Dossier: Planning a family

Falling pregnant: How to calculate your ovulation

A woman’s ‘fertile window’ is short, lasting just six days per cycle. What’s more, the probability that you’ll fall pregnant isn’t the same on all days. Do you know how long your cycle is and when exactly your next ovulation is due?

If you bet on pregnancies, you wouldn’t stand a very good chance of winning, even though sperm can survive in the woman’s body for between two and five days. Studies indicate that the statistical probability of falling pregnant is between 20 to 30% per cycle – and only on three of the six fertile days (i.e. the three days that are closest to ovulation). Four days before, the chances of conceiving are only 10 to 12%, while five or six days prior to ovulation the probability falls to between 0 and 5%.

This brings us to the critical question: when exactly do I ovulate? Anyone who is trying to start a family and seeking to calculate their fertile days knows that there’s no easy answer to this question. The key is to observe your own cycle closely over a period of several months.

Fertility apps work by determining when ovulation starts

The Knaus-Ogino method – also known as the calendar-based method – has been around since the 1930s. In this case, a woman’s cycle is tracked over a period of 12 months to create an ovulation calculator. Women who keep accurate records can calculate when they are fertile. This seemingly simple method is used by today’s fertility apps as the basis for calculating ovulation and fertile days.

The problem with this is that these apps are based on an average cycle, whereby ovulation occurs 14 days after the last period and 14 days before the next menstruation. However, 70% of women don’t match the standard cycle length, having either a longer or shorter cycle (source: Ava). As you only have between three and six fertile days per cycle, it can be critical if you miscalculate ovulation by just two days. In addition, various factors, such as stress or even the time difference while on holiday, can affect the length of a woman’s cycle.

Calculating fertile days based on hormones and basal body temperature

Observing hormones is another way of calculating ovulation and a woman’s fertile days. Five days before ovulation, oestrogen levels increase. This can be determined either by observing the cervical mucus – the discharge is stretchier, more transparent and thinner prior to ovulation – or by using a commercially available ovulation test. These tests work along the same lines as pregnancy tests: you either urinate on a test strip directly or dip it in a cup of urine. The results are based on the levels of oestrogen and luteinising hormone (LH), which increases around 24 hours prior to ovulation.

The disadvantage of ovulation tests is that, if your hormone levels aren’t high enough, they show the increase too late or not at all. And frequent visits to the toilet with test strips can be annoying. Another way to determine the point of ovulation is to observe the levels of progesterone, which increase after ovulation. This is signalled by a higher body temperature. However, as soon as you register an increase in temperature, ovulation is over.

Measuring your basal body temperature is another way of determining your fertility window. After ovulation, your body temperature rises by round 0.5 degrees. If you observe your body temperature over a longer period, you can determine the days on which you are fertile. At least in theory, because this method is very imprecise and highly prone to error.