Introduction

Definition and phases of the menstrual cycle

We have always known the term menstrual cycle but do we know what exactly a menstrual cycle is and what it consists of?

A menstrual cycle starts from the first day of bleeding which is referred to as menses and ends the day before the next bleeding.

It consists of several phases like luteal phase, follicular phase, ovulation and menses.

Each phase lasts for a number of days and has a role in the formation of the ovum and ovulation.

So the menstrual cycle starts with menstruation(bleeding) with an average 3 to 6 days, the heavy flow being on the second day, then the follicular phase with an average 10 to 23 days and it shortens by 3 to 7 days over time and then ovulation which only lasts 24 hours in the cycle and then luteal phase which is the last phase with average length 7 to 19 days.¹

The duration of the menstrual cycle differs from one individual to another and it also depends on the age.

However, the average length of a menstrual cycle ranges from 21 to 34 days.¹

Importance of understanding menstrual cycle length

The connection between cycle length and overall reproductive health

There is a connection between the length of a menstrual cycle and its regularity and the reproductive health of a person assigned female at birth.

To understand this we first have to understand the menstrual cycle and how it is directly related to reproduction.

So the part of the brain called the hypothalamus every month at a certain time releases a group of hormones called Gonad releasing hormones, follicular stimulating hormones and luteinizing hormones. All these hormones play an important role in the production of the ovum. After 14 days of the production of the ovum if it was not fertilised by a sperm (which can live inside the genital tract for up to 5 days) menstruation begins marking the beginning of a new menstrual cycle.

So if any of these hormones was affected or if any phase or process was affected it can affect the success of the egg fertilisation and hence pregnancy.²

Relevance to fertility and conception

There are lots of studies that were made to find a correlation between fertility and ovarian reserves and the length of the menstrual cycle.

If a person assigned female at birth gets older and her cycle length becomes shorter this means her ovarian reserves are decreasing as well.³

But what if the menstrual cycle is always short, ranging from 21 to 29 days, will it affect fertility, the chances of pregnancy and the success of IVF treatments? 

Menstrual cycle length: what it means

Normal variability in cycle length

Factors influencing cycle length (e.g., age, lifestyle, health conditions)

There are several factors that affect the length of the menstrual cycle like age, exercise, body weight, the amount of caffeine and alcohol consumption and also smoking.⁴

People assigned females at birth who smoke or are exposed to smoke over the age of 35, it was found that their follicular phase gets shorter as smoking affects the release of the follicular stimulating hormone(FSH) which resulted in a shorter menstrual cycle but it wasn't proved that smoking affects ovulation or menstruation.⁴

Physical activity and the amount of exercise also can affect the length of the menstrual cycle.

It was found that people assigned female at birth who exercise or have a physical activity of 4 hours or more experience a longer menstrual cycle as it also has an effect on the follicular stimulating hormone thus increasing the length of the follicular phase which delays ovulation and increase the length of the menstrual cycle.⁴

Age is a known factor that affects the menstrual cycle length. With age, the follicular phase gets shorter so the whole length of the menstrual cycle gets shorter.⁴

Being overweight was found to affect the menstrual cycle length in girls in college, they have longer cycles while in older individuals it was not found that it affects the cycle length.⁴

Alcohol consumption was also found to affect the length of the menstrual cycle and cause some changes in the hormones.⁴

Differences between regular and irregular cycles

A person assigned female at birth can say that she has an irregular menstrual cycle when about a third of the cycles are more than 35 days or less than 21 days.

It is also considered normal if a cycle is longer or shorter than usual within 8 months.

It is expected that 35 to 40% of people are assigned female at birth.⁵

Short cycles (less than 21 days)

Impact on fertility

Studies have shown that there is a correlation between short cycles and low chances of pregnancy.

As the age increases the length of the menstrual cycles decreases and with it the chances of pregnancy.⁶

It was found that the total dose of follicular stimulating hormones(FSH)/human menopausal gonadotropin(hMG) needed to reach the full maturation of the follicle and in return, the egg maturation increases in people assigned females at birth with short menstrual cycles.

It was also found that the number of eggs that could be retrieved from women with short menstrual cycles was fewer in comparison with women with longer cycles.⁶

Long cycles (more than 35 days)

Implications for ovulation and conception

Fecundity or fertility was found to be more in people assigned female at birth with regular cycles of length from 28 to 31 days.

In a study of those whose cycle length varies about 10 days or less fertility equals 25% of those with regular cycles.

It was found that cycle variations had more effect on fertility and pregnancy chances than the cycle length.⁷

Correlation between menstrual cycle length and fertility

Cycle length and fertility 

Some studies were made to compare menstrual cycles of different lengths to see whether it affects fertility or not.

It was found that people assigned female at birth with short cycles have low ovarian reserves, lower fertility and lower chances of IVF success in comparison with those who experience normal cycles(28-31 days).

Serum AMH value represents the number of egg reserves. It naturally decreases by 5% every year.

When comparing the serum AMH value in individuals with short cycles and individuals with normal cycles, a significant decline in the serum AMH value was found in those with short cycles. It decreased by 8 folds.

When comparing individuals with long cycles and normal cycles, no noticeable or massive difference in the serum AMH level.

But some differences were noticed when it came to the success chances in IVF, AFC values which is the number of small follicles that are found in both ovaries in the early stage of the follicular stage and also a decreased number in retrieved oocytes which have an effect on fertility.

Health conditions affecting cycle length and fertility

Polycystic ovary syndrome (PCOS)

Description and symptoms

Polycystic ovary syndrome is dysfunction in ovaries where hyperandrogenism(excess amount of sex hormones) happens and the appearance of the ovaries using ultrasound is polycystic.⁸

Some signs associated with polycystic ovary syndrome are irregular cycles, an increase in sex hormones and an increase in weight.⁸

Impact on cycle length and fertility

People assigned females at birth with polycystic ovary syndrome and metabolism defects have steroid dysfunction and decreased fertility.

It also affects the success rate of fertilisation of eggs in lab.⁹

Thyroid disorders

Role of thyroid hormones in regulating the menstrual cycle

Thyroid hormones play an important role in reproduction as they act on the ovaries, uterus and the placenta.

They also interact with other hormones like oestrogen and prolactin and affect the release of gonadotropin-releasing hormone (GnRH), which makes any changes in the levels of thyroid hormones affect fertility causing infertility or subfertility.¹⁰ 

How hyperthyroidism and hypothyroidism affect fertility

When a person assigned female at birth suffers from hypothyroidism( decrease in the level of thyroids) or suffers from anovulation( inability to ovulate), ovarian cysts and menstrual cycle irregularity.

A person also might suffer from infertility or increased chances of abortions or giving birth to premature infants with congenital anomalies.¹⁰

Suffering from hypothyroidism in early life may delay puberty.¹⁰

Lifestyle factors and their impact on cycle length and fertility

People assigned female at birth who used to have regular cycles and then noticed changes in their cycles are usually asked about weight gain or loss, whether they had a very strict diet, excessive exercise, or had a very stressful period of time.¹¹

Some  suffer from Functional Hypothalamic Amenorrhea(FHA) which is the no menstruation due to one of the reasons we just mentioned.¹¹

Summary

The menstrual cycle is a physiological process people assign females at birth go through once a month It consists of 4 phases: follicular, ovulation, luteal and menstruation.

The normal cycle length is 28 to 31 days. Some experience short menstrual cycle length and some experience long cycle length.

Cycle length is correlated to fertility and chances of pregnancy and also the success of IVF results especially when the cycle is short.

There are reasons that can affect the length of the menstrual cycle like lifestyle factors or some health disorders.