Fertile window calculator: how to find your ovulation day, and read mucus and LH tests

Your fertile window is the five days before ovulation plus ovulation day itself, and you can work it out from your cycle length: ovulation usually happens about 14 days before your next period starts. SGH's example is a 26-day cycle, which means ovulating on day 12, so the fertile days run from day 7 to day 12. Our ovulation and fertile window calculator does the maths from the first day of your last period and shows your next six cycles. A calculator gives an estimate, so pair it with your body's signs: cervical mucus turns thin and slippery just before ovulation, and an LH test turns positive a day or so before the egg is released. KKH suggests sex every one to two days in the fertile window, and seeing a doctor if you're not pregnant after a year, or sooner from age 35.

How a fertile window calculator works

The calculator counts back from your next expected period. Most women release an egg about 14 days before a period, and the NHS puts the gap between ovulation and a period at around 10 to 16 days, which is why our calculator lets you change that number (the luteal phase) if you know yours. Ovulation day is the most fertile day, and the five days before it count because sperm can survive that long.

Day 1 is the first day of full bleeding. These assume a 14-day luteal phase. The American College of Obstetricians and Gynecologists (ACOG) counts one more day: you can get pregnant from five days before ovulation until one day after, because the egg survives about 24 hours.

NUH says a normal cycle lasts 21 to 35 days. KKH's obstetricians note that ovulation on day 14 only fits a regular 28-day cycle; if your cycle is longer or shorter, it happens at a different time, which is what the table shows.

If your cycles are irregular

A calculator struggles when cycle length jumps around, because it predicts from past dates. A SingHealth fertility specialist says irregular periods are a clue that ovulation may be variable or missing: keep having regular sex, but get fertility advice if you're not pregnant after six months of trying. Body signs such as mucus and LH tests help more than dates in the meantime.

Cervical mucus: the free daily sign

ACOG describes the change. Just before ovulation, your cervix makes noticeably more mucus, and it turns thin and slippery, often compared to raw egg white. Just after ovulation, there's less of it and it becomes thicker. Slippery, stretchy mucus is your signal that the fertile window is open, and it shows up before the egg is released, which is when sex counts most.

Check it when you go to the toilet, on your fingers or on toilet paper, and note it alongside your period dates. ACOG warns that some things change how mucus looks and make it hard to read: medicines, feminine hygiene products, douching, sex, breastfeeding, and lubricant from a pelvic exam.

Ovulation tests (LH strips): what they show and what they miss

An ovulation test picks up the surge of luteinising hormone (LH) in your urine that comes before the egg is released. KKH's obstetricians say the egg is released about 36 hours after the surge, and MedlinePlus, the US National Library of Medicine's health site, says a positive result means you should ovulate in the next 24 to 36 hours, though not for every woman. Start testing three to five days before you expect to ovulate; in a 28-day cycle that's around day 11. Strips and digital tests are sold over the counter at pharmacies.

What an LH test can't tell you:

• That an egg was actually released. It shows LH is high, nothing more.
• That you'll get pregnant. The US FDA describes it as a yes-or-no test of hormone levels.
• When you're safe from pregnancy. The FDA says it isn't reliable as contraception.
• Your surge, if you skip a day or your cycles are irregular. MedlinePlus says both can make you miss it.

Things that throw it off: drinking a lot of fluid beforehand dilutes your urine, the fertility drug clomiphene raises LH, and false positives happen occasionally. An ovulation strip isn't a pregnancy test either; our guide on when to take a pregnancy test covers that. The FDA says these tests detect the hormones reliably about 9 times out of 10 if you follow the instructions carefully. If you never see a surge, or you're not pregnant after several months of using kits, MedlinePlus suggests talking to your doctor.

Basal body temperature

Your resting temperature rises slightly after ovulation, by about 0.5 to 1 degree Fahrenheit according to ACOG. To track it, take your temperature every morning after waking, before you get out of bed, eat or drink anything, and chart it.

The catch is timing. ACOG points out that a temperature chart shows ovulation only after it has happened, and that your most fertile days are the two or three before the rise. A fever, broken sleep or a different waking time throws the reading off. Charting over a few cycles shows your pattern, which helps you set the calculator's luteal phase or spot cycles where you may not be ovulating.

Signs that you're ovulating

• Clear, slippery, stretchy cervical mucus
• A positive LH test
• A small rise in basal temperature, seen only after ovulation
• A mild one-sided twinge low in your tummy, sometimes called mittelschmerz
• A slight rise in sex drive, or breast tenderness, for some women

The mucus change and a positive LH test are the most useful for timing, because both come before the egg is released.

Timing sex for the best chance

KKH suggests sex every one to two days during the fertile window. SGH's fertility centre suggests trying throughout the cycle at least every third day, and says there's no benefit in holding back to "build up" sperm. A SingHealth specialist adds that regular sex, whatever the ovulation day, is often easier than trying to pinpoint it.

• Focus on the two to three days before your likely ovulation, plus the day itself.
• Every other day works if daily feels like too much.
• You don't need to lie with your legs up afterwards.
• Keep it relaxed. A rigid schedule adds stress for many couples.

KKH says a healthy couple both under 40 have about an 80% chance of pregnancy within a year of regular sex, so a few months without success is normal. KKH also recommends 400 mcg of folic acid a day from before conception to 12 weeks; see our guide to prenatal vitamins. For the bigger picture, read trying to conceive in Singapore.

Ovulation myths

• Everyone ovulates on day 14: only some 28-day cycles do.
• You can get pregnant any day: only in the fertile window, though ovulation can shift, so no day is guaranteed safe.
• A regular period proves you ovulate: usually, but it's possible to bleed without releasing an egg.
• More sex after ovulation helps: once the egg's day has passed, that cycle's chance has closed.
• Period apps are always accurate: they predict from past dates and are often off for irregular cycles.

When to see a doctor in Singapore

KKH advises seeing a doctor if:

• You're not pregnant after a year of regular sex without contraception
• You're 35 or older, in which case go earlier
• You have a medical condition that might affect fertility

See a doctor sooner, whatever your age, if your periods have stopped for more than three months and you're not pregnant, if bleeding is very heavy or comes between periods or after sex, or if period pain is severe and painkillers don't help, all of which KKH lists. NUH says about 1 in 6 couples have difficulty conceiving, so asking for help is common.

You can book a fertility consultation through KKH Central Appointments on 6294 4050, see a gynaecologist, or ask a polyclinic for a referral. Our guide on how to choose a gynae in Singapore helps, and IVF and fertility treatment in Singapore explains what comes next if you need it.

Frequently asked questions

How do I calculate my fertile window?

Take your usual cycle length and subtract 14 to get your likely ovulation day. Your fertile window is that day and the five days before it. In a 28-day cycle that's days 9 to 14. Our ovulation calculator does it for you.

How long does the fertile window last?

About six days: the five days before ovulation and ovulation day. ACOG adds the day after, because the egg survives about 24 hours.

Can an ovulation test confirm I ovulated?

No. It shows the LH surge that usually comes before ovulation, not that an egg was released. A rise in basal temperature afterwards gives a better idea, and your doctor can check further if you're worried.

What does fertile cervical mucus look like?

More of it, thin, clear and slippery, a bit like raw egg white. ACOG says that after ovulation there's less of it and it turns thicker.

My cycles are irregular. Can I still find my fertile window?

Yes, but lean on mucus and LH tests rather than dates; our guide to the early signs of pregnancy helps once your period is late. A SingHealth specialist suggests getting fertility advice if you're not pregnant after six months of trying with irregular periods.

Is a 28-day cycle necessary to get pregnant?

No. NUH says normal cycles run from 21 to 35 days. What matters is that you ovulate.