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Ovulation calculator: How to spot the signs of ovulation and find your fertile window

Knowing the signs of ovulation by using a fertility calendar can help you to work out the best time to get pregnant. Once you can spot the symptoms and know your cycle, you'll be able to identify the time when you're at your most fertile.

By Ruby Relton BSc MSc | Last updated May 23, 2023

Use our quick and easy calculator tool below to find out the days each month that you are at your most fertile.

Simply enter the date your last period started and the length of your menstrual cycle- this is the number of days from the start of one period to the day before the next one. The calendar below will highlight the dates when you're most likely to conceive.

Want to know more about increasing your chances of conception? Read on as Ruby Relton BSc MSc, Research Associate at Hertility Health lays down the facts. 

What is Ovulation?

Ovulation is the point in your cycle when a mature egg is released from your ovary and into your fallopian tube - ready to be fertilised by a sperm.

What is the fertile window?

Your fertile window is the time in your cycle where you have the highest chance of conceiving. It's a six day window that includes the five days leading up to ovulation and the day of ovulation itself.

Once the egg has been released from your ovaries, it only survives for around 12-24 hours. However, the sperm can survive  for up to five days, so it’s possible that the egg can be fertilised as soon as it is released. The American Society for Reproductive Medicine (ASRM) states that 'highest pregnancy rates are seen when couples have unprotected intercourse every 1-2 days during their fertile window.'

How can I tell if I’m ovulating?

Contrary to what some biology textbooks may tell you, most women don’t ovulate like clockwork exactly 14 days before their period. Whilst it is often true that ovulation occurs halfway through your menstrual cycle, it’s important to remember that not everyone’s cycles are 28 days long.

Cycles can vary in length from person to person and even from one cycle to another. Anything from a 21-day cycle to a 35-day cycle is normal.

Did you know? A recent study of over 600,000 menstrual cycles showed that only 13% of those who took part had the 'average' 28-day cycle length. 

If you have a 35-day cycle for example, ovulation is more likely to occur between day 18-20 than on day 14.

How can I determine when my fertile window is?

One way you can identify your fertile window is by using our ovulation calculator and tracking your cycles. If your cycle is fairly regular, simply input the date your last period started and the length of your cycle into the calculator and it will tell you when your most fertile days are most likely to be. But if your cycles are sometimes irregular, don’t despair, there are other ways you find out if you’re ovulating, here are a few to consider:

1. Try an ovulation kit

Ovulation kits can be purchased at a pharmacy or online. These are used to detect a surge in the hormone called Luteinising Hormone (LH). The LH surge occurs around 24-36 hours prior to ovulation and is what triggers ovulation. So, when you chart your results, it will essentially be able to tell you if ovulation has happened in that cycle. Taking it is similar to how you would take a pregnancy test, you pee on the test strip portion of the ovulation kit at the same time every day after you've finished your period to check your LH levels.

2. Track your Basal body temperature

Your menstrual cycle can affect your body temperature in a biphasic manner, that means it changes with the two phases of your menstrual cycle - the follicular phase and the luteal phase. After ovulation, your body temperature can rise by up to a degree (this is because of progesterone release). It’s difficult to detect this in a standalone measurement so you have to measure your temperature every morning to detect a rise.

Read next: How to measure your Basal body temperature

3. Monitor your cervical mucus

Throughout your menstrual cycle, the changes in your hormone levels affect your cervical mucus. As your oestrogen levels rise, your body produces more cervical mucus and the consistency changes.

Around 1-2 days before you ovulate your cervical mucus becomes stretchy and a  similar consistency to egg whites so that it is easier for the sperm to move towards the egg. Having sex on a day when your cervical mucus is like this, is associated with the high pregnancy rates in comparison to the dry mucus seen at the beginning and end of your cycle.

4. Ovulation pain

Some women experience pain around the time of ovulation or Mittelschmerz. Although this doesn’t happen to everyone or in every cycle, people usually report experiencing it as a one-sided, lower abdominal pain. It shouldn't be overly painful, so if it feels like more than mild discomfort, you should visit your GP.

5. Saliva ferning pattern

The rises in oestrogen levels can affect the concentrations of electrolytes in your saliva. By changing the levels of sodium chloride and potassium in your saliva, the fern or pattern it makes on a slide changes. There are specific saliva ferning kits, similar to LH-based ovulation kits that help you spot the distinct ferning pattern associated with ovulation.

Before you start tracking your ferning patterns though, it's worth knowing that ferning shouldn’t be used on its own to detect ovulation as it can be affected by many things such as smoking, brushing your teeth or the foods you eat. A small study in 2020 showed that this actually had 53.7% sensitivity, although when used together with basal body temperature it could be a powerful indicator of ovulation.

6. Listen to your body

There are other signs of ovulation that are not specific to ovulation as well such as tender breasts, bloating, spotting and increased sexual desire. These all occur as a result of the fluctuations in your hormone levels that occur with your menstrual cycle. For example, in the days leading up to ovulation as oestrogen levels rise and trigger the eventual release of LH it's common to notice an increase in sexual desire - just another way your body is telling you it's time to start trying.

As you begin tracking your cycles you may spot more tell-tale signs that you're ovulating and begin to get a greater awareness of the subtle ways your body communicates with you.

What can affect ovulation?

Sometimes lifestyle factors, such as a particularly stressful month at work or changes to your weight or exercise routine can affect your cycle regularity by directly impacting ovulation. This means it is extremely important to track your cycles and look out for the signs of ovulation to be able to time your attempt at conception accurately.

If you notice any irregularities after three months or if your cycles occur more often than 21 days or less often than every 35 days, you may want to seek advice from your GP. Hertility recommend taking a hormone and fertility test - to help give you an insight into the inner workings of your cycle. 

How does knowing your hormones help you identify your fertile window?

Alongside knowing the signs you are ovulating, knowing your hormones can be extremely useful for calculating your fertile window.

Collectively, knowing the signs of ovulation, tracking your cycle and PMS symptoms and your hormones can help you learn so much about your body and reproductive health. For example, measuring hormones like Anti-mullerian hormone (AMH) can give you an indication of your ovarian reserve i.e. how many eggs you have left. If your tracking data shows your cycles are very irregular, doing a hormone and fertility test can reveal if you have any hormonal imbalances or reproductive health conditions like PCOS (there are even specific kits targeted at detecting PCOS) or thyroid dysfunction which may be stopping ovulation from happening. Learning this information can provide you with actionable insights to not only help you try to conceive but also to improve your overall health.

Should I only have sex during my fertile window?

If you’re trying to get pregnant, the NHS recommends that you should be having sex throughout your cycle in order to increase your chances.  Every other day is the optimum. However, the best time to conceive is in the two days before ovulation (usually days 12 to 14 if you have a 28 day cycle) in preparation for the egg being released. If you use lubricant, you may want to consider switching to a sperm-friendly option while you're trying to get pregnant.

Read next: Ready to test? Discover the best pregnancy tests as rated by Mumsnetters and maternity experts.

How can Hertility Health help?

This article was written in collaboration with Hertility Health and was reviewed by Dr Tharni Vasavan Bsc (Hons), MSc, PhD. Hertility Health are on a mission to make information about female health accessible and affordable through at-home hormone and fertility fitness tracking, consultancy and care.

If you're experiencing any symptoms or want to understand where your fertility and reproductive health is currently at, Hertility Health can help you get the answers you deserve. The Hertility Health journey includes a tailored at-home hormone test, fertility and gynaecology triage service, digital personalised results, access to highly-rated experts and clinics, educational content and a community of care. They're here to support you throughout your reproductive journey, from menstruation to menopause.


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