Current ovulation and luteal phase tracking methods and technologies for fertility and family planning: a review

Abstract

Ovulation is critical for both conception and overall health, but many people who may ovulate are not tracking ovulation or any other part of their menstrual cycle. Failure to track ovulation, especially in those trying to conceive, can lead to fertility challenges due to absent ovulation, mistiming intercourse, or an undetected luteal phase defect. Ovulatory disorders and mistiming intercourse are both primary causes of infertility, and tracking ovulation is shown to decrease average time to conception. While there are many tracking methods and apps available, the majority are predictive apps or ovulation predictor kits and do not test or track both successful ovulation and the health of the luteal phase, leading to missing information that could contribute to diagnosis or successful conception. Here, we review why ovulation tracking and a healthy luteal phase are important for those trying to conceive. We present currently available ovulation tracking methods that detect both ovulation and the luteal phase, including cervical mucus, urinary hormone testing, and basal body temperature, and discuss the use, advantages, and disadvantages of each. Finally, we consider the role of digital applications and tracking technologies in ovulation tracking.

INTRODUCTION

The menstrual cycle is often recognized as the fifth vital sign for females, giving insight into overall health and underlying hormone patterns.^1,2 The importance of understanding the menstrual cycle is being increasingly recognized, as the number of menstrual tracking applications and interest in fertility awareness-based methods continues to increase^3,4. While the most visible sign of the menstrual cycle is menstruation, perhaps the most important part of the menstrual cycle is ovulation, or the release of an ovum (egg) from the ovary. This occurs roughly in the middle of the menstrual cycle, and the precise day of ovulation varies from individual to individual and cycle to cycle.⁵

Understanding when ovulation occurs, as well as tracking the health and success of that ovulation (that is, does it produce in sufficient progesterone to maintain a pregnancy), can be beneficial in several ways. If an individual or couple is trying to achieve pregnancy, ovulation tracking may improve chances of conception as well as clarify when other testing is appropriate.^6–8 Ovulation tracking also plays a key role in successfully avoiding pregnancy using fertility awareness-based methods.³ Finally, even for individuals with no current family planning goals, understanding and tracking ovulation as a sign of health can help identify potential health or hormonal concerns^9–11. Identifying one’s current menstrual cycle phase is important for understanding sleep,^12,13 sports medicine/exercise physiology,^14,15 addictive behaviors and self-medication tendencies that may vary by menstrual phase,¹⁶ mood disorders and psychiatric symptoms (including premenstrual dysphoric disorder, PMDD),^17,18 and nutritional needs,¹⁹ and current research continues to expand our understanding of the role of the menstrual cycle in female physiology.

Here, we review the factors that contribute to normal ovulation and the hormonal processes involved, as well as pathologies of ovulation and problems that arise related to ovulation or failure to track ovulation. We also summarize the currently available methods for individuals to track ovulation at home, and their advantages and disadvantages of each. Finally, we address the role of menstrual cycle/ovulation tracking apps and emerging technologies.

PARAMETERS OF NORMAL OVULATION

Ovulation is the release of an ovum (colloquially, egg) from the ovaries approximately once each menstrual cycle. Ovulation is necessary for conception and successful pregnancy. This is due to both release of an ovum to fertilize and for successful formation of the corpus luteum, thereby producing sufficient progesterone to maintain the uterine lining for implantation. Regardless of family planning goals, the cycling of hormones involved in and resulting from ovulation is beneficial to many aspects of overall health.^9,10

Biology of ovulation

The menstrual cycle is counted from the first day of an individual’s menstrual period, at which point hormone levels and ovarian activity are relatively low (Figure 1). Communication and hormone secretion along the hypothalamus-pituitary axis and with the ovaries then control the activity of the rest of the cycle⁹. In the first half of the menstrual cycle, termed the follicular phase, follicle-stimulating hormone is released from the pituitary gland, resulting in recruitment and development of several follicles in the ovary²⁰. Follicular development increases estrogen levels, causing selection of a dominant follicle, proliferation of the endometrium, and indirect induction of luteinizing hormone (LH) secretion from the pituitary. LH induces formation of the corpus luteum from the dominant follicle and a slight pre-ovulatory rise in progesterone,²¹ and 12–36 hours following the LH surge,²² ovulation will occur and the corpus luteum produces both estrogen and progesterone during the second half of the menstrual cycle, termed the luteal phase.^23–25 Progesterone maintains the endometrial lining and induces gene expression that leads to it becoming receptive for potential implantation of a fertilized egg. If implantation does not occur, estrogen and progesterone levels will drop, and a new menstrual cycle will begin.

Figure 1. A typical menstrual cycle.

The menstrual cycle begins with a rise in FSH (green) during the follicular phase. After several days, estrogen will begin to rise (teal) followed by a surge in LH (blue). This triggers ovulation, and the end of the fertile window. 6 to 9 days following ovulation, implantation is possible, and progesterone is elevated (purple).

Timing of ovulation: the fertile window

Ovulation occurs roughly in the middle of the menstrual cycle, but timing varies between individuals and even from cycle to cycle. The follicular phase lasts an average of 14–19 days,⁵ and is the most variable phase of the cycle. Therefore ovulation occurs on average 14–19 days after the start of the menstrual cycle, and following ovulation the egg survives for 12–24 hours (Figure 1).²²

The timing of ovulation is a key component in establishing the “fertile window,” or the time during which intercourse may result in conception.⁷ While the term fertile window is used widely to refer to a range of times, it may be more accurately separated into the biological fertile window and the clinical fertile window. The biological fertile window is the empirically determined 6 days, based on 5 days of sperm survival and an additional day of egg survival, during which intercourse may result in conception.^6,7,26 The clinical fertile window refers to that time determined by symptoms of ovulation (especially hormone testing and/or cervical mucus) during which it appears ovulation is impending, and intercourse is highly recommended if trying to achieve pregnancy (or cautioned against if using a fertility awareness-based method to avoid pregnancy).^26,27 The clinical fertile window is nearly always longer than the biological fertile window, and it is only in hindsight and with precise ovulation tracking measures that the biological fertile window may be determined.

Importance of the luteal phase and progesterone

The second half of the menstrual cycle, the time following ovulation, is termed the luteal phase after the corpus luteum. Health of the luteal phase is critical to achieving and maintaining pregnancy, but many methods of tracking ovulation do not account for the role of the luteal phase. The luteal phase begins with ovulation, and a healthy luteal phase lasts 11–17 days.^25,28,29 Concurrently with ovulation, the corpus luteum will begin producing progesterone, which remodels the endometrium and modulates the immune system to allow for potential implantation of a blastocyst.²⁸ Approximately 7–10 days following the LH surge, this implantation may occur, and if pregnancy results, the blastocyst will begin producing human chorionic gonadotropin (hCG) and the corpus luteum will continue to produce progesterone until the placenta completely takes over production by the end of the first trimester.²⁸ If implantation does not occur, both estrogen and progesterone will decrease, reducing feedback inhibition on FSH and beginning a new menstrual cycle.

IDENTIFYING PROBLEMS WITH OVULATION

Tracking the timing and success of ovulation can be critical in successfully achieving pregnancy and diagnosing several concerns that contribute to infertility. Ovulatory disorders (which include anovulation, polycystic ovary syndrome, and luteal phase defect) are the leading cause of female factor infertility.^30–32 Additionally, failure to track ovulation may lead to mistiming of intercourse, which is a leading cause of infertility as well, leading to lack of fertilization entirely.³³ The most common problems associated with ovulation and infertility that may be alleviated or diagnosed by ovulation tracking are further discussed below.

Anovulation

Anovulation, or lack of ovulation entirely, is a common cause of female factor infertility (Figure 2A).³⁰ As meeting of sperm and egg is necessary to achieve pregnancy, anovulation of course makes conception impossible and often requires intervention to alleviate. Anovulation may be accompanied by amenorrhea, in which case it may not be challenging to diagnose, but in some cases a female may have consistent menstruation and not be ovulating. Even in those with normal cycles and no known reproductive pathology, around 1/3 of cycles may be anovulatory.³⁴ Ovulation tracking is therefore important to confirm that an individual is ovulating or assist with diagnosis of anovulation. Treatment of anovulation will depend upon the cause—that is, if anovulation is accompanied by amenorrhea and likely caused by lifestyle or diet (i.e. overexercise, disordered eating),^14,35,36 or certain endocrine conditions,^14,37,38 the cause should be treated and the anovulation will likely resolve. If anovulation is due to PCOS or is unexplained, ovarian stimulation by letrozole, clomiphene citrate, or gonadotrophin therapy is common.^39–42 Currently, letrozole is preferred for PCOS as studies suggest it may be more effective than clomiphene citrate and may result in lower rates of multiple pregnancies.^40,43–45

Figure 2. Ovulatory concerns relating to infertility.

A) Anovulation, or lack of ovulation, often presents as a lack of menstrual cycle entirely, although it may have occasional hormone fluctuations. B) If intercourse is not targeted properly around ovulation, this may result in trouble conceiving. C) A luteal phase defect may be caused by a shortened luteal phase, missing implantation entirely, or D) low progesterone, making implantation challenging.

Mistiming Intercourse

When individuals are not tracking ovulation accurately, it is also common to mistime intercourse and fail to conceive as sperm never meets egg (Figure 2B). This may be due to complete lack of tracking ovulation and so coincidentally never having intercourse near ovulation, or it may be due to faulty understandings or assumptions about when ovulation is occurring (i.e. assuming day 14 ovulation when this is not the case, or relying on period tracking apps that estimate ovulation).^5,46–48 For instance, a woman with a 30 day cycle may assume ovulation occurs on day 14, and have intercourse on days 10–15 accordingly. It is entirely possible, however, that she ovulates instead closer to day 19 and has a relatively short luteal phase, and therefore intercourse is not targeted correctly for conception as sperm typically only survive 3–5 days in the female reproductive tract. Additionally, even if intercourse is targeted closer to ovulation, fertile cervical mucus is required for sperm survival and transport and so intercourse may not result in conception if cervical mucus was not present at the time.⁴⁹

The biological understanding of the fertile window as five days of sperm survival plus one day of egg survival lines up with study results showing that only intercourse within five days before or one day after ovulation results in pregnancy.^6,7 Additionally, tracking ovulation decreases the average time to conception, supporting the idea that mistiming intercourse is a cause of perceived infertility.^3,27,30,50

Implantation Failure and Luteal Phase Deficiency

The luteal phase and role of progesterone in achieving pregnancy are critical in two ways. Firstly, the luteal phase must be long enough to allow implantation; that is, if the uterine lining sheds too quickly due to low progesterone, there will be no chance for a blastocyst to implant. This is the traditional clinical definition of a luteal phase defect (LPD), with a luteal phase of 10 days or fewer (Figure 2C).²⁹ Secondly, even if the luteal phase is long enough, progesterone must be sufficiently high to remodel the endometrium, allow implantation, and maintain the pregnancy (Figure 2D). This is increasingly recognized as a luteal phase defect as well, although it must be diagnosed biochemically rather than by luteal phase length alone.^29,51 In this case, progesterone may be tested directly by serum or urinary measurement. While a single measurement is sufficient to confirm ovulation, an integrated or scored progesterone value (that is, multiple progesterone measurements over multiple days, reaching at least 30 ng/mL total) may be more useful in assessing if progesterone is sufficiently high throughout the entire luteal phase.^51–53

Low progesterone and LPD are associated with increased risk of pregnancy loss in many studies, both of assisted reproductive technologies/in vitro fertilization and natural conception/unassisted cycles.^54–58 Additionally, LPD and/or low progesterone are often associated with comorbidities that contribute to infertility and/or recurrent pregnancy loss, and it is unclear if the LPD is a cause or effect of subfertility.^28,29 That is, while certainly a LPD may prevent implantation, it is hypothesized to be a result of an earlier hormonal defect, such as an insufficient LH surge and/or low FSH levels.^28,29 Therefore, it has also been suggested that determining the cause of the LPD is an important approach to improve fertility.²⁹ Strategies to mitigate LPD and restore fertility are an active area of research with mixed results for both direct luteal phase support (progesterone supplementation) and support of corpus luteum formation (typically ovarian stimulation). Progesterone supplementation has been shown to increase birth rate in women with threatened miscarriage who have had prior miscarriages,^59,60 and also improves live birth rate in natural cycle frozen embryo transfers,^61,62 but there is currently no consensus on application of progesterone supplementation to treat luteal phase defect alone.²⁹ This could be due to varied efficacy based on in timing of progesterone administration, which can be during the luteal phase or early in pregnancy,^63,64 the route of administration (vaginal, oral, or intramuscular),^65–67 formulation, and dosage. More research is needed to truly understand how progesterone administration can most effectively improve pregnancy rates. Ovulation stimulation may also be used, but similarly has limited evidence for success.²⁹

METHODS OF TRACKING OVULATION AND THE LUTEAL PHASE

Ovulation tracking by some means is therefore crucial for the correct timing of intercourse, diagnosis of luteal phase defects or anovulation, and determining the time of fertility in irregular cycles. How, then, does one effectively track ovulation? While multiple methods and technologies are available in both the clinical and consumer markets, not all methods detect the same thing or provide the same information about ovulation and the health of the luteal phase.

In the remainder of this review, we will discuss the evidence for and use of urinary hormone testing, cervical mucus monitoring, and basal body temperature measurements, as these are the only currently established methods of tracking that can accurately detect the timing of ovulation and length of the luteal phase. We also discuss the role of menstrual cycle tracking apps in ovulation tracking.

Urinary Hormone Testing

Hormones produced by the pituitary (FSH, LH) and ovaries (estrogen, progesterone) are metabolized and excreted in the urine, at rates proportional to their serum concentrations.^68,69 Peaks of hormone concentrations in urine are only slightly delayed compared to in serum, providing relatively accurate timing of rises and surges.^24,68–70 Urinary hormone measurements are relatively robust and rarely affected by contaminants, and can be obtained accurately by at-home users, making testing over many days logistically feasible.^25,71 As water consumption and other factors may affect urine dilution, the most accurate ways to obtain measurements (particularly for estrogen and progesterone metabolites) are to dilute urine to create a standard volume per hour²⁴ or use first morning urine.⁷² Urinary hormone testing to track ovulation is popular with patients and clinicians alike,⁷³ and may be used with regular or irregular cycles.⁷⁴

How to use:

Whole cycle analysis using estrone-glucuronide (E1G; metabolite of estrogen), LH, and pregnanediol-D-glucuronide (PdG; metabolite of progesterone) is most effective for ovulation tracking. An E1G rise signals the beginning of the fertile window,^75,76 the LH surge indicates ovulation will occur within 12–36 hours,⁷⁰ and a PdG rise and elevated and sustained PdG levels during the implantation window indicate successful ovulation and a healthy luteal phase.^53,71 These are more commonly used separately as well, and the specifics of each will be discussed further below. Instructions for accurate use vary based on manufacturer and method of measurement (i.e. test strips alone vs. monitor or smartphone), and so users should follow the directions that come with the urinary hormone testing method they choose.

Estrone-3-Glucuronide (E1G)

As estradiol levels reflect follicular growth, the urinary metabolite estrone glucuronide (referred to as E1G here for estrone (E1) glucuronide, but also as E3G, for estrone-3-glucuronide, elsewhere in the literature) directly reflects ovarian physiology. A rise in E1G relative to an individual’s baseline levels may be used to predict that ovulation will occur approximately 5 days later.^24,71,76 While there may be significant variation in the precise timing of ovulation relative to the E1G rise, use of estrogen to open the fertile window to try to conceive is accurate roughly 90% of the time.⁷⁵ Using urinary hormone testing alone to open the fertile window if using a fertility awareness-based method to avoid pregnancy is typically not reliable enough to yield high efficacy, and additional rules should be followed in accordance with instruction in a fertility awareness-based method (such as the Marquette Model or Boston Cross-Check methods of Natural Family Planning).^77–81

If trying to conceive, intercourse should occur after an E1G rise has been detected and through predicted or confirmed ovulation (by LH, PdG, or another method). Currently, there is no specific threshold that is considered a “rise,” and instead a relative rise should be used compared to baseline.²⁴ In the United States, there are currently five methods available for testing E1G. These include two standalone monitors, one smartphone-assisted monitor, and two smartphone-supported testing kits. The Clearblue monitor uses test sticks that measure LH and E1G, and provides a “low,” “high,” or “peak” result based on a user’s baseline.^80,82 Mira is a monitor that may test E1G, LH, FSH, and PdG depending on the chosen test sticks, and provides a quantitative result.^80,83 Inito is a smartphone-assisted monitor that provides the same hormone information as the Mira monitor.^84,85 Oova is a smartphone and test-strip based kit that tests E1G, LH, and PdG and provides quantitative results. Finally, Proov Complete is a test-strip and smartphone-based kit that tests FSH, E1G, LH, and PdG and provides quantitative results.⁷¹

Luteinizing Hormone (LH)

Luteinizing hormone testing is one of the most common methods of ovulation tracking, although considerable variability exists in its usefulness. Detection of an “LH surge” (typically, greater than 25 μg/mL) results in ovulation the majority of the time, but it is possible to ovulate without a detectable LH surge or before the detection of the surge,^86–88 or to detect an LH surge that does not result in ovulation.⁸⁹ Therefore, use of a corroborating method such as cervical mucus or E1G testing increases accuracy of LH testing, and confirmation of ovulation by PdG testing or basal body temperature is generally recommended (and discussed later).^70,86,88 Most LH tests, often referred to as ovulation predictor kits, are standalone test strips that may typically be read by eye (considered “positive” if a test line is as dark or darker than a control line) or with an app designed for use with particular tests. While most users will find the standard positive threshold to be predictive of ovulation,⁸⁸ in some individuals an LH surge will be below this threshold and therefore it may be necessary to identify a lower concentration LH peak followed by a drop in LH levels.⁸⁰ Additionally, LH testing alone only provides the last 1–2 days of the fertile window, and an LH surge may occur post-ovulation, making timing intercourse challenging with only LH testing.^86,87

For LH testing with individual test strips, users should follow the instructions included but note that these typically do not require first morning urine. The LH surge is considered to have begun with the first positive LH test, not the highest or latest LH tests.^88,90 Therefore, intercourse should be targeted as soon as an LH surge is detected if trying to conceive. LH testing is also available as part of the monitor and/or complete testing systems described for E1G, and in these cases, instructions should be followed for the monitor/complete testing system.

Pregnanediol-D-Glucuronide (PdG)

PdG is the urinary metabolite of progesterone, and is detected in the urine at higher levels following ovulation.⁹¹ A serum progesterone level of > 3ng/mL confirms that ovulation occurred,⁵² and a urinary PdG threshold of 5 μg/mL mirrors this serum progesterone rise and is a highly specific way of detecting ovulation and the beginning of the luteal phase.^53,92 Use of at-home PdG test strips confirmed ovulation with 80–100% specificity in pilot studies.^92,93 Use of PdG testing to confirm LH testing results provides additional confidence that ovulation has occurred, providing greater specificity and sensitivity than either test alone.^92–94

PdG testing may also be carried out during the implantation window of the luteal phase, not necessarily to confirm that ovulation occurred but to determine the health of the luteal phase and the quality of ovulation (“high quality ovulation” defined as a sustained increase in PdG that continues through the implantation window). That is, it is possible for ovulation (egg release) to have occurred but produce only low progesterone levels, which could lead to luteal phase defects and increase the risk of early miscarriage (Figure 2).^55,57,95 PdG testing may be carried out between days 7–10 following a positive LH test, consistent with when a clinical progesterone blood draw would occur.⁹⁰

PdG testing is currently available as standalone test strips (Proov Confirm) or part of a complete cycle analysis test strip kit (Proov Complete). Two monitors, Mira and Inito, also include PdG testing in several wand/test strip options, as does the Oova test strip and smartphone system. To use PdG to confirm ovulation occurred, PdG tests should be carried out using first morning urine following a positive LH test, with the highest probability of a positive test 4 or 5 days later.⁹² All PdG tests currently available are marketed to confirm ovulation. If using PdG during the implantation window to determine the success of ovulation and likelihood of a luteal phase defect, testing should occur 7–10 days following a positive LH test.⁷¹ Proov Confirm and Complete are currently marketed to test PdG levels during the implantation window to assess luteal phase health, and provide an Ovulation Score (based on total positive PdG tests across the implantation window), as this is a better reflection of luteal phase health than a single PdG measurement.^51,71

Cervical Mucus

Throughout the entire menstrual cycle, secretory structures in the cervix (typically referred to as “crypts”) respond to changing hormone levels and secrete cervical mucus. Cervical mucus is a hydrogel made up of mucus molecules, water, salts, proteins, and cells, the composition of which changes in response to estrogen and progesterone. There are four types of cervical mucus, each coming from their respective crypts: G, L, S, and P. G mucus is produced in response to progesterone, either at the beginning of the follicular phase before estrogen begins to rise, or during the luteal phase following ovulation. G mucus contains the greatest concentration of leucocytes and lymphocytes and is not conducive to sperm movement. L, S, and P mucus are secreted in response to estrogen rising, and contribute to fertility and sperm transport^96,97.

Fertile-type cervical mucus (L, S, and/or P) is required for sperm to travel through the cervix and reach the egg naturally. Therefore, observation of cervical mucus can be used to determine/estimate the fertile window^96,98,99. While methods of tracking mucus (the Billings Ovulation Method, Creighton System, Marquette Model, and symptom-thermal methods, for instance) differ in their specific categories, universally it is understood that the highest quality, most fertile type mucus is slippery, clear, and stretchy (often described as “egg-white”). The highest probability of conceiving is on days with this high-quality cervical mucus,⁹⁹ and the last day of such mucus is most likely to be ovulation day^96,99 and will almost always be within 4 days of ovulation.^26,98

Use of cervical mucus monitoring has been shown to increase chances of conception due to correct timing of intercourse and detection of the biological fertile window.^26,27 While the fertile window as determined by cervical mucus is often longer than that of the 6-day biological fertile window, there is significant overlap. Additionally, days of highest quality cervical mucus (that is, the most slippery, stretchy, and clear) are likely to be closest to ovulation and this helps narrow down timing for intercourse to achieve pregnancy.^26,99 For instance, one study of over 2000 cycles in 501 women found an average of 12 fertile days by mucus alone but only 6 days of peak-type/highest quality mucus.²⁷ The same study found that as age increases, the number of days of peak type typically decreases for nulliparous females, consistent with earlier observations that over time, L, S, and P-type crypts transition to less fertile types.⁹⁶

How to use:

Cervical mucus may be tracked by observing secretions/discharge found at the vulva and/or sensation observed upon wiping. If trying to achieve pregnancy, intercourse should occur on days with slippery, clear, and/or stretchy cervical mucus. Ovulation is estimated to be the last day of such peak-type cervical mucus. One device is currently on the market to track cervical mucus, sold under the brand name kegg. This device analyzes electrolyte levels in cervical mucus to provide users with an estimated fertile window.

Basal Body Temperature

Another popular method for ovulation tracking measurement of basal body temperature (BBT), the natural temperature of the body at rest. Following ovulation and the rise of progesterone, the increase in metabolic rate leads to increased body temperature.^3,100,101 Once established, this rise in body temperature is 0.2–0.5 °C and may be confirmed by the observation of three high temperatures that are above the previous six and meet the threshold of at least 0.2 °C above the low temperatures (Figure 3).^102,103 The rise in BBT typically occurs between one day before and two days after ovulation,¹⁰⁴ although greater variation has been observed.^100,105,106 Some evidence suggests that there is often a decrease in BBT that corresponds with the fertile window and/or ovulation, but this is considered unreliable and not always observed.^105,107 Therefore, as an ovulation tracking method BBT is primarily useful to confirm that ovulation has already occurred and the luteal phase has begun. Any “use” of BBT to predict ovulation is simply estimating when ovulation might occur based on previous cycle data, which is often unreliable given natural variation in the length of the follicular phase. Because of this inability to predict the beginning of the fertile window, if used to avoid pregnancy BBT is most effective as part of a symptom-thermal method of fertility awareness/natural family planning.^3,102 There are some suggestions that BBT patterns may be used to help address infertility concerns such as PCOS or luteal phase deficiency, but even here BBT would not replace existing diagnostic criteria.¹⁰⁸ Additionally, while the presence of the BBT rise confirms ovulation,^100,101 the height of the rise is not directly related to urinary progesterone levels and is not sufficient to diagnose a luteal phase defect.^51,106

Figure 3. Analysis of basal body temperature for ovulation tracking.

Body temperature is recorded every morning, and upon a temperature shift, a coverline is placed on the highest of the prior low temperatures. Ovulation occurs between one day before and two days after a temperature shift, and the fertile window is considered closed after three high temperatures. Counting from the first high temperature is one way to measure luteal phase length.

How to use:

BBT should be measured first thing in the morning upon waking, before getting out of bed and around the same time each day with a highly accurate thermometer (i.e. not a regular fever thermometer).^102,104 If trying to achieve pregnancy, intercourse may be targeted immediately after a temperature shift, although a predictive method should be used as well. There are also several wearable/insertable thermometers available in the United States for BBT reading, including Oura, Ava, FitBit and Apple Watch (all wrist/finger skin temperature),^{12,107,109,110} Tempdrop (skin temperature on the arm), and OvuSense (vaginal).¹⁰⁸ Some of these devices, as well as other tracking apps that include BBT such as Natural Cycles, may also include algorithms that attempt to predict time of ovulation based on timing of past temperature shifts.^111,112

Digital Tracking Technologies

Digital health tools and mobile health applications continue to increase in popularity and create a large and profitable market, including menstrual cycle tracking apps and devices.¹¹³ Polls suggest that cycle tracking apps are used by at least 1/3 of women in the United States and United Kingdom, with use by approximately 2/3 of women aged 18–35 in the UK.^114,115 Individuals use apps for many reasons, but some of the most common include menstrual cycle tracking to prepare for/predict future periods and to try to conceive.^113,116 Most fertility-related wearables and tracking devices also have their own apps designed to integrate with their product, leading to many users having multiple apps. Regulation of health-related smartphone apps varies greatly, as some are regulated as medical devices and registered with the FDA but most are not.¹¹⁷ Menstrual cycle tracking apps also often provide users with inaccurate or misleading information.^46,118,119 However, smartphone and health apps also have powerful data collection potential to assess population-level menstrual cycle patterns and characteristics,^120,121 and may help diagnose reproductive health conditions.¹¹⁹ With at least 100 apps available that include some sort of menstrual cycle tracking,^4,122 it is challenging to concisely characterize all potential apps and features, but a few broad categories do emerge that are relevant to ovulation tracking and that we will address further.

Predictive features of ovulation tracking apps

A significant reason for use of menstrual cycle tracking applications is to predict future periods and/or fertile days. One study found that 91% of apps tested included predictive features.¹²³ However, many app predictions are inaccurate and multiple apps often give predictions that do not agree with one another.^4,46,47,118 Additionally, nearly all apps assume that a given cycle is ovulatory and include predictions based on regular cycles.¹¹⁸ As has been discussed above, predicting the fertile window based on past cycle data or rhythm method predictions alone may lead to mistiming intercourse if trying to conceive. Relying on apps and their predictions alone to avoid pregnancy is also ineffective.⁴ Therefore, predictive features of tracking apps should be used with caution, and in general preference given to apps that allow users to input symptoms that may affect future predictions. For instance, if an app allows a user to input confirmation of ovulation, prediction of a future period based on a user’s typical luteal phase length is likely to give a more accurate prediction, compared to an app merely estimating based on a regular cycle length. Additionally, apps that are clear on where their predictions come from, and providing education and information, are likely a better choice for most users. In one study that focused heavily on users of fertility awareness-based methods, these users largely preferred apps that had no predictive features.¹¹⁹ This is consistent with users already educated on ovulation tracking preferring to input their own hormonal data instead.

Incorporation of hormonal biomarkers/integration of wearables

Many apps are designed for use with either ovulation tracking via a fertility awareness-based method or for integration with wearables, hormone monitors, or other tracking devices. These applications vary greatly with respect to their predictive features, and so caution should be taken for the reasons described above. However, use of apps to support ovulation tracking with evidence-based methods is much more likely to be successful in both achieving pregnancy and avoiding pregnancy (the latter for those trained in a fertility awareness-based method).¹²² For example, many of the fertility monitors and testing systems for urinary hormone detection have apps designed to integrate with their products, and these typically incorporate user data to provide feedback about menstrual cycle characteristics and the fertile window. Many temperature tracking wearables and systems also include applications to support the wearable, although they vary in how much temperature analysis is done automatically. Finally, some apps are not designed with a particular device or method in mind, but instead allow users to input cycle tracking information and analyze it themselves. These apps then become useful repositories for symptom tracking as well as sharing information with health care providers, without potentially misinforming users about their fertility status.^46,113

Overall, those who choose to use menstrual cycle tracking and/or ovulation tracking will likely find it difficult to avoid app usage entirely (especially in the United States, given the prevalence of smartphone usage and sheer number of apps), particularly when it comes to interpretation of urinary hormone testing and efficient data storage. Paper cycle charting is possible, but apps are often required to read urinary hormone tests, and make cycle charting throughout the day much more feasible. Users should find apps that best fit their needs and be cautious of predictions provided by apps and ensure that they understand their chosen ovulation tracking methods themselves rather than relying on an app to interpret their fertility status for them.

CONCLUSION

Interest in ovulation tracking and fertility awareness-based methods is increasing, and parallels the increase in available menstrual cycle tracking apps. Overall, ovulation tracking may provide users with many benefits, including potential detection of health concerns, increased likelihood of conception, and a better understanding of their menstrual cycle. There are many methods available for ovulation tracking, but those that are most accurate at detecting ovulation at this time are cervical mucus monitoring, basal body temperature, and urinary hormone testing. Urinary hormone testing with PdG can also help determine the overall health of the luteal phase, while other tracking methods provide only the length of the luteal phase.

While there are other tracking methods available, including heart rate, cervical position, and salivary ferning, these are currently neither as common nor as well established. Additionally, there are more robust tracking devices and apps dedicated to use of basal body temperature, cervical mucus, and urinary hormone testing, as well as effective protocols for conceiving or avoiding pregnancy using fertility awareness-based methods. Ultimately, those tracking ovulation should choose the method(s) that are affordable, and accessible to them, and that they are most likely to adhere to. Menstrual cycle tracking apps are likely to play a role in ovulation tracking, but users should be cautious of predictions that are not based on real hormonal biomarker data (i.e. hormone testing, BBT, or cervical mucus). Finally, the health of ovulation and the luteal phase should not be ignored, especially for those trying to conceive, as sufficient progesterone in the luteal phase is necessary for implantation and successful pregnancy.