators work best for women with regular, predictable cycles. They are an estimate based on average luteal phase length (14 days). Actual ovulation can vary by several days due to illness, stress, or cycle variation. For more accuracy, combine a calculator with ovulation tests or BBT charting.

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Last updated: February 2026. This calculator is for informational purposes only and does not replace medical advice. If you have concerns about your fertility, consult your GP. Written by Mustafa Bilgic.