A Needle in a Hay Stack…Finding Your Most Fertile Day

There are close to 2 billion women of reproductive age on our planet. Every month, these women's bodies are designed to grow and release an egg that can be fertilized by sperm to form a new baby - except, of course, if the woman is pregnant, nursing or taking hormonal contraceptives. This egg release is called ovulation. Ovulation occurs when a small sac (the "follicle") forms on the ovary. During a woman's six fertile days each month, the follicle becomes larger and the wall thinner, until "POP" - a signal from the brain causes a rupture of the sac and the woman's egg is swept into her Fallopian Tube. As the egg rolls down the tube it hopes to meet and merge with a healthy sperm so that conception can occur.

Doing the math, with all of us ladies around the world, there could be as many as 278,000 human ovulations a second, each day, 365 days a year. That is a lot of popping and a lot of potential baby-making!

All of this makes ovulation sound very exciting and sexy. But, in reality, it is a quiet and unobservable process, which means that most of us can only guess when it happens. The only way to see ovulation happen is to use advanced medical imaging equipment to track the forming follicle and the rupture (the "pop") that leads to egg release. So unless you plan on buying a very expensive ultrasound machine to look at your ovaries every day (which has been done!), knowing exactly when you ovulate each month can feel like looking for that proverbial needle in a haystack.

Why is ovulation prediction important?

Predicting Ovulation, A Needle in a Haystack

If a woman wants to become pregnant, identifying when she is fertile (that is, when she is approaching ovulation) is critical because sperm and eggs only live for a short time. Specifically, the egg lives between 12 and 24 hours after it is released from the follicle. As the egg ages over this one day, it can degenerate, leading to problems with the pregnancy or even the baby's health.

On a side note, we are just beginning to understand what helps an egg stay healthy longer after ovulation, but antioxidants, such as the ingredients in FertilAid for Women and OvaBoost may be beneficial.

In contrast, sperm from SOME men can live more than 5 days after intercourse, patiently resting in the woman's Fallopian Tube waiting to meet that egg …but sperm from other men may live less than 24 hours.

It is easy to see that for some couples, for example if the woman's egg only lives 12 hours and her man's sperm live less than one day, timing sex just right so that a viable sperm and egg meet in the Fallopian Tube can be challenging.

To make things even more complicated, two recent studies are turning what we know about a woman's fertile window upside down. Warning, you may be reading this breaking science before your doctor does!

The basics of the menstrual cycle

To explain the new understanding on women's fertility and chances of conceiving, we need to review a wee bit of physiology.

Every woman's monthly cycle is counted in days. Day 1 is the first day of your period (actual bleeding). The average menstrual cycle in the US is 29 days (from the start of one period to the start of another). The average variation for an individual woman within her own cycle is 7 days. This means a woman could have a 21-day cycle one month and then a 35-day cycle the next! Some of us are regular as clockwork, others - not so much! Illness, stress, diet, certain medication, and having sex can impact your cycle length.

Another side note: It is important to remember if you are taking oral contraceptives - your body's natural cycle is not happening. The Pill is giving you non-bleeding and bleeding days, so don't look to understand your own cycle while on the Pill or other hormonal contraceptives.

After we are done with our period, the follicle sac in the ovary grows and begins making estrogen, the female sex hormone. As the egg becomes ripe and our estrogen levels rise, we enter into the 5 days before ovulation, which are part of our fertile time. Because sperm from most men live at least this long, sex during this time can lead to conception. Sex during the five days before ovulation plus the day of ovulation (six days overall) gives us the highest chance of pregnancy. These six days are our "fertile window."

The day before ovulation, a hormone with an unfortunate name, luteinizing hormone or LH, is released as the brain senses the follicle is ready to rupture and release an egg. Once this LH level peaks, often called the "LH surge," ovulation will occur within 24 hours for over 80% of women and within 48 hours for 98% of women. The remaining few percent of women get their signals mixed up and may not have normal follicle rupture and ovulation… L

The LH surge is nature's marker of impending ovulation. It can be measured in blood or urine and is the basis of most home ovulation prediction kits.

Now that you know a bit more about what is happening during ovulation, let's look at why finding your fertile window is so very critical to fertility.

Most women get pregnant mid-cycle, but we don't all follow the rules

In one recent study of over 6,000 women, the chance of being in the fertile window went from 2% on Day 4, to over 50% on Days 12-15, and back down to 5% on Day 21 of the cycle. Remember that Day 1 is the first day of bleeding during a period.

The probability of conception from intercourse was also highest at 13% if sex occurred on Day 15 of the cycle. Sex before or after this midpoint resulted in a much, much lower chance of pregnancy. Most doctors and our sex education textbooks teach that women are most fertile on Day 14 or 15. This study confirmed that half of all women will be fertile mid-cycle (Days 2-15). But that also means half of all women WILL NOT be fertile during this time!

One can see by looking at this chart (kindly supplied by Dr. D. Li, 2015) that chances of conceiving from one act of intercourse dramatically rise and fall during the "average" woman's monthly cycle. Overall, one act of intercourse even on a woman's peak fertile day only has about a 10% chance of a pregnancy. The secret to maximizing pregnancy odds during the fertile window is having sex frequently, so that cumulative odds come into play. Even then, the very highest chance of a pregnancy during each cycle is still only about 33%.

Probability of clinical pregnancy after observing one unprotected act of intercourse relative to the day of the menstrual cycle. Courtesy of Dr. Li 2015

Probability of Clinical Pregnancy

But not everyone's body reads the book or follows the average. This same study found that older women became pregnant earlier in their cycles. Women under 25 had more pregnancies from later ovulations, closer to Day 21. Women with irregular cycles also conceived later in the cycle. And Asian women had a greater chance of conceiving later in their cycle than did Caucasian women.

In my experience, women who ovulate late in the cycle often miss having sex when they are fertile, because they don't realize that it is even possible that they could be ovulating as late as day 21, or even later.

Fertile women are friskier around ovulation, and "fooling around" causes ovulation

A second study is a reanalysis of findings from a benchmark 2001 publication often cited to tell women their odds of becoming pregnant. If you have ever heard that women have a 20% chance of pregnancy each cycle, this number came from the 2001 study.

Recently, these authors realized that when they calculated the chance that women would get pregnant from one intercourse event on any given day of the cycle (the "benchmark pregnancy rate"), they missed two important biological facts.

The first missed fact is that women have sex more frequently when they are fertile. The rising estrogen levels from our developing follicle makes us feel frisky. We seek out our men and likewise they are more attracted to us. There are many studies showing the changes in both men and women that occur during the woman's fertile time. A couple of my favorite changes are that women wear 5 times more pink and red during their fertile window and men drink more alcohol when exposed to T-shirts from fertile women. These changes have no scientific explanation beyond the fact that humans are still animals behind all our fancy lifestyles.

Interestingly, more unprotected, unplanned sex occurs during women's fertile time than during their non-fertile time, because of this biologic draw between the sexes. This explains the seemingly high number of couples (like at our house) with "surprise" conceptions from just one time of not using birth control.

Secondly, although rabbits and cats don't ovulate until after mating with a male, scientists thought humans were far removed from any link between intercourse and ovulation. But newer data suggests that having sex helps cause ovulation in humans too!

In fact, the probability that ovulation will occur after a single intercourse event increases from 9% on the day after intercourse to a whopping 30% six days after intercourse. During this time, intercourse triggers hormone changes in the woman that recruits a growing follicle so an egg can mature. The woman's LH levels begin to rise and ovulation is triggered. It is as if the woman's body knows "mama has a honey" and redirects ovary function to rapidly make a mature egg so his sperm do not go to waste.

Taken together, more sex and more ovulation after this sex means healthy couples who are trying to conceive have a higher chance of conceiving than doctors previously thought, up from 20% to over 30% each cycle!

Many of us don't know how to tell if or when we ovulate

The research discussed above is good news for couples actively trying to conceive. Unfortunately, recent research reveals a bit of bad news as well.

Specifically, only about 20% of women can accurately estimate their day of ovulation. And only 55% of women were even in their fertile window when they thought they were ovulating. That means almost half of all women may be having sex to try to conceive when they are not fertile.

In more detail, no women in this study believed they ovulated earlier than Day 9 (although some women do ovulate before this). Also, no women thought they ovulated later than Day 25, when in reality some women ovulated as late as Day 44.

Most women (56%) estimated their ovulation day either two days too early or two days too late from the true day. If women are with men who have short-lived sperm, 2 days too early won't leave mama with living sperm to meet the egg. Two days too late, results in a dead egg and no one to greet the sperm. In either situation, no baby!

My favorite words to conceive by include: "Timing intercourse to coincide with the fertile period is a simple method which can maximize the chances of natural conception. However, to be effective, timed intercourse requires a good awareness of the day of ovulation." (Zinaman et al, 2012)

An introduction to tools to help with ovulation prediction

ovulation prediction chart

By now you understand that having sperm meet the egg during a 12-hour window each month is critical to conception. But you may be wondering how to identify that fertile window and ovulation. There are several methods to help optimize baby-making sex.

The LH surge in urine is a reliable indicator that ovulation will occur within 12 to 36 hrs. Ovulation prediction kits such as Fairhaven Health's BFP ovulation tests can track this LH surge, but remember there isn't much advanced warning with these (usually only about one day in advance) and you also have to know which day to start using the test strips. Also, if you ovulate late, it is easy to give up on testing or run out of test strips before you ovulate, and doing so means you will miss your chance to have intercourse during your fertile time.

Additionally, peak LH surge values in urine vary dramatically between women from between 14 mIU/ml to 73 mIU/ml. The lowest level a urine test can detect is about 25 mIU/ML. Some tests are very weak and only measure 50 mIU/ML. This means that some low quality LH tests will only show half of all women when they are ovulating. Always look up or call to know a manufacturer's kit limit using the correct units "mIU/mL". Fairhaven's BFP test measures LH to 25 mIU/ML, so it is quite sensitive.

Another way to find your fertile window is by detecting changes in electrolyte (minerals like sodium and potassium) concentrations in the saliva and cervical mucus as you move through your menstrual cycle. Fairhaven Health's OvaCue fertility monitor uses this electrolyte method to provide up to 7 days advance notice of ovulation. This patented OvaCue technology is now available as OvaCue Mobile, a fertility monitor that connects directly to your iPhone.

One of my favorite methods of detecting the fertile window is by tracking cervical mucus production. This topic is worth its own blog post, and I highly recommend Toni Weschler's best-selling book Taking Charge of Your Fertility, which describes in detail how to identify the changes in cervical mucus that indicate that ovulation is about to occur. Cervical mucus tracking is cheap and reliable in women who consistently produce cervical mucus each cycle.

If trying to predict your fertile time intimidates you, just have intercourse consistently 3 times a week, spread out every other. For most couples this will get the job done, unless your partner's sperm don't live as long as average, healthy sperm. Intercourse every 2 to 3 days is actually a part of the NIH guidelines for increasing the chance of conception. More intercourse helps trigger ovulation. So having sex frequently is a good starting place for many TTC couples.

How effective are ovulation prediction gadgets? P.S. They really do work!

Before you invest in a fertility monitor or ovulation tests strips, you might be curious about how much these tools actually increase pregnancy rates for trying-to-conceive couples? The unfortunate truth is that evidence-based medicine has not yet provided an answer to this question. When you see a doctor for heart issues, he/she can look to Level 1 evidence (the very best evidence there is) to know that statin drugs reduce mortality (as shown in 19 high quality studies with 57,000 patients) or that taking aspirin reduces strokes (8 high quality studies with over 9,000 patients). On the other hand, it is disappointing to note that only a handful of studies, involving a total of only a few hundred patients, have been done to determine if ovulation prediction tools decrease the time to conception, which leaves us with only a low level of evidence for their effectiveness. I am sharing this because some doctors may tell you "there is no evidence" these methods improve conception. While this statement is technically true, it doesn't tell the whole story. It is not that these methods have been proven to not work. Rather, the lack of scientific evidence about these products is due to the fact that our federal research dollars are not being used to fund well-designed studies on ovulation prediction methods. Over 50% of our tax funded research dollars in reproduction goes towards methods to improve the assisted reproduction used to make 1.5% of babies born by IVF. We need to pressure our legislators to fund studies that improve NATURAL reproduction and fertility for the other 98% of us!

The studies that have been done show an increase in conception rates from 13-16% per cycle without timing ovulation to a possible 23-33% chance of pregnancy with ovulation prediction methods. This is close to a 100% increase in relative pregnancy rates using products such as the BFP ovulation test or the OvaCue monitor.

Ovulation is a normal part of female physiology. Unfortunately, many of us are not taught how to accurately find our fertile window or ovulation day. This can make it unnecessarily hard to conceive a child or to avoid an unwanted pregnancy. With all those 278,000 eggs being released from women around the world every second of every day, understanding how our bodies work and finding the right tools to help us identify ovulation can be exciting instead of frustrating!


This original article was written by Joanna Ellington, PhD. Dr. Ellington is an internationally-recognized researcher in the field of andrology and serves as medical advisor to Fairhaven Health. She has had numerous featured blogs on women's health at BlogHer, as well as other media channels.

Disclaimer: Please note that the information in this website is an educational resource and represents Dr. Ellington's personal opinions. All decisions about any treatment you need must be made in consultation with your doctor or your healthcare provider who has examined you. Nothing in this post is meant to be used to diagnose or treat any person.