Monday, August 30, 2004

Period Math

The length of the monthly cycle varies. Usually, during the first 5 to 7 years after your first menstrual period, the cycles are more irregular and the interval between them is longer than for cycles later in life. Typically, during your twenties and thirties, the cycles become increasingly shorter and more regular. When a woman enters her forties, the cycles begin to lengthen again.

So, how do you know if you're cycle is normal? An average cycle length is 28 days, but if your cycle lasts anywhere between ~24 to 35 days, it's considered normal. A normal period flow lasts anywhere from 4 to 6 days, causes an average blood loss of 25-60 ml, and can be light, moderate, or heavy. Flow lasting longer than one week and blood loss of more than 80 ml are considered abnormal. Also, passing the occasional blood clot is normal.

By convention, the 28-day cycle is considered the ideal cycle. (Only 10-15% of cycles last exactly 28 days.) This doesn't mean that if your cycle isn't exactly 28 days there's something wrong with you; 28 days is just an average. [Not to mention that it makes the math simple--in a 28-day cycle, ovulation happens at the halfway mark, on the 14th day.] Also, by convention, the first day of bleeding is considered the start of the cycle, and is denoted as Day 1; the period lasts for 5 days (in an ideal cycle), or from Day 1 to Day 5; and the cycle ends on Day 28. Of course, real life doesn't always conform to conventions: if you're cycle isn't 28 days, how do you number the days of your cycle? Let's use an example to illustrate.

Say your period starts on the tenth of the month, your bleeding lasts for 4 days, and your cycle length is 30 days.

The day the bleeding starts, the tenth of the month, is Day 1 of your cycle. The blood flow days are Day 1 through Day 4, or the tenth through the thirteenth of the month. Your cycle ends on Day 30, or the ninth of next month.

One more useful calculation: your ovulation day. (Ovulation = the release of the egg from the ovary.) In an ideal, 28-day cycle, ovulation happens on day 14. Obviously, if you're cycle isn't 28 days, that doesn't help you. Fortunately, ovulation day is remarkably constant from woman to woman, and you can calculate your probable ovulation date based on the length of your monthly cycle. Here's one helpful way to think of the monthly cycle: a cycle divided into two intervals. The first interval (preovulatory) lasts from the end of your period to ovulation. The second interval (postovulatory) lasts from ovulation to the start of your next period. The preovulatory interval can vary widely; it may last for four days, or nine days, depending on the length of your cycle. In contrast, the postovulatory interval tends to be fairly constant at 14 days, regardless of how many days your cycle lasts. Crystal clear, right? You can now calculate your ovulation day, even in your sleep if you have to. [Huh? - ed. Now you know how I feel when someone tries to explain tech-related stuff to me.] Let's go through this step-by-step:

First, you need to track your cycle. Mark the day your period starts; this is Day 1 of your cycle. Then count the number of days until your next period; this is your cycle length. Do this for about three cycles in a row (more than three is fine, less, not so much). [The reason you need a minimum of three months is because cycle length might not always be the same each month. After three consecutive months you should be able to see a cycle length pattern.] Once your fourth period starts, count forward from Day 1 the number of days in your cycle length and mark that date. Then, from that day, count backwards fourteen days. This is your presumptive ovulation day. [You never count forwards, from the start of your period, because the preovulatory interval varies in length. You count backwards, because the postovulatory interval is fairly consistent at 14 days.]

For example, let's say your cycle lasts 24 days and your best friend's last 31 days. Both of you will most likely ovulate 14 days before the start of the next period. For you, this means ovulation is on Day 10 of your cycle (24 - 14 = 10), while for your friend, ovulation happens on Day 17 of her cycle (31 - 14 = 17).

Keeping track of your monthly cycle isn't useful just for planning a pregnancy. It's also beneficial if you plan to manage your period. In particular, it's useful if you plan to use period control occasionally, like for a scheduled event. If you know you have an upcoming event (vacation, exams, business trip) and you don't want to have a menstrual period around that date, the best time to suppress your real or fake period is about three months in advance. The advantage: it lowers the likelihood of nuisance side effects, like breakthrough bleeding/spotting.


At 12:25 AM, Anonymous Anonymous said...

This is useful, but still an oversimplification. If one is going to go to the trouble of charting one's cycles, there are better indicators of ovulation than counting backward. (Which, by the way, is not necessarily 14 days either; in my own case it's 13 days; some women's are as short as 10 days.) Changes in basal temperature, changes in cervical mucus, changes in the cervix itself, ovulatory twinges, changes in lymph nodes in the groin... All these things allow you to figure out when you are ovulating _as it happens_ rather than two weeks later.

(They also help you predict your period, to the day. It's great.)

(I have links to more specific information if you are interested in them.)


At 9:21 AM, Blogger ema said...

Rana, my charting post was apropos of menstrual management, not birth control. In this context, my two main points were: 1) tracking cycle data helps with period control, and 2) monthly cycle has a variable preovulatory interval and a, more or less, fixed postovulatory one. (Agree about individual variations, but this is a general info blog, hence my use of averages.)

In the context of birth control, you are correct—the calendar method is not reliable. I have a more detailed post on Natural Family Planning methods here:

If you have good reference links you think might be useful to other women, please share. I’ll check them out and post them.

At 5:15 PM, Anonymous Anonymous said...

Well, I tend to think of it in terms of a whole system. :)

(I like your site overall, by the way.)

The online site I like most (accurate, good explanations, no preaching) is this one: this book is similarly good:

Taking Charge of Your Fertility Rev Edition
by Toni Weschler


At 6:53 PM, Blogger ema said...

Rana, thank you. I'm familiar with that site and I'll add it.

At 1:21 PM, Anonymous Anne said...

This article was a good crash course for someone who is just starting to learn about their cycles, but it is important to note that some women have short luteal phases (the time from ovulation to period) and although they may be ovulating successfully, they are still having fertility problems because a short luteal phase doesn't give enough time for implantation.
In other words, if your post-ovulatory phase is LESS than 10 days, do some research and consult with a doctor when you are ready to conceive a baby.

At 3:59 PM, Anonymous Emma said...

For those interested in menstrual cycle charting (ie. cervical mucous, BBT & cervix position changes) you can also see the online e-use guide at

I am currentle training as an intructor and run a small charting circle.

I think any accurate information that encourages a women to tend to her body is good information.

At 6:44 PM, Anonymous Anonymous said...

i dont know much about birth control pills, do they make u have less periods?? if so, how often would u get one?

At 2:11 AM, Blogger Rainswolf said...

This is nonsense. You do not necessarily ovulate at the mid-point of your cycle.

Try reading a book like "Taking Charge of Your Fertility" that will actually tell you the truth.

At 4:03 AM, Blogger ema said...


Sometimes it's more about reading comprehension (the preovulatory interval varies in length...the postovulatory interval is fairly consistent at 14 days) than it is about conspiracy theories (that will actually tell you the truth -- bwahahaha).

At 10:09 PM, Anonymous Anonymous said...

This was great info! Thank you for breaking it down you really made it simple for anyone who reads this to do a little planning. P.S. We all get that nothing is going to be exact for everyone. But you gave perfect basics THANKS!!!

At 6:12 AM, Anonymous Anonymous said...

well it was very helpful.Very clear to understand.Thanks.

At 6:26 PM, Anonymous Anonymous said...

Brilliant info very helpful thankyou! I might be getting a touch paranoid after taking in so much different info but im desperate to conceive and have realised i have a 24 day cycle. Now im worried that my post ovulatory phase could be to short and will cause a problem for inplantation should conception take place. Any advice?? Am i just being over paranoid and need to chill!! Thankyou,

At 2:40 AM, Anonymous Anonymous said...

Hi All - this has been an extremely useful site so thanks to ema and all others who have posted a comment. I have a quick question for you all. My husband and I are currently attempting to concieve. This month I used an ovulation predictor to determine when I was ovulating. Around 10 days after ovulation I completed pregnancy test which was negative. It's now been a minimum of 23 days since ovulation and I still dont' have my period. I'm trying not to get my hopes up, but what are the changes of a postovulatory period lasting this long?

At 3:22 AM, Anonymous Anonymous said...

What could cause a sudden increase in cycle length? My normal cycle length is 27-32 days but over the last couple months it has changed to 45 day cycles.

At 7:46 AM, Anonymous Kirthika said...

One useful article I've read..Thanks :)

At 10:13 AM, Anonymous sarah said...

Hi thank you for ur article, was very useful and informative. My monthly cycle usually ranges from 28 to 31 however last month was 26. We have been ttc for 6 months, with no luck despite doing dtd every 2 days over ovulation period. I was told i have pcos years ago, can that affect ovulation? Unlike alot of women with pcos i have a good bmi and do have regular menstruation as in monthly and have 3 year old. With her it happened straight away why could it be taking longer this time?hly cycle usually ranges from 28 to 31 however last month was 26. We have been ttc for 6 months, with no luck despite doing dtd every 2 days over ovulation period. I was told i have pcos years ago, can that affect ovulation? Unlike alot of women with pcos i have a good bmi and do have regular menstruation as in monthly and have 3 year old. With her it happened straight away why could it be taking longer this time?

At 8:09 PM, Anonymous Anonymous said...

Im still a little confues..ok so lets say that my last period from the month February started on the 11 and ended on the I have my period for seven whats my cycle length...



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