In the BMI range 18—35 the cycle length variation is flat and above a BMI of 35 it increases although the confidence intervals are very large. Flow diagram of user and cycle selection for study. In this study we analysed the key characteristics of more than , menstrual cycles. This large analysis of menstrual cycle parameters provides insight into the physiology of the menstrual cycle amongst the general population, which is not widely known.
It demonstrates significant variability in cycle and follicular phase length amongst a large group of women with wide age and BMI ranges. Using this large data set, our analysis reveals important information on menstrual cycle characteristics in a real-world population of women. Knowledge and understanding of the menstrual cycle, ovulation day and the fertile period is important for both individual women and healthcare professionals providing services in reproductive health.
These data are valuable for fertility educators to support educational activites around female fertility that address knowledge gaps across both the general population and the medical community. It is a common belief that ovulation occurs on day 14 of the cycle, but our analysis has shown that for the majority of women in the real-world that this is not the case.
Cycle length differences were found to be predominantly caused by follicular phase length differences i. For women with a typical cycle length 25—30 days the follicular phase length was on average For women with normal but longer cycles 31—35 days , it was In very short cycles 15—20 days the mean follicular phase length was These findings demonstrate that the widely held belief that ovulation occurs consistently on day 14 of the cycle is not correct.
Clinically, it is important that women who wish to plan a pregnancy are having intercourse on their fertile days. In order to identify the fertile period it is important to track physiological parameters such as BBT and not just cycle length.
Anecdotally most healthcare providers believe that the luteal phase is consistently 14 days in length but we found a mean of The data in this study showed that luteal phase lengths across the population do vary, albeit less than follicular phase lengths. Variation in luteal phase lengths has previously been observed in controlled clinical studies 7 , 24 , 28 ; however, this is still not widely acknowledged amongst nonspecialists.
The results from this study are important in order to highlight variations in phase lengths amongst the general population. It is remarkable that short cycles had a significantly reduced luteal phase relative to normal length cycles, but conversely very long cycles had a significantly long follicular phase and the luteal phase did not vary much. The use of a menstrual cycle tracking app that utilises BBT and other important physiological parameters to identify ovulation day and in turn luteal phase length can give insights into individual fertility and potentially support early identification of subfertility.
Strong linear correlations between menstrual cycle length and follicular phase length with increasing age are demonstrated. Although it is known that cycle length is likely to decrease with age, the linear correlation outlined in our analysis has never been described in such detail.
The mean cycle length dropped by 3. Above 40 the variation increased dramatically. These results are in alignment with those of reference studies. It is well-established that obesity is related to menstrual disorders, infertility, miscarriage, obstetric complications, live birth rate and can affect the success of assisted reproductive technology. This is likely due to underrepresentation of women with high BMI within the study population.
This effect is expected because pre-existing medical condition PCOS is associated with obesity and causes erratic menstrual cycles. The main limitation of this study is that the study population is derived solely from users of the app who may not be representative of the wider population. Of the 1. Nevertheless, there is a bias caused by excluding these cycles.
We also acknowledge the potential for human error in identification of the start of the cycle, the start and peak of the LH surge and the BBT rise based on self-reported bleeding, urinary LH test results and temperature measurements respectively.
Study participants were able to purchase approved LH tests from the app developers, however, it is known that some users prefer to buy other commercially available tests between which there may be small variations in LH threshold values for a positive result. Given the variations in cycle length and follicular phase length that we have described, especially for cycles outside the average range 25—30 days , an individualised approach to identification of the fertile window should be adopted.
There are more than fertility tracking apps freely available for download. Many of these apps claim to identify fertile days based on traditional assumptions about key menstrual cycle parameters such as regularity of cycle length, follicular phase length and luteal phase length. Apps giving predictions of fertile days based solely on an outdated understanding of ovulation day variation could completely miss the fertile window.
It is, therefore, unsurprising that several studies have shown that calendar apps are not accurate in identifying the fertile window. Some fertility apps are based on sophisticated algorithms for individualised identification of the fertile window relying on physiological parameters such as BBT which are more acceptable for large numbers of women. The addition of BBT and the use of a fertility app may help to narrow down testing days and therefore be more convenient and cheaper.
Individualised identification of the fertile window based on BBT and menstruation dates can help to reduce the time to conception in some cases. With women globally delaying fertility 39 the potential value of fertility tracking apps as a platform for delivery of individualised fertility education and preconception care should not be underestimated. Anecdotally there is poor understanding of fertility amongst the general population, which can lead to both unintended pregnancies and delayed time to conception with associated psychological suffering for those wishing to start a family.
The value of fertility apps as educational platforms to achieve public health benefits through standardised health promotion messages during key stages of reproductive life such as preconception, pregnancy and birth spacing is also being explored.
Finally, the widespread use of mobile phone apps for personal health monitoring is generating large amounts of data on the menstrual cycle. Provided that the real-world data can be validated against traditional clinical studies done in controlled settings, there is enormous potential to uncover new scientific discoveries.
This is one of the largest ever analyses of menstrual cycle characteristics. These initial results only scratch the surface of what can be achieved. We hope to stimulate greater interest in this field of research for the benefit of public health. Physiological data, including daily BBT sublingual measurement , cycle by cycle dates of menstruation, and urinary LH test results, were collected prospectively from users of the Natural Cycles app.
Participant characteristics including age and BMI were determined through mandatory in-app questions that must be completed during the sign-up process. Users are recommended to measure their temperature on 5 out of 7 days per week as soon as they wake up.
They are requested to report whether a temperature measurement may be deviating for reasons such as disrupted sleep or alcohol consumption the night before. The algorithm also identifies deviating temperatures if the value is outside the range All users in the study had consented at registration to the use of their data for the purposes of scientific research and could remove their consent at any time. A surge in LH is responsible for triggering follicle rupture.
At the onset of menses, marking the start of the follicular phase, the corpus luteum collapses and progesterone levels fall back to a low level until the next preovulatory increase. Progesterone has a thermogenic effect so its levels can be tracked by measuring BBT. BBT is at a relatively constant low level during the follicular phase, reaching its lowest level the nadir prior to ovulation, 43 and then displays a distinct rise of 0.
The algorithm within the app detects ovulation retrospectively based on BBT measurements, menstrual cycle parameters and additionally on positive urinary LH tests. The algorithm can identify the BBT rise associated with ovulation in the presence of measurement errors, missing data and BBT rise occurring over a variable length of time.
The horizontal grey line is the cover line. Comparisons are made using standard statistical techniques taking into account sample size and standard deviation. If ovulation is not detected in this initial test then more tests are performed with a rolling average over an increasing number of days up to 1 week. If a positive-LH test has been recorded, fewer high temperatures are required in order to detect ovulation since the LH test provides extra confidence that ovulation has occurred.
The app recommends which days to take an LH test, considering the uncertainty of the ovulation day such that it minimises the number of LH tests used while ensuring that the user will not miss her surge.
For users on Plan mode the app always recommends which days to check for LH since Plan users are in general more keen on finding the surge, even if it requires a large number of LH tests. The app will, however, only recommend to start checking LH 10 days prior to the earliest recorded ovulation day even if the total uncertainty is larger.
As the LH surge typically lasts for several days 42 the probability of missing the surge if only testing every other day is relatively small. The app, therefore, recommends to only test every other day until close to the expected ovulation day.
If one positive LH test has been entered, but no positive or negative LH test entry exists on the day immediately before, then the user is encouraged to test the following day to establish whether the positive test corresponds to the first or second day of the surge.
If no such test is entered, the app assumes the first LH test marks the first day of the surge. Cycles in which ovulation has been detected are hereafter referred to as ovulatory cycles.
If ovulation has been detected in the current cycle then the algorithm selects the most suitable candidate day to call the First High Point FHP using a system of measurements based on comparisons of each temperature to the phase averages. This is the day on which the temperatures immediately before and after are most consistent with the follicular and luteal phase averages respectively. On average the FHP temperature is just below the cover line.
In a previous study the FHP was 1. An evaluation of the timing of the FHP and the LH peak relative to the data of Ecochard et al is available in Supplementary materials. This means that ovulation itself is estimated to occur on the day of the last low temperature before the rise as suggested by Hilgers and Bailey 46 and Mouzon et al. Progesterone helps thicken your uterine lining so that if there is a fertilized egg, it has a nice, soft bed in which to implant itself.
If no egg implants, the corpus luteum stops producing progesterone after about 10 — 16 days, and you shed your uterine lining in your period.
When we think about pregnancy, most of our minds jump straight to conception—when the sperm enters the egg. For implantation to happen, your body must be making enough progesterone in order to build up a thick, healthy lining. Anything shorter than 10 days can make it difficult to achieve pregnancy.
This is called luteal phase defect. The length of the luteal phase can sometimes serve as a proxy for your progesterone levels. Adequate progesterone is crucial for maintaining healthy bones, long-term heart health, sleeping well, and feeling your best. Read more about the health benefits of progesterone. It can be tempting to write off any abnormality in the luteal phase as irrelevant — after all, our area of primary concentration always seems to be focused on predicting the day of ovulation — but did you know that this stage is actually the unsung hero of conception?
Because it is during this time that the all-important fertilization and implantation occur. Since this is when the baby-growing magic happens, any disruption in this phase can affect fertility and should be taken seriously.
Not only might variances in length be cause for concern, but a luteal phase that is too short may be worrisome as well, noted Natural Fertility. It is generally agreed upon in the world of fertility that if this phase is less than 12 days long, your uterus does not have enough time to form an appropriately nutritive lining for a fetus. If that's the case, even if implantation has already happened, there is a good chance that a miscarriage may occur.
Then the entire cycle will repeat. A normal luteal phase can last anywhere from 11 to 17 days. In most women , the luteal phase lasts 12 to 14 days. Your luteal phase is considered to be short if it lasts less than 10 days. In other words, you have a short luteal phase if you get your period 10 days or less after you ovulate. As a result, it can be harder to get pregnant or it might take you longer to conceive. A long luteal phase may be due to a hormone imbalance like polycystic ovary syndrome PCOS.
But your progesterone levels during this phase may drop as you get closer to menopause. A short luteal phase can be a sign of a condition called luteal phase defect LPD. In LPD, the ovary produces less progesterone than usual. LPD can lead to infertility and miscarriage. Certain lifestyle factors might also be behind a short luteal phase.
In one study , women with a short luteal phase were more likely to smoke than those with a longer phase. This is your temperature right when you wake up, before you even get up to use the bathroom or brush your teeth.
During the first part follicular phase of your cycle, your BBT will likely hover between When you ovulate, your BBT will go up because progesterone stimulates heat production in your body. The luteal phase, which is when the body prepares for pregnancy, can be an important indicator of fertility. They can identify any medical problems affecting your cycle and recommend treatment.
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