The very first cycle with NaturalCycles

We get a lot of questions about how many green days to expect in the beginning when using NaturalCycles, since NaturalCycles needs some data to get to know you and your body. Every woman is different, with different cycle characteristics and different body temperatures, so we cannot make any reliable assumptions about your body until you start entering your own data in your NaturalCycles account.

The number of green days a woman gets in the first ovulatory cycle depends a lot on those first data points. What cycle day did she start measuring at? Does she measure every day or does she skip a lot of days? How much is her temperature fluctuating? Which cycle day does she ovulate? Has she recently been on hormonal contraception and will this postpone ovulation? I’ve already written a separate blog entry concerning how much the pill affects your cycle, here:

NaturalCycles requires 6 low temperature values before looking for a rise due to ovulation. This means that if she ovulates around day 15, she needs to start measuring around day 10 at the latest. If she starts measuring after the temperature rise, then it is of course not possible for NaturalCycles to detect ovulation. Below is such an example, where she started measuring on day 21:

One could argue that most probably ovulation had happened there and NaturalCycles could give green days, but before the following menstruation, there is really no way of knowing whether ovulation has happened and if the temperature is high, or if ovulation is for some reason delayed and the rise is still to come. As safety comes first, these days should not be green!

Below you see another example of a user that started measuring her temperature on cycle day 11, ovulated on day 20 and then got green days on day 23. She also entered a first positive LH test (ovulation test) on day 20, which helps to get easier green days.

In some extreme scenarios, where the woman does not measure very often in combination with highly fluctuating temperatures, it can take up to two cycles to get the first post-ovulatory green days.

To summarize: on average, if a user starts measuring before ovulation, she gets 40.2% green days during her first cycle. So even though the number of green days in the first cycle varies greatly, depending on several factors, the users do on average get a significant amount of green days.

Accuracy of perception of ovulation day in… [Curr Med Res Opin. 2012] – PubMed – NCBI

Accuracy of perception of ovulation day in… [Curr Med Res Opin. 2012] – PubMed – NCBI

The fertile window


The woman’s body is really a remarkable thing. Cycle after cycle, the uterus does its job and releases another egg, expecting it to be fertilized. Today, since we have such great knowledge through medical research on what’s going on in the uterus, we can use it to our advantage to either prevent or plan a pregnancy. The time frame when a woman is fertile only occurs once per cycle and is called the fertile window.

The fertile window includes the 5 days prior to ovulation and the day of ovulation.

Fertile Window - NaturalCyclesThe 5 days period prior to ovulation is determined from the longest time sperm can survive in the uteral environment. Note, however, that most sperm have a lifetime significantly less than 5 days – more like 2 or 3 days, but to completely exclude a possibility of pregnancy one must take the longest living sperm into account. For sperm to survive any significant time at all, the uterus must contain the fertile type of cervical mucus, which helps the sperm to live longer as well as to be able to travel up the uterus and fallopian tubes. Without the presence of fertile cervical mucus sperm typically only survive about 4 hours.

The fertile cervical mucus is triggered by a rise in estrogen prior to ovulation. The amount of fertile cervical mucus does not only vary from woman to woman, but also on the age of the woman. The older you get, the fewer days you produce cervical mucus and hence the narrower your fertile window becomes.

Once released through ovulation, the egg can maximally survive up to 36h, but typically only 12-24h. Studies have shown that the quality of the egg deteriorates very quickly; causing the probability of conception to decrease rapidly every hour once the egg has been released. For optimal chances of conception, sperm should thus already be present in the fallopian tube once the egg is released. Therefore, the most fertile day of the woman’s cycle is rather the day prior to ovulation than the day of ovulation itself.

To prevent pregnancy through detecting ovulation and predicting the fertile window, one must assume the longest living sperm and egg. This sums up to 6 days in the cycle. What’s more tricky is to accurately calculate the uncertainty of the ovulation day for a woman. That is, when do we think she will ovulate and what’s the earliest possibility of ovulation to occur. Luckily this is what NaturalCycles‘ algorithm does for you. When you start measuring your temperature, NaturalCycles will be very cautious as it does not know around what time you usually ovulate. With more and more data, NaturalCycles is able to better estimate your ovulation day, the variation of your ovulation day and the uncertainty on the estimated variation of your ovulation day. All this is required for a full-proof birth control method using natural family planning. So don’t chart by hand to prevent getting pregnant ladies – it is doomed to end up in an accident. Use the mathematical tools provided for you and you’ll save both time and hassle.