The Best Scientific Way To Calculate Your Ovulation Day


18 Min Read

How To Calculate Your Ovuation Day

Ovulation day can be any day between Day 7 and 20, taking the 1st day of your last period as Day 1. Look out for raw egg white–like cervical mucus (EWCM), rising luteinizing hormone (LH) levels in the urine, and high libido. Usually, ovulation occurs 1 day after the LH peak, which is also the last day of EWCM. Have sex daily for 5 days before and on the day of ovulation or every other day from Day 7 to 20.

A successful pregnancy depends on how vigilant you and your partner are and how well you understand your body. Of course, this can’t be a random guesswork. There’s a bit of maths involved. But before we get cracking on the maths, let’s proceed step by step to your goal, pregnancy.

Beginning The Journey Toward Pregnancy

First things first, making the sperm and the egg available. The sperm enters your body through sexual intercourse, and the egg is released once a month from one of your ovaries, in a process known as ovulation, into the fallopian tube where it can fuse with the sperm, in a process called fertilization.

So, the basic idea behind a successful fertilization is timing intercourse and ovulation as perfectly as you can because there’s no way to control fertilization itself.

There’s a caveat even in that. It’s not easy to find out when the ovulation is actually taking place. And unlike the sperm that can stay alive in your body for a maximum of 5 days, your unfertilized egg has a lifespan of 12 to 24 hours.

So your number of fertile days is limited to 6 days a month: 5 days leading up to your ovulation and the day after. This is known as the fertile window. But not all days are equally fertile. Fertilization, however, can happen only within a maximum of 24 hours after ovulation.


To know your fertile window and time your intercourse to it, you need to calculate your ovulation day. There are ovulation calculators aplenty on the Internet to help you nail the day, right? Sadly, no.

Understanding The Menstrual Cycle

Most ovulation calculators, which are actually ovulation calendars, work on the assumption that ovulation happens exactly 14 days before your next menstrual cycle—a menstrual cycle ranges from the first day of your period this month to the first day of your period next month.

In an ideal scenario, you have a 28-day cycle—that is you get your period every 28 days—and your ovulation happens exactly on the 14th day, month after month, without any deviation whatsoever. But not everyone has a 28-day menstrual cycle, which is quite normal, and even if they do, they might not ovulate on the 14th day.

A credible study conducted by Wilcox with the National Institute of Environmental Health Sciences on 696 menstrual cycles finds that among the 69 women with 28-day cycle, only 10 percent women ovulated 14 days before their next period. In fact, the time of ovulation was varied and ranged from 7 to 19 days before their next menstrual cycle, that is Day 10 to Day 22 of their current cycle. It also found that quite contrary to previous clinical assumptions, more than 70 percent women are in their fertile window before day 10 or after day 17 of their menstrual cycle.1

This can be owing to the length of your period. For instance, if it ends by the third day, it is possible that there are hormonal changes in your body that trigger ovulation to occur on the tenth day. There can be other physiological causes, like delay in release of or dearth of any of the sex hormones, or psychological causes like stress.

Finding The Fertile Window

As it becomes difficult to pin down the day of ovulation, the timing of the fertile window too becomes difficult to determine because a 28-day cycle this month does not guarantee a 28-day cycle next month; nor does a three-day-long period this month guarantee the same duration next month. It might be all the more difficult to predict the fertile window for women with less regular cycles, including teenagers and perimenopausal women.2

Among the 70 percent women whose fertile windows range between Day 10 and Day 22, the Wilcox study also found a pattern between the cycle length and the estimated fertile window in at least 50 percent of the cases:

Cycle LengthFertile Window
Less than 28 daysDay 7 to Day 14
28 daysDay 8 to Day 16
29 daysDay 10 to Day 18
30 days and moreDay 13 to Day 19

Fertility Window


Taking into account all these irregularities and vagaries of the body and following the Wilcox study, the American Pregnancy Association suggests focusing on the fertile phase rather than the fertile window. They estimate the fertile phase as ranging from Day 7 to Day 20. If you want to be extra-cautious, include Day 6 and Day 21 as well.

Understanding The Ovulation Calendar

Taking the first day of your period as Day 1, mark Day 7 to Day 20 as fertile period. Going by the rule of the fertile window, if your fertile window starts on Day 7, this is what happens in most cases. If it starts on Day 8, the signs will start showing a day later. But because the ovulation calculator gives you a rather broad spectrum, to hit closer to home, look out for the physiological and psychological changes ovulation and the period leading up to it bring to your body. We have mentioned some of them here. Read on to find more about them in the following section.

DayEgg White Cervical MucusLuteinizing Hormone In UrineBasal Body
10Last daySurgesDropsBest dayNo
11Thick mucusDropsStays lowYesYes
12Thick mucusLow LHRises by 0.4 to 1° FahrenheitNoNo


Ovulation Calender


You Can Tell You Are Ovulating

1. If Your Cervical Discharge Is Like Raw Egg White

Look out for: raw egg white-like clear, slippery, and runny fluid in your vagina that can stretch about 5 inches without breaking

Reliability index: High

If you see quite a bit of a clear, slippery, stretchy, and runny cervical discharge in your underwear, or when you swipe your vagina with a cotton tissue, it indicates that you are about to ovulate soon. This discharge is called cervical mucus, and because at this stage close to ovulation it is pretty much like raw egg white, it is called egg white cervical mucus (EWCM) or fertile quality cervical mucus. For most women, the last day of the EWCM, medically referred to as the peak day, is the estimated day of ovulation. For many, it is the day after. But in some, ovulation may follow after three days.3

Note that peak day doesn’t mean the best-quality mucus, it simply means the last day you find this mucus.

The quantity and composition of the cervical mucus depends on the estrogen level in the body as the estrogen makes the cervix produce mucus so that the cervical opening can become softer and wider and can allow the sperm in easily. The higher the estrogen, the greater the quantity of the mucus and the more slippery it is to give the sperm easy passage.

About 34 hours before ovulation, the mature egg-containing follicle releases the maximum amount of estrogen,4 after which it starts dropping.

Right after your menstrual period, after a period of relative dryness, your mucus will be white or yellowish, cloudy, thick, and sticky. This is because the estrogen level is quite low. With the estrogen level going up, the quantity of the mucus will keep increasing over the next few days and it will get thinner, less cloudy, and less sticky, till it reaches the egg white-like consistency around the time of ovulation.

After ovulation, progesterone levels start rising, which now gives the mucus into a creamy, thick consistency that is no longer helpful for the sperm. This explains why the egg white-like cervical mucus may show till the day of ovulation, but no further.

Keep checking the mucus from Day 7 to Day 20 of your cycle. Whether or not you have a regular 28-day cycle with ovulation on the 14th day, the last day of cervical mucus would be about a day before ovulation. Seize the day. But better still, plan your intercourse at the first sight of EWCM.

Caution: Start testing right after your period so that the difference is obvious. Don’t confuse seminal or arousal fluid with cervical mucus. If any of these is present, make sure you remove it before checking for the cervical mucus. Not every woman produces it in huge amounts, even prior to ovulation.

2. If Your Urine Shows An LH Surge

Look out for: a high amount of LH in the urine that rises each day for 3 days max

Reliability index: High

When you do a simple urine or blood test at home with an ovulation predictor test kit that measures the level of luteinizing hormone (LH) in your urine or blood plasma, does it say there’s a rise in it? If it does, and if you have been seeing it rising for the last 3 days, you are close to ovulating.

The high estrogen levels in your body before ovulation stimulates the release of LH. LH in turn triggers ovulation. Usually, ovulation occurs within 3 days of the first significant rise in the level of LH and within 36 hours of peak LH.5

Because the rise in LH is linked to the high estrogen levels, you’ll notice a link between your cervical mucus quality and LH levels. The peak day of cervical mucus coincides with the LH surge.6 This is the ideal day for copulation. But, again, to be safe, engage in intercourse as soon as you see the LH rising.

Caution: Start testing right after your period, twice a day, between 10am and 8pm. Don’t test first thing in the morning, nor after drinking a lot of water.

3. If The Thermometer Is Calling You Hot

Look out for: a drop in temperature on the day of high LH and a sharp increase within two days

Reliability index: Medium

If you’ve been tracking your basal body temperature (BBT)—your lowest body temperature when you are at rest, i.e. sleeping—right after you wake up every morning and you see a 0.4 to 1.0 degree Fahrenheit rise in it, congratulations, your egg is available for fertilization now. This rise is due to the progesterone surge in the body, and it will continue till your next menstruation.

You might even be one of the lucky few who experience a drop in BBT before or at the time of ovulation. If you do find a drop, see if it’s coinciding with your LH surge. A study finds that the BBT hits its lowest point at the beginning of the LH surge, and increases again 8 hours after ovulation, or within 36 hours.7

But for your practical purposes, this test will come handy only if you observe your temperature patterns over a few months. You can then assess your day of ovulation with some certainty.

Caution: Invest in a special basal body temperature thermometer, not your usual fever thermometer, and make it a habit to measure the temperature first thing every morning before leaving the bed. Maintain the same hours too.

 4. If You Have Difficulty Reaching Your Cervix

Look out for: centrally aligned, soft cervix, which is difficult to reach, with a slight opening in the middle

Reliability index: Low

If you try reaching a finger into your cervix when you are closer to ovulating, chances are you won’t be able to because high levels of estrogen cause the uterus to be pulled further inside the body.

If you do, you will find it centrally aligned and soft to touch, like pursed lips. You might even be able to make out a dent in the middle of the cervix, called the os, which is the opening to the uterus. If you are ovulating, the os will be slightly open.

After ovulation, as the estrogen level goes down, the cervix comes back down and might even be a little tilted to one side. The os remains closed until menstruation when it opens slightly.

Check the position of your cervix every day for several menstrual cycles to be able to compare the differences between the phases.

Caution: Unless you are a natural, understanding the cervical shift can be a tough job. So don’t set great store by this method.

5. If Your Breasts Are Hurting

Look out for: breast tenderness coinciding with egg white-like cervical mucus

Reliability index: Low

Increase in estrogen is also related to some changes in your breasts. They might feel fuller, and you might notice a dull, heavy pain along with some tenderness. This happens mostly in your child-bearing age.

Caution: The symptoms in themselves aren’t entirely reliable indicators of ovulation because these may present even after ovulation, and might be more in women with fibrocystic breast disease.

6. If You Are Spotting Or Cramping Before Your Period

Look out for: brownish or pinkish blood spots, or very mild flow, and dull pain on one side of the abdomen

Reliability index: Low

This is not very common, but some women point out that they have had light spotting and cramping on one side of the abdomen in the middle of their cycle. It’s not worrisome unless your flow is heavy and the pain is severe. Some experts suggest that the release of the egg and the fluid causes abdominal pain on the side of the ovulating ovary. One study, however, calls it a pre-ovulatory symptom, claiming that the pain, medically known as Mittelschmerz, coincides with the LH surge. The hormone might be the cause of contraction in some of the ovarian muscles.8

Caution: These signs do not appear in every woman or every month. The reason for pain may not even be related to ovulation. This is not a very reliable way of predicting ovulation, unless it matches your calendar and your LH test.

7. If Your Are Craving And Fantasizing About Sex

Look out for: increased sexual urges and fantasies

Reliability index: Medium

In the middle of your cycle, if you are feeling less lonely and finding yourself more in the mood for sex and fantasizing about it, you might be about to ovulate. A study finds that in women who initiate sexual activity, libido begins to increase 3 days before their LH rises and lasts for a total of 6 days.9

Caution: Of course, this is not foolproof, and your libido depends on your general mental health too. But hey, nature’s with you on this. So throw caution to the winds and get active.

Your Partner Can Tell You Are Ovulating

1. Your Face Appears More Attractive

If your partner tells you you’re looking pretty somewhere near your mid-cycle, it’s not flattery. You are probably about to ovulate.

A study asked men and women to judge photographs of women who were about to ovulate and who had already ovulated, without giving away any information about the women. It turned out that women in their pre-ovulation stage were deemed more attractive.10

2. And You Make Sure It Looks Attractive

A few days after you’ve emerged from the bloodbath of your menses, you do feel the urge to deck up, right? Well, it could be your ovulation alarm going off.

A study in which 30 women were photographed during their high and low fertility phases showed that women nearing ovulation dressed attractively and had fancier hairstyles. In fact, when 42 judges chose photographs of women who were trying to look attractive, without knowing anything about the women, they picked more photos of women in the pre-ovulatory phase than in the post-ovulation phase.11

3. You Sound More Attractive

Don’t be surprised if people compliment you on your voice somewhere near mid-cycle, or a few days before you start PMSing.

A study found that both men and women judged the voice of an ovulating woman as more attractive than one who wasn’t. It suggested that the hormones released during ovulation bring about certain changes in the larynx, which produces this effect.12 The pitch also tends to be higher in the two days leading up to ovulation.13

4. And You Smell Better Too

You can read “smell” any way you like because it cuts both ways. If you are close to ovulation, your partner will find your odor pleasant and more attractive14, and you yourself will be more sensitive to any kind of smell and musks. A study on 313 women in several stages of their menstrual cycle found that the odor detection threshold was lowest during the ovulatory phase.15

When Should You Have Sex?

The best day is the day before ovulation.

Now that you have found your ovulation day, the next step is timing the intercourse.

A 1995 study by Allen J. Wilcox on timing the intercourse in relation to ovulation found that the probability of conception ranged between 1o percent, when intercourse happened five days prior to ovulation, to 33 percent, when it happened on the day of ovulation. This indicates that the highest probability was on the day of ovulation. The study also found that only 6 percent of the pregnancies could be attributed to sperm that was three or more days older.16

A 1999 study, however, finds that the highest probability is when the intercourse occurs the day before ovulation and the lowest is when intercourse occurs on the day after ovulation.17 The American Pregnancy Association and many doctors suggest that the chances of pregnancy go down to 5 percent, even zero, if you have intercourse on the day after ovulation, because it’s better to keep the sperm waiting for the egg than the other way around. By that same logic, intercourse a day before ovulation makes more sense than on the day of ovulation.

And How Frequently?

Don’t take chances. Have sex on all days of the fertile window or on alternate days from Day 7 to Day 20, counting the first day of your last period as Day 1.

But if you don’t want to take chances, plan your intercourse on all the days of the fertile window, rather than just the day before ovulation or on alternate days leading up to ovulation as some doctors suggest.

Though there is some concern that the sperm count drops with more successive intercourse, research suggests that it doesn’t drop so much that it would affect the chances of pregnancy.

There’s also another theory that it’s best if you abstain from intercourse for five days before the fertile window as that maximizes the sperm count on the first day of intercourse, but that needs more research.18

The American Pregnancy Association takes it one step further and recommends that you engage in sex every alternate day in the fertile phase, that is from Day 7 to Day 20, rather than only in the fertile window.

Alas, These Are Not The Only Criteria

Be warned though that intercourse during the fertile window is not the only parameter of a successful pregnancy. There are other factors such as the sperm and egg health and how well the uterus can receive the fertilized egg.

Also, fertility starts decreasing in women in their late 20s, and those in their late 30s experience a significant reduction. While age doesn’t affect men’s fertility as much, men in their late 30s show decline too.19

But don’t let negative thoughts add up to your stress and skew the odds of pregnancy further. Do your maths right but don’t let it steal from the fun and excitement of baby-making.

References   [ + ]

1, 2.Wilcox, Allen J., David Dunson, and Donna Day Baird. “The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study.” Bmj 321, no. 7271 (2000): 1259-1262.
3.Fehring, Richard J. “Accuracy of the peak day of cervical mucus as a biological marker of fertility.” Contraception 66, no. 4 (2002): 231-235.
4.Pauerstein, C. J., C. A. Eddy, H. D. Croxatto, R. Hess, T. M. Siler-Khodr, and H. B. Croxatto. “Temporal relationships of estrogen, progesterone, and luteinizing hormone levels to ovulation in women and infrahuman primates.” American journal of obstetrics and gynecology 130, no. 8 (1978): 876-886.
5.Organization, Health, and Special Programme of Research. “Temporal relationships between ovulation and defined changes in the concentration of plasma estradiol-17β, luteinizing hormone, follicle-stimulating hormone, and progesterone: I. Probit analysis.” American Journal of Obstetrics and Gynecology 138, no. 4 (1980): 383-390.
6.Nulsen, John, Carol Wheeler, Militza Ausmanas, and Luis Blasco. “Cervical mucus changes in relationship to urinary luteinizing hormone.” Fertility and sterility 48, no. 5 (1987): 783-786.
7.de Mouzon, Jacques, Jacques Testart, Brigitte Lefevre, Jean-Luc Pouly, and René Frydman. “Time relationships between basal body temperature and ovulation or plasma progestins.” Fertility and sterility 41, no. 2 (1984): 254-259.
8.O’Herlihy, C., H. P. Robinson, and L. J. De Crespigny. “Mittelschmerz is a preovulatory symptom.” British medical journal 280, no. 6219 (1980): 986.
9.Bullivant, Susan B., Sarah A. Sellergren, Kathleen Stern, Natasha A. Spencer, Suma Jacob, Julie A. Mennella, and Martha K. McClintock. “Women’s sexual experience during the menstrual cycle: Identification of the sexual phase by noninvasive measurement of luteinizing hormone.” Journal of Sex Research 41, no. 1 (2004): 82-93.
10.Roberts, S. Craig, Jan Havlicek, Jaroslav Flegr, Martina Hruskova, Anthony C. Little, Benedict C. Jones, David I. Perrett, and Marion Petrie. “Female facial attractiveness increases during the fertile phase of the menstrual cycle.” Proceedings of the Royal Society of London B: Biological Sciences 271, no. Suppl 5 (2004): S270-S272.
11.Haselton, Martie G., Mina Mortezaie, Elizabeth G. Pillsworth, April Bleske-Rechek, and David A. Frederick. “Ovulatory shifts in human female ornamentation: Near ovulation, women dress to impress.” Hormones and behavior 51, no. 1 (2007): 40-45.
12.Pipitone, R. Nathan, and Gordon G. Gallup. “Women’s voice attractiveness varies across the menstrual cycle.” Evolution and Human Behavior 29, no. 4 (2008): 268-274.
13.Bryant, Gregory A., and Martie G. Haselton. “Vocal cues of ovulation in human females.” Biology Letters 5, no. 1 (2009): 12-15.
14.Kuukasjärvi, Seppo, CJ Peter Eriksson, Esa Koskela, Tapio Mappes, Kari Nissinen, and Markus J. Rantala. “Attractiveness of women’s body odors over the menstrual cycle: the role of oral contraceptives and receiver sex.” Behavioral Ecology 15, no. 4 (2004): 579-584.
15.Navarrete-Palacios, Evelia, Robyn Hudson, Gloria Reyes-Guerrero, and Rosalinda Guevara-Guzmán. “Lower olfactory threshold during the ovulatory phase of the menstrual cycle.” Biological psychology 63, no. 3 (2003): 269-279.
16.Wilcox, Allen J., Clarice R. Weinberg, and Donna D. Baird. “Timing of sexual intercourse in relation to ovulation—effects on the probability of conception, survival of the pregnancy, and sex of the baby.” New England Journal of Medicine 333, no. 23 (1995): 1517-1521.
17.Dunson, David B., Donna D. Baird, A. J. Wilcox, and C. R. Weinberg. “Day-specific probabilities of clinical pregnancy based on two studies with imperfect measures of ovulation.” Human Reproduction 14, no. 7 (1999): 1835-1839.
18.Stanford, Joseph B., George L. White Jr, and Harry Hatasaka. “Timing intercourse to achieve pregnancy: current evidence.” Obstetrics & Gynecology 100, no. 6 (2002): 1333-1341.
19.Dunson, David B., Bernardo Colombo, and Donna D. Baird. “Changes with age in the level and duration of fertility in the menstrual cycle.”Human reproduction 17, no. 5 (2002): 1399-1403.
CureJoy Editorial

The CureJoy Editorial team digs up credible information from multiple sources, both academic and experiential, to stitch a holistic health perspective on topics that pique our readers' interest.

CureJoy Editorial

The CureJoy Editorial team digs up credible information from multiple sources, both academic and experiential, to stitch a holistic health perspective on topics that pique our readers' interest.