Believe it or not, there's only a very small window each month when you can actually conceive, typically no longer than four days. This time, when you are most fertile, leads up to ovulation. Each month, 15 to 20 eggs mature inside one of your ovaries. The largest is pushed out into the pelvic cavity and swept into the fallopian tube. Simply put, ovulation is the process that results in the release of one or more eggs from your ovary. 

Your egg survives for only 12 to 24 hours if it is not fertilised. While it is possible for sperm to survive inside you for up to seven days, the healthiest sperm with best motility (moving swiftly in a straight line) results from intercourse every three to four days. That's why, to increase the likelihood of conception, you need to ensure that you have sex sometime in the few days leading up to ovulation - your "fertile window".

One way to approach this is to make sure that you have sex a couple of times a week throughout your cycle. It means there's a better chance that a healthy supply of sperm will be waiting in the fallopian tube whenever the egg is released. However, sex every few days - just in case - may not be possible or practical for all couples. 

Ovulation kits can help you to identify your fertile window, but there is no evidence that using them will improve your chances of success when it comes to getting pregnant naturally. Experts recommend regular sex throughout the cycle as the way to maximise your chances of conception. However, they recognise that some couples prefer to identify when they are most likely to conceive. There are a number of reasons why women may choose to try to predict their fertile period: 

• If one or both partners frequently work away from home, scheduling sex during the fertile period could make all the difference. 
• Hectic schedules and caring for existing children mean that sex is less of a priority; having sex every two to three days can feel like just another chore. 
• An irregular cycle can make it more difficult to conceive - sex every few days throughout the cycle is ideal in this situation but, after a few months of not conceiving, it can become stressful. 
• Suspicions that ovulation is not happening at all can be investigated in the privacy of your own home before you seek help. 

There are all sorts of ways to determine your fertile window. You could record yourmenstrual cycle, chart the monthly cycles of your basal body temperature or study changes in your cervical mucus . But it can be a bit tricky to pinpoint ovulation using these methods, especially if your menstrual cycles are irregular. This is where ovulation predictor kits (OPKs) can help.