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I don't understand when they say the 11th day of your 28 to 30 day cycle. Can someone please break this down in layman terms for me. My last period was the first of October and I finished it on the 6th. If necessary, the period before that one was August 26 and that finished September 1st.

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Ok, for this you'll need a slide rule (yes I'm that old) or a calculator - wait come back - ok : first things first: principles.

(Thinks: this would be a really cool iPhone app).

Your cycle is deemed to begin on the first day following your period, and goes like this:

Every month, your body goes through a makeover, clearing out the old, preparing a new bridal suite for Mr Right Sperm to come along.

Here's the sequence:

[End of period]
Day 1-14 (approx):

Your body has completed its clearout: now it's time to prepare the suite.

Day X (ovulation day, approx mid way through your 'cycle' we're describing)

Once the uterine walls are in place, the couch is nice and comfy, here comes the Bridal Ovum.

Day X - 21 (approx, notice - pill is 21 days, here's why)

Waiting for Mr Right Sperm to show up...

Day 21 - 28: tears, tantrum (cramps) and period

No show. Tear down the suite, let's try again.

That's an idealised (and slightly tongue in cheek) view of the period, but that's the cycle - and your 'length' is basically the average or typical number of days between the first and last date of your period, or strictly the first day off period to first day off period. Like months, and not by coincidence, periods are idealised at 28 days for convenience (or vice versa, we use a week because in life, that's a natural length of time split conveniently in four between our women-folk inviting us in from the fields - take your pick).

If your period is Ms In A Hurry, with a 28 day cycle or less, then your Day X is going to be approx half way through (say 14, but more in a moment) your cycle, so two weeks into your cycle.

If your period is Ms Relaxed, with a 36 day cycle, say, just to make the math clear, then Day X will again be approx half way through, but that is now 18 days, not 14, so you will be expecting to ovulate approx two weeks and four days, nearly three weeks into your cycle.

Hence the rhythm method of birth control, more appropriately the rhythm method of having a baby - if you have sex before Ms Bridal Ovum is in place, no worries - trouble is, she's unpredictable.

Hence all the approx (approximately) - nature doesn't blow a fanfare, so if you swear blind you were on the first day of your cycle, just finished your period, some how, some way, some where, someone has gotten pregnant that way.

That's the cycle, now for your cycle... there are cycle calculators (google fertility calculator or similar) on the web, so in a moment I'll go and check. Bear in mind that you need a more reliable figure than just one set of dates: make a note in your diary for six months, a year, and you'll have a more accurate idea.

Since months are, not coincidentally, approx the length of your cycle, you can do simple math for the days between period:

A few examples, look at the examples, you'll soon get the hang of it: let's do six months:

1st day (off period) | 1st day (next off period) | days in the (starting) calendar month | simple difference | actual days

12 Jan | 13 Feb | 31 (days in Jan) | +1 day (13 - 12 = 1) so 31 + 1 = 32 days in that cycle
13 Feb | 15 Mar | 28 (days in Feb) | +2 days (13th to 15th is 2 days) so 28 + 2 = 30 days in that cycle
15 Mar | 14 Apr | 31 (days in Mar) | -1 day (14th is one day before 15th) so 31 -1 = 30 days in that cycle
14 Apr | 16 May | 30 (days in Apr) | +2 days (14th to 16th) so 30 + 2 = 32 days in that cycle
16 May | 16 Jun | 31 (days in May) | +0 days (same same) so 31 + 0 = 31 days in that cycle
16 Jun | 17 Jul | 30 (days in Jun) | +1 day (16th to 17th) so 30 + 1 = 31 days in that cycle

You see why it's a good idea to make a note in a calender, or just a line like the above in your pc.

Once a month just make a note in a document, notepad, calender on your pc:

Notice the date on the left is always the same as the date on the right, one month earlier. ie: you only come 'off' period once a month, so you can do the same arithmetic as above, simply, in a note as follows:

Start with what you know, months and day counts, then fill in the rest as the year progresses, viz:
Calendar | Your dates
Mth Cal Prev | Off Diff Math Cycle
Jan 31 - | 12 na - -- start your 'diary' now we wait till next month, to calculate the first cycle
Feb 28 31 | 13 +1 31 + 1 = 32 - remember, it's the calendar days for the month end you cross (Previous month cal)
Mar 31 28 | 15 +2 28 + 2 = 30
Apr 30 31 | 14 -1 31 - 1 = 30
May 31 30 | 16 +2 30 + 2 = 32
Jun 30 31 | 16 +0 31 + 0 = 31
Jul 31 30 | 17 +1 30 + 1 = 31
Aug 31 31
Sep 30 31
Oct 31 30
Nov 30 31
Dec 31 30

Hopefully I got that right: I'd normally do this on a spreadsheet!

If it seems confusing, take heart: I put the wrong month - next not previous - until I realised the math wasn't working!

Play with it: if you can run a social life, you can definitely master this!

So, for the six months you've tracked, above: you got 32, 30, 30, 32, 31, 31 days.

Your personal cycle is the average of those: you can either get a calculator, or make an informed guess.

With a couple of 32, a couple of 31, and a couple of 30 - I'd say 31 was reasonable, wouldn't you?

Put that into the fertility calculator, and you get a Day X, or ovulation day (expected) of - whatever (approx 15, half) - let's go see.
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OK - your cycle - and yes, we need at least two dates, otherwise we can't take the difference between them:

So you finished Oct 6 and before that Sep 1.

It doesn't matter whether you count between finish dates, or start dates as long as your consistent:

Your start dates would be Oct 7 and Sep 2, but since you naturally quoted your finish dates (which is a 'big deal', rather more noticeable, so easier to remember and put in your diary), let's work with those:

Sep (previous month) has 30 days: Oct was 6th, Sep 1st, so 6 - 1 = 5, so your cycle was 30 + 5 = 35 days.

So roughly, you're going to be ovulating half way through that, at 17 - 18 days (two weeks three days into your cycle).

Now let's see what the official web calculators say, based on experience, and assuming that 'every' cycle you had was 35 days.

You already have a starting point, and your first cycle calculated, and you'll know roughly if that is reasonable (35 days), so you're already 75% certain of the appropriate answer.

Let's check the web.
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OK, just to make things confusing, they take the 1st day of your period online (google ovulation calculator or fertility calculator).

Typical result seems to be ovulation on the 22nd Oct, with fertility from 18 - 28, a couple of days before (she's early, or mr right may be hanging around - wouldn't bet on it - like sperm like grown male, they tire pretty quickly).

So, finishing Oct 6, ovulating 22, that's 16 days in to your cycle, and we said 17, just by dividing by two. Hardly rocket science.

Or, as the calculators do, you had a 5 day period, so its 16 days into your cycle + 5 days period, or 21 days from first day of your period.
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