How To Calculate Ovulation: Know When You’re Most Fertile

Calculate your Ovulation days

What’s the very first thing you should do to ensure a successful pregnancy? Calculate your ovulation day. Because, ladies, let’s face it, getting pregnant is a lot simpler when you understand the monthly rhythms of your body and do the math right.

The Sperm And The Egg

While the sperm comes from your male sexual partner, your body already has all the eggs you ever need. Every month, one of your ovaries releases an egg. This process is known as ovulation. The egg then travels to the fallopian tube, where it fuses with the sperm, provided the sperm is already waiting or will arrive soon. This fusion is called fertilization.

Your fertile window is of 6 days: your ovulation day, 4 days before it, and 1 day after it.

You cannot control whether or not the fertilization will happen; but what you can do is have sex around the time of your ovulation so that the sperm at least has a chance of reaching your egg. While the sperm can stay alive in your body for a maximum of 5 days, your egg lives for just 12 to 24 hours after ovulation.


So, there are basically just 6 days in a month when having sex can make you pregnant: 4 days before the ovulation, the day of ovulation, and the next day. This is known as your fertile window. Do remember that fertilization can happen only within 24 hours of ovulation, because after that the egg dies.

That brings us to the vital question: how to calculate your ovulation day?



If You Have A Regular 28-Day Cycle

If you have a regular 28-day menstrual cycle, you will ovulate 14 days before your next period. Have sex from day 10 to day 15.

In an ideal scenario, you have a 28-day menstrual cycle, that is you get your period every 28 days. Your ovulation would happen exactly on the 14th day of this cycle. So your fertile window would be day 10 to day 15, counting the first day of your last period as day 1.


If You Don’t Have A 28-Day Cycle

But not everyone has a 28-day menstrual cycle, which is quite normal. Even if they do, they may not ovulate on the 14th day.

Scientist Allen Wilcox did a study with the National Institute of Environmental Health Sciences on 696 menstrual cycles. He found that among the 69 women with 28-day cycle, only about 7 women ovulated 14 days before their next period. Some ovulated 19 days (that is on day 10 of their current cycle) before their period, and some just 7 days prior (day 22).


The Wilcox study also found that more than 70% women are in their fertile window before day 10 or after day 17.1

This can be owing to the length of your period. For instance, if it ends by day 3, it is possible that hormonal changes in your body that trigger ovulation to occur on day 10. There can be other physiological causes, like lack of sex hormones or their delayed release. There can be psychological causes like stress too.


Finding The Fertile Window

It’s difficult to predict the fertile window for women with less regular cycles, especially teenagers and perimenopausal women.2

Among the 70% 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 half the cases.

Cycle Length Fertile Window
< 28 days Day 7–Day 14
28 days Day 8–Day 16
29 days Day 10–Day 18
>30 days Day 13–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 phase. If day 7 also starts your fertile window, you will notice the following signs:

Day Egg White Cervical Mucus Luteinizing Hormone In Urine Basal Body
Intercourse Ovulation
7 Appears Regular Regular Yes No
8 Continues Rises Regular Yes No
9 Continues Rises Regular Yes No
10 Last day Surges Drops Best day No
11 Thick mucus Drops Stays low Yes Yes
12 Thick mucus Low LH Rises by 0.4 to 1° Fahrenheit No No


Ovulation Calender


If it starts on day 8, the signs will start showing a day later. But the ovulation calculator gives you a rather broad spectrum. To hit closer to home, look out for the physical and psychological changes.

Signs Of Ovulation

1. Raw Egg White-Like Cervical Discharge

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

Check your underwear for mucus or swipe your vagina with a cotton tissue. Do you see quite a bit of a clear, slippery, stretchy, and runny cervical discharge somewhat like raw egg white? This is called egg white cervical mucus (EWCM) or fertile-quality cervical mucus. This indicates you will ovulate soon.

For most women, the last day of the EWCM (peak day), is the estimated day of ovulation. For many, it is the day after. But in some, ovulation may follow after 3 days.3 Note that peak day doesn’t mean the best-quality mucus, it simply means the last day you find this mucus.

How much and how slippery your cervical mucus will be depends on how high your estrogen level is. Closer to ovulation, your ovary releases estrogen to keep the cervical opening wider and softer so that the sperm can travel easily. Your estrogen level reaches its maximum about 34 hours before ovulation.4 Then it starts dropping. After ovulation, progesterone starts increasing, making the mucus creamy and thick. This is 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 comes 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. But be warned that 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 significantly enough to 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 math right but don’t let it steal from the fun and excitement of baby-making.