In connection with both our guide for couples who are trying to conceive and our guide for family planning, this page provides a foundational discussion that is meant to help you understand fertility. On this page we’ll talk about:
The Female Cycle
The cycle of fertility in a woman is intricate and multifaceted. In order to explain what happens during this cycle, we’ll first talk about what happens in a few of the major organs of the reproductive system during the cycle, including the uterus, ovaries, and cervix. Then, we’ll take a step back and look through the lens of the hormones that regulate this cycle to see how these all work in concert to support healthy conception, implantation, and pregnancy.
While our page on timed intercourse provides a more practical discussion of how to recognize and understand your (wife’s) individual cycle, we’ll talk briefly here about the variability of cycle length and what portion of the cycle generally contributes to this variability.
The Uterus: Menstruation
Certainly, the most obvious event in a woman’s cycle is menstruation. The first day of the woman’s cycle is marked by the first day of bleeding (excluding any spotting that may occur prior to menses). But, what causes this bleeding? At the center of this mystery lies the largest of the reproductive organs, the uterus. So, let’s begin by understanding what happens in the uterus during the cycle.
The uterus will ultimately become the temporary home for any child conceived during the cycle. In order for it to do so, it must essentially prepare a bed for the baby embryo. This bed, so to speak, is the lining of the uterus (known as the endometrium). Early in the cycle (in the proliferative phase), estrogen is released which stimulates the growth of the endometrium. As the cycle progresses (to the secretory phase which begins after ovulation), estrogen begins to wane some while another hormone, progesterone, begins to increase. This progesterone essentially helps to mature the endometrium so that it is not only thick enough but also “soft” enough to receive the baby during implantation. Thus, healthy levels of progesterone help to support implantation (as well as early pregnancy) by supporting a healthy endometrium (uterine lining).
If implantation does occur, then the body continues to produce progesterone. This puts a hold on this cycle and menstruation does not occur. However, if implantation does not occur, then both estrogen and progesterone begin to decrease. This decrease signals to the body that implantation has not occurred and that it’s time to prepare a new bed for another cycle. The uterus then begins menstruation where it sheds the endometrium, or, by analogy, takes the covers off the bed it made and throws them out. This is done so that in the next cycle, the uterus can make a brand-new endometrium rich with fresh nutrients for the child that might be conceived in that cycle. This shedding of the endometrium is the menses, or bleeding, that marks the beginning of a new cycle.
The Ovaries: Ovulation
While the most obvious event in a woman’s cycle is menstruation, likely its most significant event is ovulation. Ovulation is the release of a developed egg (or ovum), which is the female counterpart to sperm. This egg is released into the fallopian tubes where fertilization can occur. Before this egg is released, however, it must develop. While the uterus is going through the phases of its cycle (menses, proliferative phase, and secretory phase), the ovaries are going through a synchronized cycle of their own.
In the first phase of the ovarian cycle (known as the follicular phase), an ovarian follicle within the ovary is working hard to mature the egg (a hormone known as FSH, or follicle-stimulating hormone, is responsible for encouraging this process). Once the egg is mature, the hormone LH (Luteinizing Hormone) surges which triggers that developed egg to release (ovulation).
After triggering the ovarian follicle to release its egg, LH also causes this follicle to change its form and function. Whereas its previous purpose was to develop a singular egg, its new purpose is to produce progesterone. (This new function also comes with a new name, corpus luteum.) In short, the ovarian follicle that developed and released the egg now becomes the corpus luteum that produces progesterone. Thus, ovulation marks the start of the second phase for the ovaries, the luteal phase.
The corpus luteum continues to produce progesterone (for about two weeks) but eventually begins to break down (unless implantation occurs). As such, the production of progesterone stops. As we mentioned in the uterine cycle, this fall in progesterone triggers the shedding of the endometrium which marks the beginning of a new cycle.
However, should implantation occur, then hCG (the hormone measure by home pregnancy tests) is released. hCG helps to sustain the corpus luteum so that it continues to produce progesterone. It is this continued production of progesterone that puts a hold on this cycle by preventing menstruation.
The Cervix: Cervical Mucus
While the uterus and ovaries get center stage in the woman’s reproductive cycle for their roles in menstruation and ovulation, there is another reproductive organ whose reaction to this cycle is critical to fertility: the cervix. The cervix serves as the passageway between the uterus and the vagina. Its primary function is to control access to the uterus. In order to serve in this function, the cervix will open during menstruation (to allow menses to leave) and prior to ovulation (to allow sperm to enter). In addition to this, the cervix also will release different kinds of mucus to encourage (or prevent) sperm reaching the uterus, bringing them one step closer to finding and fertilizing the egg.
Prior to ovulation, the prevalence of estrogen causes the cervix to produce mucus which helps to facilitate the sperm’s path to the cervix. This kind of cervical mucus functions like swimming lanes at a pool, keeping the sperm swimming in the right direction. The cervical mucus also helps to filter out any irregular sperm so that only the best ones have the possibility of fertilization. Additionally, the mucus helps the sperm to survive by providing a safe (alkaline) environment and even nutrition to the sperm. This helps the sperm to survive as much as 7 days while they wait for ovulation and the possibility of fertilization.
Sperm have even been shown to rest (or hibernate) in the cervical crypts where this mucus is produced for hours or even days before continuing their journey to the uterus and hopeful fertilization. By providing these well-painted lines and nice rest stops, healthy cervical mucus can help the cervix function as a fast-moving highway for sperm.
Shortly after ovulation, the cervix changes the type of mucus it produces in response to higher levels of progesterone. Since the fertile window has now closed, this type of mucus prevents the sperm from reaching the uterus. If implantation does occur, then progesterone continues to support the production of this kind of mucus to keep sperm from entering the uterus which is now occupied. (This kind of mucus is what constitutes the “mucus plug” that is passed in the early stages of delivery.)
Recap: The Hormones
They say a picture is worth a thousand words. I can say that it’s certainly true in this case. This image provides a succinct illustration for the woman’s cycle of fertility. While the bottom of the image depicts the uterine cycle by showing the variation in the thickness of the endometrium, the top of the image portrays the ovarian cycle by showing the development and release of the egg as well as the transition of the corpus luteum. At the center of this image rightfully sits a graph of the various hormone levels as they change throughout the cycle. These hormones regulate and synchronize the entire cycle so that everything works together in concert to support a healthy conception and pregnancy. Allow me to provide a summary of the fertility cycle by describing the roles that these hormones play.
After the start of the cycle, estrogen begins to climb, causing the endometrium to start thickening and causing the cervix to produce mucus that convey the sperm to the uterus.
While Follicle Stimulating Hormone (FSH) remains relatively stable throughout the cycle[1] helping the egg to develop, the LH surges just prior to ovulation. This surge triggers the release of the egg and transforms the ovarian follicle into the corpus luteum, leading the ovaries into the luteal phase of their cycle.
This corpus luteum immediately begins producing progesterone. As you can see from the image, ovulation is a demarcation line that defines the phase for both the uterine and ovarian cycles. This is made possible by this production of progesterone. You may recall that progesterone triggers the endometrium to mature which leads the uterine cycle into the secretory phase. It also causes the cervix to change the type of mucus produced, effectively closing the highway for new sperm to reach the uterus. So, the production of progesterone by the corpus luteum helps to synchronize the uterus, ovaries, and cervix. This is so that the endometrium in the uterus is prepared for the baby embryo (should fertilization occur) and so that sperm aren’t interfering at all in the process of implantation.
If fertilization and implantation do not occur, then the corpus luteum signals the end of the cycle and steps down as conductor by reducing the production of progesterone and being reabsorbed by the body. This reduction in progesterone triggers the shedding of the endometrium causing menstruation and marking the start of another cycle.
However, if fertilization and implantation do occur, then hCG (the hormone measured by home pregnancy tests) is released. This hormone sustains the corpus luteum which then continues to produce progesterone, keeping the symphony going. This continued production of progesterone also keeps the cervical road closed to new sperm. The corpus luteum continues to play this role of producing progesterone until the placenta is able to produce enough on its own to sustain the pregnancy (usually the first 8 to 12 weeks of pregnancy).
While this discussion serves primarily as a foundation for our guide for couples trying to conceive, I would be remiss if I didn’t stop here to behold the amazing beauty, unbelievable complexity, and magnificent order in God’s design for conception and pregnancy. It is miraculous!
Irregular Cycles Are Normal Cycles
The term “irregular” I think is often misunderstood. It seems to carry a notion that if a woman’s cycle doesn’t run like clockwork, consistently 28 days in length, then her cycle is abnormal. This is far from true. The reality is that most women’s cycles are variable in length and 28 days is more of an average than the definition of a healthy cycle. In fact, the definition of a “regular” cycle allows for cycles to range from 24 to 38 days[2] and to vary up to 7 to 9 days between cycles[3]. What this means is that a woman’s cycle might be 26 days one month, 34 days the next, and 29 days after that and those cycles would still be considered “regular”. Variability then is not, on its own, an indication that something is wrong. So, what causes this variability?
If our discussion here has revealed one thing it’s this: ovulation is the center piece of the woman’s cycle. It marks a distinct change in the hormones being produced: facilitating the increase in progesterone and the related decrease in estrogen. So, with this importance in mind, allow me to simplify all these ovarian and uterine phases into two overarching parts of the woman’s cycle: prior to ovulation and after ovulation. Prior to ovulation, the whole cycle is on pins and needles awaiting that LH surge which triggers ovulation and an increase in progesterone. After ovulation, the whole cycle watches that progesterone level closely to see if pregnancy has occurred or if it’s time to hit the reset button and start another cycle.
While the length of the cycle prior to ovulation can vary from one cycle to the next, the length of the cycle after ovulation is actually quite consistent from cycle to cycle. This is in part because there are many environmental factors (like stress) that affect ovulation, while the rest of the cycle is less sensitive to these things. So, once a woman has learned her own cycle, she can have confidence that from ovulation to the start of another cycle is going to be nearly the same number of days each time.
For us personally, this meant that once we used a fertility monitor for a few months, then we slowly began to learn Monica’s cycle. While the day of her cycle that ovulation occurred varied from cycle to cycle (day 16 one cycle, day 14 the next, and day 20 after that), the number of days after ovulation before menses was surprisingly consistent (16 days in the first cycle, 16 days in the second, and 17 days in the third). So, while the total length of her cycle varied (32, 30, and 37 days), all this variability came from the portion of her cycle before ovulation.
The fertility monitor took the guesswork out of knowing when she ovulated and learning her cycle took the guesswork out of knowing when her next period would begin. Her cycle went from feeling inconsistent to predictable. This predictability helped us know what to expect when we wanted to conceive (and avoid conception); it even helped our marriage as we were both more aware and I could be more sensitive to when her cycle might contribute to a “discussion”.
Male Fertility
So, we’ve talked quite extensively at this point about a woman’s cycle of fertility, but what about a man’s fertility. Men do not share the same cyclicality to their fertility. Rather, a man will consistently produce semen throughout his lifetime month in and month out. While age does affect the level of his fertility, generally a man is fertile from puberty onward. I’ll spend less time here, but our discussion would be incomplete without a short background on the male contribution to the process.
First of all, it might be necessary to mention that semen does not consist only of sperm. While the testes produce the sperm in semen, the seminal vesicles, prostate, and bulbourethral glands contribute the rest of the seminal fluid that composes most of the volume of semen. This seminal fluid provides various components that help to protect the sperm, enhance their mobility, and even provide nutrition for their journey. Of all of these components, the sperm is certainly the most critical to male fertility. The quantity and quality of sperm in semen are the primary measures used to determine male fertility.
The frequency of intercourse does indeed affect both the quantity and quality of semen that a man will release. While less frequent intercourse increases the amount of semen/sperm, it can also lead to sperm DNA fragmentation thereby reducing the quality of the sperm and overall fertilization rates and outcomes.[4] We’ll save the more detailed and practical discussion of how this impacts timing of intercourse for couples who are trying to conceive for our page dedicated to that: Timing Intercourse to Conceive.
Additionally, there are some controllable environmental factors that can affect the production of sperm and thereby influence male fertility. These including the use of alcohol, drugs, and smoking. Additionally, stress and exposure to pesticides, heavy metals, and other toxins can also affect sperm production. Exposing the testes to heat can also temporarily affect sperm production including saunas, hot tubs, sitting for long periods, wearing tight clothing or underwear, and using a laptop (i.e. on your lap). [5] Finally, regular exercise has also been shown to improve male fertility.[6] Being mindful of these things can help to avoid the reduction in male fertility they can cause.
The Couple’s Fertility
The Fertile Window
Perhaps the most practical application of this is to understand when a couple is fertile during the month. The fertile window is the period in each cycle when intercourse can result in pregnancy. As we’ve seen, a woman ovulates only once per cycle as a rule. This egg that is released survives for only 24 hours. This means that, throughout a woman’s cycle, fertilization (conception) can only occur in that 24-hour period after ovulation. However, because sperm can survive inside a woman’s body for 7 days, the fertile window is longer than this. In other words, if a couple has intercourse 7 days before ovulation up to 1 day after ovulation, then fertilization can result. This is the fertile window.
This fertile window is still a relatively short period of time during each cycle where intercourse can result in conception. This is also the time of a woman’s cycle, however, where she experiences the most interest in sex which can serve not only to encourage intercourse but also can help signal that conception is possible. So, can a couple use all of this information to time intercourse to support conception? Indeed, they can. We’ve reserved the practical discussion for how a couple can do this on our page dedication to Timing Intercourse to Conceive.
Conception, Implantation & Early Pregnancy
Before we end, it made sense to conclude our discussion on fertility by talking about what happens once conception has occurred. Let’s see how God’s design helps support conception as well as implantation and the first weeks of pregnancy.
Once the sperm reaches the egg, fertilization does not always occur. The egg has brought half of the DNA from mom and the sperm has brought half of the DNA from dad. This is where sperm DNA fragmentation that we mentioned earlier as well as the overall quality of the sperm and the egg come into play. The quality of the sperm and egg determines whether or not fertilization is successful. If the sperm and egg have everything they need, then the miracle of fertilization (conception) occurs!
Conception is precisely the moment when a new person’s life begins. From the moment of conception a person is alive. (You can see some discussion of this in our post for Mother’s Day; we have more research on this topic that we hope to release over time.) That first cell formed at fertilization, called a zygote, already begins to release an endocrine signal telling mom “I’m here”! Even from the moment of conception, the baby that’s conceived is able tell mom that they are alive! That’s incredible! Mom’s body is essentially listening for this signal and it triggers the release of a hormone known as EPF (early pregnancy factor). EPF then works to suppress mom’s immune system to protect the baby! Even from conception, a woman’s body is able to recognize she is carrying her child and chooses to protect that child even at the risk of inhibiting her own immune system. This truly is an amazing design.
Once the baby makes their way down the fallopian tube to the uterus, they begin to nestle in to the endometrium (the lining of the uterus). This process is called implantation and occurs about 8-9 days after conception. The endometrium has been prepared for implantation by progesterone and is actively releasing nutrients for the baby.
Implantation triggers the release of the hormone hCG (the one that is measured by home pregnancy tests) which sustains the production of progesterone to keep the endometrium thick. This hormone, hCG, also serves to suppress the mother’s immune system to protect the baby from being falsely identified as an infection. Eventually, once the placenta is established around 8-12 weeks, it takes over the production of progesterone.
This amazing process is not without its challenges. While everything in God’s design is there to support conception, implantation and a healthy start to pregnancy, it’s easy to see how the sheer complexity of the process allows so many opportunities for a misstep. As a result, early miscarriages happen far more often than most of us are aware. This can be a difficult burden for parents, especially for those who have been trying to conceive for some time. In the overview of our guide for couples who are trying to conceive we talk more about the emotional and spiritual challenges of this process as we ask the hard questions: “Why is conception so hard?” and “Where is God in the process?” If you are struggling to conceive, I’d encourage you to take a look at that page.
[1] FSH remains prevalent even after ovulation in part because the other ovary is busy developing an egg for release in the next cycle.
[2] Fraser IS, Critchley HO, Broder M, Munro MG. The FIGO recommendations on terminologies and definitions for normal and abnormal uterine bleeding. Semin Reprod Med. 2011 Sep;29(5):383-90.
[3] Munro MG. Practical aspects of the two FIGO systems for management of abnormal uterine bleeding in the reproductive years. Best Pract Res Clin Obstet Gynaecol. 2017 Apr 01;40:3-22.
[4] Borges E Jr, Braga DPAF, Zanetti BF, Iaconelli A Jr, Setti AS. Revisiting the impact of ejaculatory abstinence on semen quality and intracytoplasmic sperm injection outcomes. Andrology. 2019;7(2):213-219. doi:10.1111/andr.12572
[5] Mayo Clinic: https://www.mayoclinic.org/diseases-conditions/male-infertility/symptoms-causes/syc-20374773
[6] Gaskins AJ, Afeiche MC, Hauser R, et al. Paternal physical and sedentary activities in relation to semen quality and reproductive outcomes among couples from a fertility center. Hum Reprod. 2014;29(11):2575-2582. doi:10.1093/humrep/deu212