Your menstrual cycle explained – the basics.

Understanding your menstrual cycle – the basics.

When I first started to try for a baby I knew very little about my body and my cycles. I had been on the contraceptive pill for 12 years with my main goal being to not get pregnant. I just took my pill every day for 3 weeks and then stopped for a week and a few days after stopping I got my period or ‘withdrawal bleed’. I had no idea whether I would have a normal or average cycle length or whether I would have any problems conceiving a baby once I stopped my pill. To say the least I was pretty naive and I honestly thought the first time I had unprotected sex after stopping my pill I would get pregnant. How wrong I was!

Luckily for me I have a biology based degree and I am very interested in the human body and how it works. I also love to do research. I vaguely remembered doing a module on reproduction while at University but it was a long time ago and I only knew really what most women know when starting to try for a baby. I knew that a cycle should be around 28 days and at some point an egg is released which needs to be fertilized. I had no idea about fertile windows, the hormones that play key roles in fertility, implantation of the egg or that your fertility levels decrease with age. After stopping the pill for 3 months and having no sign of a period but countless negative pregnancy tests I got my research cap on and started to expand my knowledge. I soon realised that things were a lot more complex than I first thought and that averages were just averages and should not be taken literally. I am going to break things down for you below so you can get a basic understanding of what happens during a normal menstrual cycle but please do not get too worried if it all sounds too complicated as I will also give you a summary at the end.

The 4 key phases of your menstrual cycle.

There are 4 key phases in your cycle and understanding what each phase is composed of and when each occurs in your cycle can give you a great advantage on your journey to getting pregnant. I will begin from the first day of a cycle which is often called cycle day 1 (or CD1) on most pregnancy phone apps and websites. The average menstrual cycle length is 28 days but the normal menstrual cycle range is between 21 and 35 days. It is useful when starting to try for a baby to start recording your cycles using a menstrual cycle calculator or menstrual cycle calendar in order to keep track of your cycle length and when you are predicted to ovulate but you can enter your daily temperatures and signs and symptoms of ovulation. You can download a free ovulation calendar to your phone or there are lots of free pregnancy apps available such as fertility friend (…), which is the one I use but there are many alternatives out there.

We see hormonal fluctuations during the menstrual cycle involving many hormones but I am going to introduce just 4 to keep things simple i.e follicle stimulating hormone (FSH), Estrogen, Luteinizing hormone (LH) and Progesterone.

Phase 1: Menstruation – Menstruation basically means the periodic shedding of blood and other tissue from the uterus through your vagina. The blood and tissue you lose is the lining that builds up inside your uterus throughout each cycle and is shed if you fail to fall pregnant. This lining needs to be discharged so your uterus can start a fresh ready for the next egg that is released during your following cycle. The first day of your period is called CD1 and your period usually lasts from 3 days to 7 days but can last a little longer.

Phase 2: The follicular phase – When you are born you will have millions of immature eggs inside your ovaries and each cycle usually only one of these eggs is released for fertilization. The follicular phase also starts on CD1 and ends when you ovulate i.e. release this egg from the ovaries. The process is very complex but it basically involves a hormone called follicle stimulating hormone (FSH) which stimulates your ovaries to start maturing around 5 to 20 of these immature eggs inside follicles on their surface. Each of the follicles that contain an immature egg is stimulated to develop in the run up to ovulation. As they develop, the ovaries also start to produce increasing amounts of the hormone estrogen which hits a peak mid cycle.

Phase 3: Ovulation- You are probably already aware that a woman normally has 2 ovaries which are connected to your uterus by tubes called the Fallopian tubes (see diagram below). Usually one of these 20 or so follicles on the ovaries develop into a mature egg and is released, however if a woman is pregnant with multiple babies this is because multiple eggs have matured and have been released together. Once the lucky egg has developed to maturity and is ready to be released and fertilised and your estrogen levels are nice and high, another hormone called lutineizing hormone (LH) starts to increase and it reaches a threshold level. This is the hormone that stimulates ovulation and what we are testing for when we pee on an ovulation stick. The threshold level is usually reached 12 to 24 hrs before the egg is released and happens on average 14 days before your period is due (so mid cycle, around CD14 if you have the average textbook 28day cycle) but can occur earlier or later. When the ovaries release the egg, an empty sac is left behind which develops into a structure called the corpus luteum. The egg starts to flow from the ovaries down the Fallopian tubes in the hope to meet some sperm cells and it will be fit for fertilisation for around 24hrs. After this, it is unlikely the egg is viable.

It is important at this stage not to get too worried about names of hormones etc. as it is more the basic process you need to get your head around. Just take away that there are a series of events that happen between your period and ovulation and each is stimulated by hormones. One of these hormones we can thankfully test for (LH) with ovulation sticks which will warn us of the impending release of an egg within 24 hours or so.

Phase 4: The luteal phase- During this final phase the corpus luteum left behind by the released egg begins to produce progesterone and small amounts of estrogen which will maintain the lining of your uterus in the hope that a fertilised egg will implant. As the levels of progesterone increase, it causes our core body temperature to increase and this increase in temperature is what we measure when we talk about charting temperatures on ovulation charts. If the egg is not fertlised and does not implant then after an average of 14 days post ovulation (14dpo) the corpus luteum dies off and our progesterone and estrogen levels decrease along with our core body temperature. The drop in estrogen and progesterone triggers menstruation and the whole cycle begins again from CD1. You will clearly see this rise and drop in body temperature on your charts if you decide to do temping so you can pinpoint the exact day you ovulated.

The role played by each of the main hormones involved in your cycle is very important and you will by monitoring 2 of them during your cycle if you decide to use ovulation tests and chart your cycle. If you have been trying to conceive for a while it is definitely worth reading the section on ‘charting your cycle’ as it could uncover any menstrual cycle problems e.g. annovulation (no ovulation). This is when you are having regular periods but are not actually ovulating. If no egg is released then there is no corpus luteum producing progesterone and no rise in your body temperature (see the section on ‘fertility issues’ for more information on this topic). You may find also find that you have an average 28-day cycle but actually ovulate earlier than cd14 giving you a shorter follicular phase and a longer luteal phase. Alternatively you could ovulate later than cd14 giving you a short follicular phase or a short luteal phase. Assuming you ovulate on cd14 and by having sex outside of your fertile window can prolong your trying to conceive journey and can prevent you from getting pregnant. More information about this can be seen within the ‘luteal phase’ section.

So what happens if you fall pregnant?

If an egg was fertilized by a sperm cell within the Fallopian tubes then the corpus luteum would live on and continue to produce progesterone and estrogen. The high levels of progesterone and estrogen support the uterine lining and once the egg has traveled down the tubes and into your uterus it implants into the lining. The placenta then develops which starts to produce another hormone called HCG. This hormone along with the high levels of progesterone and estrogen prevents menstruation and you would miss your period. HCG is the hormone that we test for when we pee on a pregnancy test which should start to turn a pregnancy test positive about 12 -16 days after the egg was fertilised. This can vary greatly however as some eggs don’t implant until later on and so the hormone levels can take longer to reach a testable level. Once the placenta has developed and the egg starts to develop into an embryo our levels of HCG rise and this is what usually causes morning sickness. Levels can double every 24hours so reach very high levels very quickly. From around 12-16 weeks the placenta is fully developed and takes over the production of progesterone and estrogen and the corpus lutuem dies.

Can you tell if you are pregnant from your ovulation chart?

By charting your temperatures you can be almost certain that you are pregnant if your temperatures stay high after your period is due even if you do not yet have your positive pregnancy test. This is because your luteal phase stays the same length from cycle to cycle and it is the follicular phase that fluctuates if you have irregular cycles. So if you are recording you daily temperatures and you know your luteal phase is 14 days long every cycle and you can see on your chart that you ovulated on e.g. cd14 you can assume your period is due on cd29 (which will then become cd1 once your period arrives). If however you have not come on and your temperatures are still high past day 29/30 you are probably pregnant. This is because the levels of progesterone are maintained and our core body temperature stays high throughout the 9 months of pregnancy. I will go into more detail on this in the ‘Charting your cycle’ section but this is another perk to charting your ovulation.

A quick recap:

In this section ‘Understanding your menstrual cycle – the basics’ I have explained the following:

Menstruation and the follicular phase – On CD1 you get your period and the hormone FSH starts to stimulate 5-20 follicles to develop on your ovaries. Estrogen levels increase from CD1 and reaches a peak mid cycle followed by a rapid rise in LH to a threshold level around 14 days before your period is due. The day in your cycle that this threshold level of LH is reached depends on the average length of your cycles and on your personal luteal phase length which should stay the same every cycle. The length of the follicular phase can vary causing many women to have irregular cycle lengths.

Ovulation – The rise of LH to the threshold level stimulates the release of the egg from the ovaries leaving behind an empty sac called the corpus luteum.

The luteal phase – The corpus luteum releases progesterone which increases your body temperature and maintains your uterine lining for a further 14 days on average. If the egg is not fertilised the corpus luteum dies around 14 days post ovulation (this depends on your luteal phase length) and your progesterone levels drop causing a drop in your temperature. This is when you get your period the whole cycle starts again. If the egg is fertilised then your progesterone levels remain high along with your core body temperature.


















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2 thoughts on “Your menstrual cycle explained – the basics.

  1. Michelle says:

    Such a great post. Love how you have simplified everything. I now feel more confident now I know what my body is doing. Hope it increases my chances of conceiving this cycle.

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