Rethinking our menstrual cycle

So when do you ovulate?

Our whole reproductive lives women are either trying not get pregnant or trying to get pregnant. In schools we are taught that most women have a 28 day cycle, and that they ovulate around day 14. The first part of the cycle is called the follicular phase and the second part is the luteal phase – each being about 14 days long, with ovulation separating them. My sons were taught this in biology – I read it on their handouts. The text book definition of our menstrual cycle has been formed from small studies done many years ago, mainly on women with 28 day cycles! But is this really true?  How many women have cycles longer or shorter than 28 days and how many ovulate on day 14?

One thing we know for sure is that not all women ovulate on day 14 – if they did it would be really easy for women to know their fertile days and they would get pregnant easier. But they don’t and this is why over the years a number of methods have been established to try to help women find their fertile window.

The fertile window is defined as the day a woman ovulates and the five days before. This is because once ovulated, the egg is only viable for about 24 hours. So the window is shut 24 hours after ovulation. But sperm are hardy and can live for up to five days in the female reproductive tract, so if you have intercourse up to five days before ovulation, some sperm might still be around to fertilise the egg. But many studies have shown that the closer to the day of ovulation intercourse occurs, the higher the chance of a pregnancy.  With more women waiting until they are in their 30s to even start to try and have children, it makes sense to be sure intercourse is happening at the right time.

Three main methods have been devised to help women identify when they are ovulating; taking the basal body temperature (BBT), checking the cervical mucus and measuring lutenising hormone (LH).

The basal body temperature is your temperature as you wake up, which rises around the time of ovulation. But the rise is very small and so a sensitive thermometer is needed and accurate record taking.

If women use the cervical mucus check, they need to monitor this daily. Before and after the period there might not be any discharge. Before ovulation the vaginal discharge is cloudy and sticky. Leading up to and at ovulation, the amount of cervical discharge increases and becomes thin, clear, slippery and stretchy (which can be tested between the fingers) and it resembles raw egg white. After ovulation the discharge decreases and becomes thicker. Obviously having sex and using a lubricant will change the appearance of the mucus. Ideally women need to be taught how to check their mucus.

LH rises 36-40 hours before ovulation. There are kits which are really easy to use. Women just need to urinate on a test stick which will indicate if the surge has happened. If trying to get pregnant, intercourse is advisable on the day of the LH surge and two days later to catch ovulation.

Recently my Masters students told me that they are using menstrual cycle apps that just monitor their cycle dates and these apps tell them when they ovulate. I was dumbfounded. Looking at just dates is not predictive of the day of ovulation. There are about 100 apps that are termed fertility apps, with over half of them just looking at dates. The others look at BBT, cervical mucus changes and/or LH to identify the day of ovulation and to learn about an individual women’s cycle. The latter apps should be reliable, as long as they are using this information, but calendar apps cannot tell a woman when she is ovulating.

Last year I decided to contact one of the fertility app companies to look at the data they have on thousands of women’s menstrual cycles. So I contacted Natural Cycles and last week our paper looking at over 600,000 menstrual cycles was published in Nature Digital Medicine.

We found that 9% of women had cycles of 24 days or less, 65% had cycles of 25-30 days, 19% had cycles of 31-35 days and 7% had cycle lengths of 36 or more days. Only 13% of women had cycles of 28 days.

The average follicular day length was 16.9 days and the average luteal phase length was 12.4 days. The day of ovulation ranged from day 10 to day 20 in women with cycles of 25-30 days. The average cycle length decreased by 0.18 days and average follicular phase length decreased by 0.19 days per year of age from 25 to 45 years. The average variation of cycle length for obese women was 0.4 days or 14% higher. Cycle length variability was observed to a lesser extent in non-obese and underweight women.

The main message from this work is that our text book understanding of the menstrual cycle is outdated. If women want to know the day they ovulate, using an app that just looks at their menstrual cycle dates will not be useful. So if they are trying to get pregnant, and want to identify their ovulation day, using BBT, cervical mucus and LH will be useful and using an app that uses this information, might be even more useful and convenient.

I am doing further studies into FemTech – the use of digital media for women’s health. Do you use any and how are you finding it? Have you used a fertility or menstrual cycle app?

Read more:

The full paper can be read here.

Image is from the Natural Cycles web site

Joyce Harper is a scientific consultant for Natural Cycles to undertake research.