The 'Perfect' cycle: What are the most fertile characteristics according to the evidence?

What does the 'perfect' cycle look like?

Characteristics of a perfect and fertile menstrual cycle

This question slaps controversy round the face, shakes her by the shoulders and screams ‘pull yourself together you silly tart’.  Let’s not shy away from controversy but face her head on.  True, there are many variations of cycles that are considered to be healthy and fertile.  Yes, women can achieve pregnancy despite the characteristics of their cycles varying considerably from other women.  Indeed, women can have some, many or all aspects of their cycles outside of the description in this article and fall pregnant no trouble.  Totally agree, other woman can have ‘perfect’ cycles and still struggle to achieve a bump naturally.  There are plenty more balls in the fertility lottery than just those related to the cycle. 

But many of us are coaxing, teasing, encouraging (and sometimes downright manipulating) our cycles to try to optimise our chances of conceiving.  So what are we aiming for?  If we could wake up tomorrow and pick every aspect of our future cycles, what does the research suggest we should chose to be the most fertile?

What is a 'normal' menstrual cycle? Total cycle length: 28 - 33 days

When raising concern with two different GPs about my irregular 35 to 45 day cycles I was told that cycles vary in length, so what is normal for me is ‘normal’.  The message was consistent, but was it correct?  Maybe it is just semantics but perhaps I should have asked my doctor whether it was healthy or fertile to have long cycles.  Long cycles could be ‘normal’ in the sense of ‘common’, but that is not the same as being healthy (in fact they are not actually common either as 95% of cycles are shorter than 36 days).  Maybe clarity in language would have elicited a different response, as during my first consultation with a fertility specialist she sucked her teeth and said she would ideally like to see my cycles shorter.  Unsurprisingly, the evidence supports the specialist.

There are a number of studies on cycle length and their findings vary on the ideal length for peak fertility.  One study found 32 to 33 days to be optimal, whereas two other studies (by Small CM and Solstad HA)  both found that peak fertility is a 30 to 31 day cycle.  A further study found 28 days to be optimal.  The research therefore shows that between 28 and 33 days is the goal.

Cycles shorter than 25 days or longer than 35 days are between 10% and 30% more likely to be anovulatory (meaning that you don't ovulate) than cycles between 25 and 35 days in length. Evidence shows that shorter cycle lengths are associated with reduced fertility.  As women get older the length of the cycle tends to reduce naturally, with studies showing that cycle length shortens from a population mean of 30.1 days for women in their 20s to 27.3 days for women in their 40s. Despite this natural shortening of the cycle length over time, the research indicates that the adverse effect of short cycle length on fertility persisted across age groups, showing that the negative association between short cycles and fertility is not solely attributable to the shorter cycles being from older women.

My cycles have managed to move from one end of the spectrum (erratic 40+ day cycles) through the peak fertile length and pop out on the other side (now 27 day cycles).  I feel like I have passed go without collecting a baby.  In a ridiculous turn of events I am now trying to lengthen my cycles by coaxing a reluctant luteal phase towards the 14 day mark.

Time to ovulation (follicular phase): 14 – 21 days

The textbook cycle would have us ovulate on day 14, but research has shown that only 25% of women had all of their fertile window between days 10 and 17.  So ovulation on day 14 is a myth, which is perpetuated by many doctors and the misleadingly named 'day 21 progesterone test' for the blood test in the mid-luteal phase.  

The Infertility Cure, a fabulous book by acupuncturist and herbalist Randine Lewis, says that the follicular phase should be between 12 and 15 days for optimal fertility.  Ideally ovulation should not be before day 10 or after day 17.  The Infertility Cure suggests that if the follicular phase is too long then this may be the result of low oestrogen production, compromising egg quality and delaying ovulation.

There are few readily available studies on the peak days of ovulation for fertility.  Considering that the overall length of the cycle should be between 28 and 33 days, and the luteal phase between 12 and 14 days, this means that ovulation should ideally occur between days 14 and 21, depending on the length of your cycle and luteal phases, with the caveat that Randine Lewis would suggest peek fertility would be earlier than day 21 (no later than day 17 is best).

How long does the luteal phase need to be? Time between ovulation and start of period (luteal phase): 12 - 14 days

If the relationship between the luteal phase and fertility had a FaceBook status, it would be 'it's complicated'.  On the issue of adequate luteal phase I have managed to bore my husband to tears and drive to distraction multiple medical professionals.  Views expressed to me over the years vary from the length of luteal phase having no impact on fertility (NHS fertility consultant) to at least 12 days being integral to maintaining a pregnancy (fertility awareness practitioner). So what’s the deal?

The normal luteal phase ranges from 11 to 17 days with most luteal phases lasting 12 to 14 days.  Research has used varying definitions of a short luteal phase: fewer than  9 days,  10 days, or 11 days .  Research found that all cycles with a luteal phase of fewer than 9 days were abnormal, and that 74%, 22% and 2% respectively of cycles with luteal phases of 10, 11 and 12 days were also abnormal, which would strongly suggest that a luteal phase of 12 days or longer would be the most fertile.  However, confusingly other research has shown that having a short luteal phase of fewer than 11 days does not have a correlation with unexplained infertility.  The murky picture is further dirtied by plenty more conflicting evidence on luteal phase length and fertility.  A good summary of the conflicting evidence, issues and difficulties with testing for and treating supposed luteal phase deficiency has been published by the American Society for Reproductive Medicine.  What is clear from the research is that between 12 to 16 days is definitely considered to be adequate and removes you from the tedious and frustrating debate about the existence of luteal phase defect.

Variation in cycle length: little to none

It is good for fertility to have regular cycles (low menstrual cycle variability), meaning little or no variation in cycle lengths.  When the combined effect of cycle variation and cycle length was assessed, cycle variation was a persistently strong predictor of fertility.   So the aim is for your cycles to run like clockwork.  During 2015, my biggest variation in length from one cycle to the next was 12 days (a 44 day cycle followed by a 32) and on average they varied by 6 days from month to month.  Cycles bouncing around like a space hopper indicates an underlying hormonal imbalance.  If this is you, you are far from alone, with a study finding that in almost 43% of women had more than more than 7 days variation in their cycles lengths

As explored above, being common is not the same as healthy or fertile.  Research suggests that women with high menstrual cycle variability had a reduced (51% lower) per cycle probability of pregnancy compared with women with minimal variability.  A study  found that fertility was approximately 25% less in women who had a cycle length that differed by more than 10 days from the usual cycle length compared with women who had no variation.  When it comes to fertility, consistency is key.

What is considered a normal period? Bleed length: 5 days

Aunt Flo.  The worst part of the cycle for women who are trying to conceive.  So what should Aunt Flo look like?  Research suggests that 5 days of bleeding is the optimal number for peak fertility, with women with a shorter period length being less fertile.  This is supported by further research that found that longer cycles can indicate higher levels of follicle-stimulating hormone (FSH) and heavy bleeding shows higher FSH and progesterone throughout the cycle.  Shorter and lighter periods are more common after anovulatory cycles.  But not all evidence supports this conclusion with alternative research showing that duration or intensity of menstrual flow is not appreciably associated with fertility.

So how many of us have these fertile bleeding patterns? On average, periods last between 3 and 6 days with a medium of 5 days.  Heavier bleeding occurs during the first 3 days with the heaviest flow of blood on the second day.   Research has shown the percentage of women with the following bleed patterns

  • 1-3 days of spotting (10%),
  • bleeding lasting 3-6 days (40%),
  • bleeding lasting 6-8 days (33%), 
  • bleeding lasting 8-12 days (17%), and
  • 4.8% of women experienced midcycle bleeding.


So there you have it, the research on the 'perfect' cycle.  But getting hung up on attaining perfection is likely to drive us mad and be counter-productive in the long run.  It is really just a gauge to see whether we are moving in the right direction.  I'm putting my focus on being healthy, healing my body and observing what that does to my cycles.  And my cycles have reflected the changes in my health.  In 2015 I only consistently met one criteria (bleed of 5 days) which improved to four criteria in 2016 after focusing on healing my body.  Now I am close, so very close, to having a 'perfect' fertile cycle.  Unfortunately, that is not necessarily the same as being close, so very close, to having a baby.