We know from extensive scientific research that cervical mucus is essential for fertility. It protects and nourishes the sperm so they retain their fertilising capacity. It forms channels which help the sperm travel through your reproductive system to meet and fertilise the egg. And it acts as a filter, destroying imperfect sperm cells.

Following menstruation there will often be a number of days when you feel dry and don't see any discharge. This is called the Basic Infertile Pattern (BIP). We know from the work of scientists that the hormone levels are low at this time and the cervix is blocked by a thick plug of mucus which prevents sperm entering the uterus. This means that you're infertile at this time.

The first indication of potential fertility will be a change from the Basic Infertile Pattern (BIP). You will feel a change in the sensation that the mucus produces at the vulva. As the days pass you'll notice that the mucus becomes thinner and clearer, and the sensation becomes wet and then slippery. Women use different words to describe this changing, developing pattern, but the mucus will always have a wet, slippery quality because of its chemical structure and composition, even when there is too little to see.

Both clinical and laboratory studies have shown that the last day of the slippery sensation is the most fertile time in the cycle. It is called the Peak of fertility because it is the day when sex is most likely to result in a pregnancy. Studies show that ovulation usually occurs within a day of the Peak. It is important to realise that the Peak isn't necessarily the day of most mucus. The slippery, lubricative sensation may last a day or two after the visible signs of mucus, which means you're still highly fertile – the sensation is the more valuable symptom.

In a fertile cycle menstruation follows the Peak 11-16 days later unless conception has occurred. This interval of approximately two weeks is called the “luteal” phase.