How to calculate ovulation and identify your fertility window

Knowing when you are ovulating is key to optimising your chances of getting pregnant, but do you know when, or how many days in a given month, this is likely to be? (Spoiler: it’s not many). To maximise your chances of conceiving, discover how to calculate your ovulation days, pinpoint your ‘fertility window’ more accurately and look for the physical signs you are ovulating…

Given that many women spend a large percentage of their reproductive years trying not to get pregnant, it’s perhaps ironic that’s there’s actually only a very short window of opportunity in a given month when a woman can conceive. Understanding the role ovulation plays in whether you are more, or less, likely to become pregnant can help you to identify the times of the month you are more likely to fall pregnant and better understand your hormonal and reproductive health generally. Put simply, a woman’s chances of becoming pregnant without ovulating are zero (without an egg to be fertilised by sperm conception cannot happen) and trying to establish a healthy and consistent pattern of ovulation is likely to improve your chances of conceiving. This is how and why:

Ovulation is when a mature egg (ovum) is released from one of your ovaries during your menstrual cycle. This cycle is controlled by your hormones and there are four key phases:

Follicular: when the brain releases a follicle-stimulating hormone (FSH) which helps eggs grow in little sacs (known as follicles) on the ovaries.
Ovulation: when the mature egg is released from the ovaries.
Luteal: when the egg leaves behind its ‘shell’ known as the corpus luteum which begins to produce the hormone progesterone to help the lining of the uterus mature in preparation for a potential pregnancy. If pregnancy doesn’t happen the uterus lining breaks down resulting in a menstrual bleed.
Menstrual: when blood and tissue are released during your period/menstrual bleed.

Ovulation typically happens around day 14 of a typical 28 day menstrual cycle.¹ This is an average, however, and just as all women are different so are their menstrual cycles which differ from woman to woman, and often from month to month. Most are typically between 23 and 35 days and ovulation can occur anytime around 12-16 days before your next period. What happens as you approach ovulation is that your body produces increasing amounts of oestrogen which triggers a sudden increase in another hormone called luteinising hormone (LH). The main job of LH is to make ovulation happen and release the most mature egg. Ovulation normally occurs 24-36 hours after a surge in LH and identifying this LH increase can help to pinpoint when you are likely to ovulate and can be a good predictor of healthy fertility.

Peak fertility time

If you are having regular unprotected sex, you can then become pregnant if the egg is fertilised by sperm. Sperm can live for around seven days inside your body, so if you’ve had unprotected sex in the five days before ovulation, sperm should be ready to leap into fertilising action when the egg is released. The egg can only be fertilised for up to 24 hours after ovulation – if it isn’t, the egg is reabsorbed into the body, and the lining of the womb is shed in the form of your menstrual period.

You are at your most fertile the day before you ovulate and the day of ovulation and these are known as your peak fertility days. The week or so around when the egg is released is often referred to as your ‘fertile window’ – a six day window of opportunity for when you are most likely to conceive. Put simply, if you can plan/time sex during peak ovulation time you boost your chances of conceiving.

What’s the difference between ovulation and the fertility window?

Ovulation is the specific moment in your menstrual cycle when a mature egg is released from the ovary and propelled quickly into the Fallopian tube. While an egg survives for up to 24 hours after ovulation, sperm can remain active for up to five days which is why it is perfectly possible for a couple to conceive four to five days before an egg is even released. The total ‘fertility window’, considering the maturation and lifetime of sperm and egg is around six days. Your fertility window or fertile days are the ones during your menstrual cycle when you have the ability to become pregnant after unprotected sex. Why this is significant when you are trying to conceive is that outside of this ‘fertility window’ your chances of getting pregnant are relatively low.

In summary, your fertility window is the timeframe you need to consider in order to maximise your chances of conception, whereas ovulation is a single event within that window when the egg is released.

How to calculate ovulation based on your menstrual cycle

When you know your average cycle length – and using a period tracking app will help with this - you can work out more accurately when you are likely to ovulate. As a general rule of thumb, a woman with a 28 day cycle ovulates on day 14 although this can vary according to every woman’s unique menstrual cycle. Keep track of your cycle on paper or using an app for at least two or three months to establish a pattern. Most women ovulate 14 days before their period begins, regardless of their typical cycle length.

Signs of ovulation to look out for:

Whilst not a cast-iron guarantee of predicting ovulation, being aware of, and recognising, what is happening to your body around the middle of your cycle can also help to build up a more 3D picture that can suggest you are ovulating. Many women report the following during ovulation:

breast tenderness and pain
bloating
mild abdominal or pelvic pain on one side of the lower abdomen (this can be either a dull ache or a sudden twinge). Ovulation pain may happen when an egg bursts from a follicle. This can also sometimes cause light bleeding or ‘spotting’.
mood changes due to hormonal fluctuations.
changes in libido – when you ovulate the hormones produced by the ovaries can increase your sex drive. This increase in libido may suggest your fertility is at its highest.
Other signs of ovulation include:
changes to vaginal secretions – discharge is likely to become thinner, slicker, more ‘slippery’ and ‘stretchy’, often described as appearing like raw egg white.
minor changes to basal body temperature (BBT) – your BBT rises slightly after ovulation.

How to track and predict ovulation

There are a range of methods for tracking and predicting ovulation but since each method has its benefits and drawbacks it’s best to use at least more than one to establish your most likely accurate ovulation time. Whilst none can offer a cast-iron guarantee of when, or if, you will conceive these tools, apps and smart devices can help to provide a really useful estimate of your fertile window and include:

The calendar method - using this method you analyse six months of menstrual cycles to help determine when you might be ovulating. To calculate possible ovulation, you take your shortest and longest cycle over the six month period and subtract 18 days from your shortest cycle and 11 from your longest. These two numbers should establish your most fertile days. So, for example, if your cycle lengths are 31 and 18 your fertile time is predicted to be between days 10 to 20 of your cycle.

Pros and cons - this is a cheap method (there are many free apps that help you record your menstrual cycle) and is fairly straightforward but it can be time consuming. It also doesn’t consider any other personal details such as your age, medical history, symptoms throughout your menstrual cycle and other physical indicators such as breast tenderness and pelvic pain (see above) that may suggest ovulation.

Taking your basal body temperature (BBT) – BBT is the waking temperature of your body before doing any activity. Taking your temperature every morning (ideally at roughly the same time) before you get out of bed (and before have eaten or drunk anything) and recording it over a few cycles can help to confirm if ovulation has taken place.² You can use a smart thermometer that will log the details into an app for you. After a few months you should see a pattern of temperature highs and lows emerge and you can use that information to predict when you are most likely to ovulate next. Ovulation has occurred after you have seen a slight but sustained rise in temperature of about 0.5 -1 degree Fahrenheit for at least three days. You are most fertile in the days before this temperature rise.

Pros and cons - It can be empowering to feel more in control of what is happening to your body, but it can be difficult to remember to take your temperature first thing every morning and/or to not to eat or drink. It has also long been recognised that many women ovulate without a clear rise in temperature.³ Many things can also affect your BBT, including stress, illness, alcohol or changes to your sleep pattern so a raised BBT doesn’t automatically equate to a released egg.

Cervical mucous monitoring - tracking daily changes in consistency (either jot them down or use a tracking app) of vaginal secretions can be a useful predictor of ovulation.⁴ When ovulation is approaching mucous becomes clearer, slippery and stretchy. This consistency makes it easier for sperm to swim up to meet your egg and indicates you are likely at your most fertile. Try to monitor yours for at least a few menstrual cycles to help identify any patterns.

Pros and cons – whilst this can be helpful to get you to understand the physical changes that occur around ovulation it is not an exact science. Plus, not all women feel comfortable doing it.

Ovulation predictor kits (OPKs) – these generally test your urine to detect the luteinising hormone (LH) surge that takes place 24-36 hours before ovulation. A positive test result should mean you are about to ovulate (usually within a 36 hour window).

Pros and cons - OPKs tend to be more accurate than calendar and temperature methods but there is a danger that some factors can cause a false positive test. For example, women going through perimenopause may see their levels of LH rise, without ovulation, as their ovaries start to deplete of eggs. Some fertility medications which stimulate the ovaries to trigger ovulation can also affect test results. Testing too early the morning may also influence the test as any surge in LH tends to begin in the early morning but may not show up in your urine until later. Drinking too much fluid can also dilute your urine so the test doesn’t accurately pick up on your LH levels. There has also been some criticism that women who use home ovulation tests to time when they have sex to increase their chances of becoming pregnant may become more ‘fertility stressed’ than women who don’t use them - but a study found that not to be the case.⁵ In fact, research suggests that ovulation kits are usually around 99% accurate⁶ and in one study women who used an OPK were shown to be increasingly likely to become pregnant within two menstrual cycles.⁷

Fertility tracking apps – these calculate ovulation using algorithms that combine your cycle length with other important fertility details such as BBT, cervical mucous and ovulation test results.

Pros and cons – These apps can help you to be more aware of your menstrual cycle and provide a more ‘rounded’ picture of your fertility to help identify your fertility window. They can also make you more body aware and understand the physical signs you might be ovulating. However, apps relying on algorithms and user data don’t take into account the effects of things that are unique to you such as stress, illness and medication which might have an impact on accuracy. Constant tracking may also lead to anxiety in some.

Ovulation calculator- there are a range of these found free online and are a form of the calendar method where you enter the first day of your last period, along with details of how long your usual menstrual cycle is and it quickly calculates when you are likely to ovulate.

Pros and cons - Most women do not conform to a 28 day cycle and the timing of ovulation can vary by a few days even in women who have a 28 day cycle that runs like clockwork. Because of the variability of menstrual cycle length, the reliability of calendar based apps has been called into question.⁸

Hormone monitoring devices – these can include wearable tracking devices or taking sweat⁹ or saliva tests to track levels of two key fertility hormones - oestrogen and luteinising hormone (LH)- in order to identify your fertility window and let you know when your chances of conception are highest.

Pros and cons – these can be costly and may have the potential to cause ‘fertility stress’ in some women due to the daily testing and focus on results. They may not always be reliable for women diagnosed with certain conditions. However, when used correctly these devices tend to be highly accurate and in one research study women using a type of fertility monitor were shown to increase their chances of getting pregnant after two menstrual cycles compared to women who didn’t use a monitor.¹⁰

FAQs

Q. How do I actually know if I am ovulating every month?

If you have a menstrual cycle that usually occurs between 21 to 35 days and experience symptoms of breast tenderness, bloating and/or pelvic/lower abdominal pain (see above) you are probably ovulating. However, if your cycles are frequently unpredictable or over 35 days you might not be ovulating consistently – if at all. Other common reasons for this (unless, of course, you are pregnant) can include a medical condition, a genetic condition that causes hormonal imbalances which can interfere with regular or consistent ovulation; being in perimenopause (so you might not ovulate on a monthly basis) or menopause, having low thyroid function and amenorrhea (when your periods have stopped possibly due to being overweight or underweight, severe stress or over-exercising). It’s also important to speak with your GP, if you have any concerns about your cycle.

Q. How many days during ovulation can I get pregnant?

Even though your egg only lives for up to 24 hours, sperm can live in your uterus for three to five days. This means you can get pregnant from about five days before ovulation to one day after it – that means you are most likely to conceive during this six day ‘fertility window’.

The ovulation takeaway

Whilst there are some things you can’t control when it comes to ovulating (such as your age, genes and medical history) there are others that can help to improve your chances of healthy and consistent ovulation including maintaining a healthy weight; exercising regularly (but not excessively); eating a nutritious diet and effectively managing stress. Helping to improve the quality of your and the father’s sperm should also help. Understanding more about ovulation and using the available apps, digital tools and devices to identify the times when ovulation is more likely to happen should also help you to feel more empowered and in charge of your hormonal and reproductive health generally.