I think we have all learned about the reproductive cycle in
health class, and perhaps some things about the birds and the bees from our
parents, siblings or friends; however,  I’ve
noticed through my participation on TTC boards, that there are a lot of women
who are confused about how to count  cycle
days.  I’m not going to even lie and say
I knew everything about the menstrual cycle. 
There were things I learned in school, and even while obtaining my
Bachelor’s of Science degree in Community Health that taught me about the
reproductive cycle and sexual health etc. 
However, nothing could have prepared me for what I’ve come to learn
about how to count cycle days and everything else that comes along with that,
until my TTC journey. 
I’m going to try and explain this as simply as possible, and
perhaps make this a series as to not put too much information into one
post.  But first, let us talk about cycle
days (CD).
What is a cycle day?
A cycle day makes up your menstrual cycle length.  Your first cycle day (CD1) is the first full flow of your period.  Think of this day as the day you need a
tampon or pad versus a day you can just wear a pantyliner due to spotting.  
How many cycle days
are in a cycle?
This is where it tends to get confusing for some women
because it varies. When we were in school, we were taught that cycles are 28
days long with ovulation occurring mid-cycle on CD14; however, most women do
not have “regular cycles”, and most women do not ovulate mid-cycle.  Some cycles can be shorter than 28 days or
longer, going up to 35+ days.  According
to the American Society for Reproductive Medicine, typical menstrual cycle may
be anywhere from 21 to 35 days.  Ovulation,
then, may occur much earlier or later than typical guidelines suggest.  There
is really no simple “one-size-fits-all” mathematical formula to calculate your
ovulation date, but there is a way to determine your own ovulation date and
fertile signs by examining your fertility signals.
I have a 28 day cycle, but, I don’t always ovulate on CD
14.  There are times when I ovulate a day
before or later. This is why I chart using my basal body temperature, and check
my cervical mucus. 
How do I know the
last day of my cycle?
CD1 is the day your period begins, and all of the days up
until the day before the start of your next cycle complete your cycle.  Therefore, the last day of your cycle is
considered the day before the start of your next period.  For me, this is usually the day I start
spotting before my period comes aka CD1.
You may see my chart

CD1 was Sunday, December 22. The red boxes (CD1-CD5)
indicate the days I had full period flow; no spotting.  CD8-10 is my predicted fertile window.  CD11-13 are my actual fertile days, and my O
day was on CD14 according to my BBT.
Your cycle after
menstruation AKA Aunt Flow (AF)
After the last day of AF, begins the follicular phase
(FP).  During this time, your body starts
to release estrogen, which, in turn, causes the uterine lining to grow.  A nice, thick, uterine wall is the perfect
environment for the implantation of a fertilized egg.  Estrogen levels rise dramatically during the
days before ovulation and peak about one day before ovulation.  The surge in estrogen triggers a spike in another
hormone – the luteinizing hormone (LH) – the reason I go in for blood work and
use OPK’s. Ovulation occurs as this increase in LH causes the follicle to
rupture and release an egg.
Also during the FP, the release of the follicle-stimulating
hormone (FSH) stimulates the growth of ovarian follicles. Each follicle
contains an egg.  By late in the
follicular phase of the menstrual cycle, only a single follicle will remain
Your cycle days after
After you ovulate you continue counting your cycle days in chronological
order; however, the stage after ovulation is the luteal phase of your cycle. The
luteal phase length is generally constant from cycle to cycle for the same
woman, lasting 10-16 days.  These days are usually counted as days passed
ovulation or DPO.  
On my chart you will
see on CD15 I am 1DPO. On the 14th DPO, if my egg wasn’t fertilized,
a new cycle begins, and I start CD1.
Reasons for irregular
When cycles are irregular, it is usually because ovulation
occurred earlier or later than usual.  It
can also be hormonal or due to an illness.
Why knowing when you
ovulate during your cycle is important?
Knowing when ovulation occurred is important as it allows
you to see if intercourse was well-timed for conception and lets you determine
your luteal phase length.  Knowing your
luteal phase length tells you when to expect your period or a positive
pregnancy test result.  For example,
since I have a 28 day cycle, and O around CD14, I can test between CD25-28 or 11-14
Whew! That was a lot! I hope after reading this you’re able
to correctly count cycle days, and, perhaps teach someone else.
My next post will talk about identifying the different types
of cervical fluid and their importance when TTC.
More information can be found on FertilityFriend.com. I owe a lot of what I now know to that site. 
FYI: I’ve created a cheat sheet of acronyms or abbreviations I will use during my TTC posts. Just look for the ‘Acronyms & TTC‘ tab at the top of the page. 
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Kim S.

Kim is the do-it-all mom (and wife) who not only works full-time and is a freelance makeup artist, but also blogs about her love of family, travel, beauty and skincare. Now that she has a kindergartener, Kim has added Class Parent to her resume. These are all tough jobs, but somehow, she makes them look easy.