5 surprising facts about the female egg cell

1. It’s the biggest cell in your body

Most cells in our bodies cannot be seen without a microscope, but the female egg cell is big enough to be visible to the naked eye. Pretty impressive huh?

2. You’re born with all of your eggs in two baskets

Every woman is born with a set of eggs in her ovaries opposed to men who produce new sperm every 90 days. You can have as many as 7 million eggs in your ovaries when you are born.

3. Eggs decline over time

Although you are born with millions of eggs they will diminish over time so you end up with around 700,000 by the time you hit puberty. Each month you continue to lose eggs, at the point of menopause you will have approximately less than 1000 eggs remaining. Opposed to common misconception neither pregnancy nor hormonal contraception slows the monthly recruitment and loss of eggs down.

4. You release an egg every cycle

For most women the body initiates ovulation every cycle, which is when you have a positive LH test – it indicates your body is getting ready for ovulation. But some women have the potential to release two eggs during one cycle, one per ovary, which is how fraternal twins are made. This is only possible within a 24 hour period.

Women who take hormonal contraception do not release an egg every cycle, as it inhibits ovulation. This is how pregnancy is prevented – if there is no egg to meet the sperm you cannot get pregnant. A regular cycle without ovulation is known as an anovulatory cycle.

5. An egg has a short lifespan after ovulation

Once released, an egg can only be fertilised over the next 12-24 hours. Sperm, on the other hand, can live up to 5 days if the sperm encounters the right environment. Natural Cycles takes these factors into account when calculating your fertile days.



Your Natural Cycles team


5 Facts About Ovulation


Whether you’re planning a pregnancy or tracking your fertility, understanding when you ovulate is key. This week we’ve gathered a couple of cool facts around ovulation that maybe you weren’t aware of. Find out below!

 1. The female egg cell is the biggest cell!

Most cells in our bodies cannot be seen without a microscope, but the female egg cell is big enough to be visible to the naked eye. Pretty cool!

eggcell2. Women are born with all the eggs that she will ever produce during her lifetime.

We are born with 1-2 million immature eggs (follicles) in our ovaries. Once a woman reaches puberty roughly 500 of these will mature into an egg cell that can be fertilized throughout our lifetime.  

3. The egg cell only lives 12-24 hours

Once released, an egg can only be fertilized over the next 12-24 hours. Sperm, on the other hand, can live up to 5 days if the sperm encounters the right environment.

4. Normally only 1 egg is released each ovulation.

However, some women have the potential to release two eggs during one cycle, one per ovary, which is how fraternal twins are made! This is only possible within a 24 hour period. Afterwards, ovulation is prevented by the high progesterone values in your body, which is what Natural Cycles detects in your temperature (rise of 0.2-0.45 °C).

naturalcycles_ovary5. You can get your period although no ovulation has occurred.

We see in our Cyclers’ data that around 5% of the cycles are in fact anovulatory – meaning that no ovulation has occurred. This is completely normal and nothing to worry about. The bleeding that follows is then due to the estrogen rising, rather than the progesterone decreasing, and is thus not exactly the same as a normal menstruation.

This was all for now Cyclers!
Please comment if you have any questions and read more in our support portal ask.naturalcycles.com.

When Will I Ovulate?

Knowing your menstrual cycle is extremely important for couples trying to conceive (TTC). The important question you need to ask yourself is, “ When will I ovulate? ”. Once you know this date, you can work out when you are most fertile and should start trying to conceive or indeed wear protection, depending on your goal. However, finding your ovulation can be difficult, especially if you have irregular cycles. In this blog post we wanted to explain how Natural Cycles can be a useful tool to track fertility and stay on top of cycle variations.

Let’s start with the basics.

Ovulation day is when an egg is released from the ovary and is available to be fertilised by a sperm. As sperm can survive inside of you for up to 5 days this creates a 6 day fertile window, which is the period a woman can get pregnant. However, the likelihood of becoming pregnant is dramatically increased if you have intercourse in the three days leading up to and including ovulation. If a woman has sex on any of these three days, she has a 27-33% chance of becoming pregnant [REF: Wilcox, A.J. et al. NEJM (1995) 333:1517].

When will I ovulate?

Calendar view: If you are using Natural Cycles to plan a pregnancy, your highest fertility and the best time to get pregnant will be shown with darker red circles.


A good first step to get to know your cycles and find your fertile period is to know how long it is on average.  Day one is the first day of the menstrual period and the last day is the day before the next period begins. A general rule is the longer your cycle is, the later your ovulation day will be. For instance, my cycle is long with 33 days between periods with an average ovulation day of CD 21, which means my most fertile days are CD 19, 20 and 21. If you have shorter cycle, say 21 days between periods, ovulation is likely to happen on CD 7 and your highest chance of getting pregnant will be on CD 5, 6, and 7.

However, these days are never set, especially for us irregular ladies. Have you ever noticed your period coming earlier or later than usual? Or ever wondered why your pregnancy test is negative even though you have no period? Well, it’s because a woman’s follicular phase, the first half of your cycle before ovulation, is often variable – my follicular phase can vary up to 6 days! What this all means is that your ovulation day can differ from cycle to cycle, which is why it is good to keep track with an ovulation app, like Natural Cycles. With a clear rise in temperature and a positive ovulation test (optional), ovulation can be confirmed and your fertile window is mapped for your upcoming cycle, which are indicated by red days. TIP: It is important to measure as much as possible in the week leading up to ovulation to ensure that we capture any variations!


Another popular question women ask about their cycles is, “When will I have my period?” or “When should I take a pregnancy test?”

Let’s review the second half of the cycle: The Luteal Phase.

This is the infertile stage of your cycle and is pretty consistent (usually around 12 to 14 days). This means it is a useful menstrual cycle characteristic that can be used to determine when you are likely to get your period, which is approx. 2 weeks after ovulation. However, you should know that the length of the luteal phase can also be quite individual and can vary between 6 and 18 days, so you should never assume that your ovulation day was two weeks prior to this. Instead it is much safer to calculate ovulation with reliable fertility indicators, such as temperature and ovulation prediction kits (OPKs), like we do.



Period tracker

Once Natural Cycles has detected your ovulation day it can accurately determine when your next period is due so you can keep calm and carry on!

Period trackers, which are based on the rhythm method, assume you are regular. Their primary goal is to let you know approximately when you can expect your next period, which will be around the same time each month as they do not detect ovulation or variations from your data. However, Natural Cycles, gets to know and adapts to your cycle no matter what shape or size it is, calculating your unique ovulation day each cycle giving you an accurate idea of when your period will come (variations included) each month.

At the end of your Luteal Phase, when you get your period, your temperatures will drop down again and the app will let you know a new cycle begins!

When will I ovulate and get my period?



Pregnancy tracker – plan users

Women usually take a pregnancy test from the first day of their missed period but not many know that the earliest point a woman can take a test is after the egg has been implanted in the endometrium at around day 9. It is fairly easy to recognise when you are pregnant if you are regular and know when your period is due but as mentioned before, if you are not, your period date could vary according to the length of your follicular or luteal phase each cycle.

Natural Cycles is a great tool for women that are planning a pregnancy, as it can determine from your data whether or not you have become pregnant! At the end of your Luteal Phase, if you don’t get your period and your temperature remains high (above the cover line) – the app will recommend that you take a pregnancy test to confirm it’s true.




If this is the case and you are pregnant, your temperature data entry dots will turn blue on your cycles chart.

When will I ovulate?

Chart view: If your temperature stays elevated at the end of your luteal phase it means you might be pregnant and should take a pregnancy test to confirm. Blue dots indicate that you are pregnant.


As part of fertility awareness week, our next blog post we will talk about how fertility tracking can help you understand your reproductive health. Ask yourself, does your period come too often or maybe it doesn’t come at all? Find out answers next week or if you want to know sooner please get in touch with support@naturalcycles.com or visit ask.naturalcycles.com

Yours naturally,

The Natural Cycles team


I miss the red and green days!

My favorite color is blue. I like having everything in blue. If I could decide all of the styling, I would have made everything blue on the NaturalCycles website.

It doesn’t work, though, since our users usually have red (fertile) and green (non-fertile) days and the styling of the web site changes accordingly.

Blue days in NaturalCycles stand for pregnant days, and as I am currently half way through my pregnancy, I am in the middle of an ocean of blue, blue and yet more blue days. In principle I should be very happy about all the blue, but to tell the truth, I really miss the red and green days.

Red and green days - NaturalCycles

I guess what I miss is really the diversity. I miss the excitement of ovulation day, after a period of red days, knowing that green days are around the corner. The green days used to feel special, a luxury, something to treasure. Now every day is the same and waking up to check what temperature I have does not have the same kind of meaning as before.

Red and green days - NaturalCycles

I also used to enjoy the subtle changes from cycle to cycle, as my body had been longer and longer without hormones and as NaturalCycles received more and more statistics from me. Will the yellow days eventually turn into more green, as they usually do with more data?

Red and green days - NaturalCycles

What is more fun now, is of course to track the development of the baby, how the movements are getting stronger every day and how the belly is growing.  But I do look forward to getting back to feeling how time passes by going from red to green and green to red.

Accuracy of perception of ovulation day in… [Curr Med Res Opin. 2012] – PubMed – NCBI

Accuracy of perception of ovulation day in… [Curr Med Res Opin. 2012] – PubMed – NCBI

Are you ovulating? (Anovulatory cycles)

Although you might get your period, it does not necessarily imply that you ovulated.

There is something called anovulatory cycles, which are cycles where ovulation simply doesn’t happen.

This might be difficult to notice unless you are tracking your cycles and charting your temperature, but might also show up as delayed menstruation and mid-cycle spotting.  Anovulatory cycles tend to occur occasionally throughout the childbearing years and are then a rather common cause of infertility. They are however most common during adolescence and in the years before menopause.

The picture below shows a temperature chart of an anovulatory cycle of a real woman. The anovulatory cycle is often longer than usual. It does still end in a bleeding, caused by the lack of estrogen rather than progesterone, as is the case for a normal ovulatory cycle.

temperature chart for anovulatory cycles - NaturalCycles

Anovulation can arise from a number of causes, ranging from severe diet and exercise or stress to complex disruptions in the relationships between tiny glands in the brain that control our most basic functions. Hormonal imbalances are the most probable cause. An intense program of exercise can interfere with the ovulatory cycle by suppressing the output of hormones called gonadotropins from the hypothalamus in the brain. The disorder may also result from hypothalamic dysfunction, hyperprolactinemia, polycystic ovary syndrome, luteal phase defects, or tumors of the pituitary gland adrenal gland or ovaries.

Another possible contributor to anovulation is the long-term use of certain medications. Steroidal oral contraceptives (the Pill) could likely be responsible. These drugs work by intentionally disrupting the hypothalamic-pituitary-ovarian axis, suppressing ovulation and thereby preventing pregnancy and this often causes prolonged anovulation also after quitting the pill.

If you suspect you might be having anovulatory cycles, you should track your cycles and detect your ovulation by measuring your basal body temperature. NaturalCycles keeps track of your anovulatory cycles for you and NC Planning even gives you fertility advice accordingly. If you discover that you have such a problem you should consult your doctor, as there are treatments and medications that could help.

The fertile window


The woman’s body is really a remarkable thing. Cycle after cycle, the uterus does its job and releases another egg, expecting it to be fertilized. Today, since we have such great knowledge through medical research on what’s going on in the uterus, we can use it to our advantage to either prevent or plan a pregnancy. The time frame when a woman is fertile only occurs once per cycle and is called the fertile window.

The fertile window includes the 5 days prior to ovulation and the day of ovulation.

Fertile Window - NaturalCyclesThe 5 days period prior to ovulation is determined from the longest time sperm can survive in the uteral environment. Note, however, that most sperm have a lifetime significantly less than 5 days – more like 2 or 3 days, but to completely exclude a possibility of pregnancy one must take the longest living sperm into account. For sperm to survive any significant time at all, the uterus must contain the fertile type of cervical mucus, which helps the sperm to live longer as well as to be able to travel up the uterus and fallopian tubes. Without the presence of fertile cervical mucus sperm typically only survive about 4 hours.

The fertile cervical mucus is triggered by a rise in estrogen prior to ovulation. The amount of fertile cervical mucus does not only vary from woman to woman, but also on the age of the woman. The older you get, the fewer days you produce cervical mucus and hence the narrower your fertile window becomes.

Once released through ovulation, the egg can maximally survive up to 36h, but typically only 12-24h. Studies have shown that the quality of the egg deteriorates very quickly; causing the probability of conception to decrease rapidly every hour once the egg has been released. For optimal chances of conception, sperm should thus already be present in the fallopian tube once the egg is released. Therefore, the most fertile day of the woman’s cycle is rather the day prior to ovulation than the day of ovulation itself.

To prevent pregnancy through detecting ovulation and predicting the fertile window, one must assume the longest living sperm and egg. This sums up to 6 days in the cycle. What’s more tricky is to accurately calculate the uncertainty of the ovulation day for a woman. That is, when do we think she will ovulate and what’s the earliest possibility of ovulation to occur. Luckily this is what NaturalCycles‘ algorithm does for you. When you start measuring your temperature, NaturalCycles will be very cautious as it does not know around what time you usually ovulate. With more and more data, NaturalCycles is able to better estimate your ovulation day, the variation of your ovulation day and the uncertainty on the estimated variation of your ovulation day. All this is required for a full-proof birth control method using natural family planning. So don’t chart by hand to prevent getting pregnant ladies – it is doomed to end up in an accident. Use the mathematical tools provided for you and you’ll save both time and hassle.

First week of pregnancy = third week of pregnancy

Most women usually do not know when they ovulate and the exact time when the miracle of life actually takes place in their uterus is therefore uncertain.

Most physicians thus rather count from the time of the last menstruation, which results in that when ovulation occurs you are actually already starting the third week of your pregnancy.

Your fetus’ age will always be about two weeks (or however long your follicular phase is) younger than your pregnancy week. Knowing more precisely when you ovulate can therefore better pinpoint the gestational age and all the other variables that come with it, such as the expected size of the fetus and due date.

fertilization of egg by sperm - pregnancy - NaturalCycles

As I ovulated yesterday, assuming that the egg was successfully fertilized, that would make me 2 weeks and 1 day pregnant. However, this sounds more serious than it is, as it takes about 9 days from the time of ovulation until the fertilized egg gets attached to the uterus. In these days, the egg that was fertilized in the fallopian tube is traveling down towards the uterus where it will hang around for a day or two until it gets implanted in the uterine wall. Until implantation occurs, my body is happily unaware of what’s going on. The embryo on the other hand is already carrying lots of interesting secret information, such as the sex of the baby, since the DNA from the sperm and the egg has already been merged into the baby’s genome.

Oh, how I’d like to take a peak to find out what’s going on in there! Did the egg really get fertilized yesterday? Will it successfully find it’s path down to where it will get implanted for the next 9 months and what kind of DNA did it get? Boy or girl? Will it look more like Raoul or like me: Blond or dark haired? Tiny or big nose? 🙂 We will just have to be patient and wait and see.

When to measure and when not to measure

One question about tracking the basal body temperature for natural family planning is when to refrain from measuring. If you are clearly sick and have a fever, NaturalCycles will detect your temperatures as abnormal and automatically exclude them. However, if you are just feeling slightly under the weather, or have been partying the day before, or simply sleeping significantly more or less than usual, your temperature measurement for that morning could be slightly shifted. This is not at all “dangerous” and it will not make you get green days during your fertile window. It could however cause you to get less green days as NaturalCycles provides the same level of safety for all types of temperature fluctuations.

It is therefore better to refrain from measuring for days where you don’t follow your normal rhythm. Especially if you haven’t just started measuring your temperature. NaturalCycles uses all of your data data from the first cycle you enter, when it analyses your new incoming temperatures and therefore, after three cycles or so, statistics become less crucial. Myself, I have now used NaturalCycles for 500 days and I have meanwhile acquired quite some statistics. I therefore only measure on days where everything is according to my normal every day schedule. For example, before this morning (Monday), I did not measure since Wednesday (see chart). Thursday was the Swiss national holiday so there was some drinking involved for both Wednesday and Thursday evening. Then, during the weekend, I slept considerably longer than usually. Since my data are not limited by statistics anymore, and I am not yet close to ovulation, it is fine for me to skip those 4 days in a row. I you have a look at my chart below you see that there is a nice line until today, probably since I skipped the days before where I’d otherwise get elevated temperatures due to sleeping in/alcohol.

Measures - NaturalCycles

I will summarize some useful rules of thumb that you could adopt for optimal temperature tracking:

  • For the first 3 cycles, try to measure as often as possible
  • For the 5 days before and after expected ovulation, try to measure almost every day
  • Outside these two scopes try to at least measure 4 mornings/week, but strictly don’t measure if:
    1. You get more than 2 hours of more/less sleep than usual
    2. You wake up more than 3 hours earlier/later than usual
    3. You drank more than usual the evening before and you feel the alcohol the following morning
    4. Skip the first morning after having traveled long distance to a different time zone
    5. You feel ill or have a fever

If you apply these guidelines, you should get nice temperature curves. There will of course always be some fluctuations, which cannot be avoided, but that’s normal and nothing to be concerned about. You should also try to avoid first measuring and then deciding whether to use the temperature value or not. That might bias your chart to look like you want it to, rather than what is actually correct.

When you have to measure the LH hormones with ovulation tests (OPKs), you don’t have to worry about more/less sleep or alcohol.

You should however try to not drink any liquids and not go to the bathroom at least 2 hours before measuring, preferentially more than 4 hours. Best is also to avoid taking the test in the morning. This might sound confusing, as pregnancy tests are best taken when you wake up, but the LH surge often starts in the morning, so if you take your ovulation test then you are more likely to miss your LH peak. Of course, if you don’t find a good time during the day to avoid liquids for a longer time, then the morning is to be preferred.

Happy measuring!

From NaturalCycles Prevention to Planning

Our NaturalCycles website is now finally finished and women can use it both as a contraception method (NC Prevention) and as a tool to get pregnant when they want to (NC Planning).

NaturalCycles web desing with beautiful natural women in nature

For the last 18 cycles, which correspond to about 1.5 years, my husband and I have been using NC Prevention as a birth control. It has been working very well for us, with usually around 10 fertile (red) days per cycle where we have to take care not to get pregnant. For those days we use a condom and for the remaining non-fertile (green) days we can enjoy not using anything. My husband says that he likes the variation, as it feels very special for the first green days after ovulation.

Fertility projection with ovulation day today

Today is my ovulation day for this cycle and probably the last time that we will use a condom in quite a long time, since as of next cycle we will switch to NC Planning and start trying to get pregnant. Our wedding anniversary is coming up on the 11th of August, which is next month, and we are going on a romantic getaway to south of Switzerland for the weekend. I am due to ovulate right after, placing our anniversary during the most fertile days in the cycle. I am a sucker for romance and I imagine it very romantic to conceive a baby on our first wedding anniversary. I realize that I might be naive thinking that things will go our way during the very first time we try, but due to my work with NaturalCycles I know exactly when I ovulate and all the details that come with it. We’ll just have to see and I will keep you posted on how it goes.

Fertility calendar with red and green days

I am also very excited to see how my temperature chart will change if I get pregnant. I will definitely notice it very early as the temperatures rise further around the time of implantation (~9 days after ovulation). I’m wondering how to tell my husband when I find out. I’d like to make some kind of romantic surprise gesture, but I’m not sure exactly what yet. Any ideas?

Exciting times are definitely ahead!