Which Tests Can Tell You When You’re Most Fertile?

by Emma Wilkinson |

Taking your basal temperature and using ovulation predictor kits – just some of the ways to work out when your body’s ready to conceive

When you know you’re ready to try to conceive (TTC), you want paraphernalia, sex (obvs), kit… basically anything that helps you feel you are doing something about it. So no wonder we all love a good at-home piece of tech. Basal body temperature and ovulation predictor kits are the most popular. But do they work?

What is your basal body temperature?

This is essentially your temperature when you first wake up in the morning before you start doing any activity at all. Hormonal changes throughout the month cause this to vary by tiny amounts and regularly plotting it can help you pinpoint whereabouts in your cycle you are ovulating.

How do I take my temperature?

Firstly you need a basal thermometer which is more sensitive than the type you put on your forehead or in your ear when you have a fever and can be bought from pharmacies. Some come ready with graphs to plot your daily results on. For it to work you have to take your temperature at the exact same time each morning, when you have just woken up and before you have eaten or drunk anything.

Plotting the results

What you're looking for is a temperature spike of around 0.2-0.4 degrees C for three days in a row compared with the past six days. This suggests you have ovulated. You will need to do this for a few months before you start to spot a pattern and use that to predict when your next fertile period will be.


This method requires dedication and all sorts of factors could skew your results, such as forgetting to take your temperature one day, or being ill (yep, that includes hangovers) or taking painkillers such as paracetamol. Also it is relying on your cycle being very regular which may not be the case for many women.

Helen Kendrew, matron at the Bath Fertility Centre, says this is not a method that fertility experts would recommend. ‘Fifteen years ago we were saying this is old hat, it is not very reliable and just adds another layer of stress.’

Ovulation predictor kits

Ovulation predictor kits are more expensive than charting basal body temperature but are more accurate and some may find them easier to use. They measure a level of hormone called luteinizing hormone (LH) in your urine to tell you when ovulation is imminent and give you a 24-36 hour warning that it is the time to have TTC sex.

Again they are not ideal for those with an irregular cycle, as you may go through a lot of tests before hitting on that ovulation window. It’s also important that you don’t get too obsessed and stressed out by focusing on aiming on having sex at a specific time rather than just once or twice a week.

Mrs Kendrew adds, ‘Remember that over the counter predictor kits cannot tell you whether ovulation actually occurred, they just predict that it did.’

You can also work out when you're most fertile by using an ovulation calculator. Ovulation calculators work by working out where in your cycle you are and when you’re likely to ovulate – and therefore conceive.

Read next: A guide to natural contraception

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