Understanding the menstrual cycle

Understanding your menstrual cycle can help you get pregnant faster.

What is the menstrual cycle exactly? When does the menstrual cycle start? How long does the menstrual cycle normally last, and when in the menstrual cycle are you most fertile?

This article aims to answer all these questions and many more.

What is the menstrual cycle?

The menstrual cycle is the monthly process in which the female body prepares for pregnancy.

The menstrual cycle can be divided into two main phases:

  • In the first phase, your body matures an egg cell so that it can be fertilized by your partner’s sperm. This is called the follicular phase. It starts on the first day of your period, and lasts until the day you ovulate.
  • In the second phase, your uterus is getting ready to receive the egg in case it was fertilized. This is called the luteal phase. It starts on the first day after ovulation, and ends on the last day before your next period.

Let’s take a closer look at exactly what is happening, from the first day of a cycle until it ends and the next cycle begins.

When does the menstrual cycle start?

Every new menstrual cycle begins on the day you get your period. This means the first day with a flow of fresh blood. It is common to have brown or pink discharge (called “spotting”) for a day or two before the actual bleeding starts, but this should not be considered the start of the period.

The first day of your period is often referred to as Cycle Day 1 (CD1).

The period, or menstrual bleeding, typically lasts between 3 and 7 days. The blood flow is usually heaviest during the first few days, gets gradually lighter, and often ends with a day or two of brown or pink spotting.

Whether your period lasts for 3, 5 or 7 days has no impact on when you will ovulate, or on the length of your cycle. The hormones that stimulate ovulation start working on CD1, not at the end of the bleeding. The day of ovulation is also counted from the first day of your period. It is therefore not particularly useful to keep track of the duration of your period.

However, if you are routinely bleeding for more than 7 days, and especially if the bleeding is heavy and/or painful, it could be a symptom of fibroids or endometriosis. If this is your case, consult your gynecologist.

The follicular phase: Getting ready for ovulation

Follicles are tiny, fluid-filled sacs in your ovaries. Each follicle contains an immature egg cell, called an oocyte in medical terms.

Every time you get your period and a new menstrual cycle begins, a gland in your brain starts producing a hormone called follicle-stimulating hormone (FSH). This hormone causes a number of follicles to begin maturing the egg cells inside them.


Menstrual cycle uterine liningThe FSH hormone also stimulates your ovaries to produce another important hormone: estrogen. Estrogen makes your uterine lining thicken and fill up with nutrients and blood, in preparation for receiving and nurturing a fertilized egg.

Estrogen also causes your cervical mucus to change and become more sperm-friendly.

Most of the time, your cervical mucus acts as a barrier which protects your uterus from bacteria and other foreign bodies, including sperm. But just before ovulation, the texture and pH (acidity) level of the mucus changes in order to help the sperm cells swim more easily towards the egg.

Read more: 10 ways to improve your fertile cervical mucus

Estrogen eventually causes the production of FSH to stop, so that only one or maybe two of the follicles that started developing at the beginning of your cycle become “dominant” and finish maturing the egg cell.

Finally, estrogen stimulates the release of yet another hormone, called luteinizing hormone (LH). LH is the hormone that makes ovulation happen, by causing the dominant follicle to release the mature egg it contains.

LH is the hormone which is detected by ovulation tests. Your body produces some LH at all times during your cycle, which is why you can almost always see a faint test line on your ovulation tests. However, the production of LH increases sharply and peaks between 12 and 36 hours before ovulation. It is this temporary extra-high level of LH which makes your ovulation tests turn positive.

Read more: How to use ovulation tests

Ovulation

The surging level of LH causes the dominant follicle to break and release the mature egg. The term “ovulation” refers to this precise instant, when the egg cell leaves the follicle.

Menstrual cycle hormones FSH LHSome women can feel a sudden, sharp pain on one side of the abdomen (known as mittelschmerz) when they are ovulating, but it’s just as common to feel nothing at all. Furthermore, twinges in the ovarian area can also be caused by the movement of the fallopian tubes as they are ushering the egg towards the uterus, and should not be taken as a sure sign of ovulation.

For most women, there is no way of telling exactly when they are ovulating.

You may have learnt that women ovulate on the 14th day of the menstrual cycle (CD14), but this is just an average. It is completely normal to ovulate on a different day, and for the ovulation day to vary from one cycle to another.

Sometimes, more than one egg is released; this is how fraternal (non-identical) twins are conceived. But both (or all) eggs will be released within a 24-hour period; you cannot ovulate twice in one cycle.

Whether the egg is released from the left or the right ovary is random. If both ovaries are healthy, there’s no pattern to which ovary will release the egg in any given cycle.

Read moreCan you ovulate twice in one cycle?

Conception

As soon as it’s been released from the ovary, the egg cell is captured by the the finger-like end (fimbria) of the fallopian tube, and brought into the tube where it starts its journey towards the uterus.

Fallopian tubeFascinatingly, if one of the fallopian tubes is missing, the other tube can still pick up the egg from the ovary on the opposite side.

If the egg is fertilized by a sperm cell, this happens in the fallopian tube.

Because the egg cell only lives for about 24 hours after ovulation, it is best if a fresh supply of sperm is already waiting in the tube when the egg is released. This is why it is recommended to have as much sex as possible in the days leading up to ovulation.

If the egg isn’t fertilized within 24 hours, it dies, and your chances of conception are gone in that cycle.

Read more: How does conception happen?

The luteal phase: Getting ready for pregnancy

The first day after ovulation marks the beginning of the luteal phase. Now that your body has successfully produced and released a mature egg cell, its focus becomes to prepare for receiving a fertilized egg and sustain a pregnancy.

As soon as you have ovulated, the now-empty follicle that housed the egg is transformed into the corpus luteum (“yellow body”), which gives the luteal phase its name. The corpus luteum starts producing yet another important fertility hormone: progesterone. This hormone causes the uterine lining to continue to build up in preparation for receiving the embryo.

Progesterone also causes your body temperature to rise very slightly. You can probably not feel the difference, but the increase can be detected by measuring and tracking your temperature every morning. This can be useful in order to confirm that ovulation did indeed happen.

Read more: Basal body temperature and ovulation

What happens next depends on whether or not the egg was fertilized.

If the egg was fertilized, it will reach the uterus and attach itself to the uterine lining (this is known as implantation) between 6 and 12 days after ovulation. Immediately, your body will start producing a pregnancy hormone called human Chorionic Gonadotrophin (hCG).

hCG signals to the corpus luteum that it must continue to produce progesterone in order to maintain the uterine lining, which is needed for nurturing the embryo until the placenta takes over this function several weeks into the pregnancy.

Because the uterine lining is maintained by the continued production of progesterone, it doesn’t come out as menstruation, and you miss your period. The missed period is often the first sign that you are pregnant.

It is the presence of hCG in your urine which makes pregnancy tests turn positive. When the egg implants in the uterine lining and hCG production starts, the concentration of hCG is still too low to be detected by pregnancy tests. But in a healthy pregnancy, the level of hCG doubles every 2-3 days, and a very sensitive pregnancy test can often detect the hormone 3-4 days after implantation (7-10 days after ovulation).

Read more: When and how to use pregnancy tests

If the egg wasn’t fertilized, no hCG is being produced. The corpus luteum receives no signal to continue producing progesterone, it starts to shrink, and your hormone levels start falling. This drop is what’s causing the symptoms of pre-menstrual syndrome (PMS), such as fatigue, irritability, bloating and tender breasts.

When the production of progesterone reaches its lowest level, the uterine lining that was built up throughout the cycle can no longer be sustained. The lining breaks down, your period begins, and the cycle starts over again.

What is the normal menstrual cycle length?

The length of the menstrual cycle varies considerably from woman to woman. The textbook 28-day cycle is just an average, and anything between 21 and 35 days is normal.

The duration of the follicular phase (the first half of the cycle) can also vary from one cycle to the next. This means that it is difficult to predict your ovulation day based on when your period started. And even if your cycles are regular and you usually ovulate on the 15th day of the cycle (CD15), for instance, you may suddenly ovulate on CD12 or CD18 instead.

Ovulation before the 10th day of the cycle is unlikely, because the egg cells need a minimum of time to mature. Late ovulation is more common, and women with very irregular cycles can ovulate several months after their last period.

Read more: How to get pregnant with irregular periods

While the length of the follicular phase is often variable, your luteal phase usually counts the same number of days in every cycle. The luteal phase normally lasts between 12 and 16 days. If your luteal phase is shorter than 10 days, discuss it with your doctor. A very short luteal phase can make it difficult to get pregnant, because the egg doesn’t have enough time to implant and start sending hormonal signals before the uterine lining breaks down.

Summary

  • In the first half of the cycle, the follicular phase, your body gets an egg cell ready for ovulation and starts preparing the uterine lining for pregnancy.
  • Ovulation is the release of a mature egg from one of your ovaries. It marks the transition between the follicular and the luteal phase.
  • In the second half of the cycle, the luteal phase, your body continues to build up the uterine lining so that it can nurture a growing embryo.
  • If the egg was fertilized, it implants in the uterus between 6 and 10 days after ovulation. The uterine lining is maintained, and you miss your period.
  • If the egg wasn’t fertilized, your hormonal levels will start dropping. When they reach their lowest level, your body starts shedding the old uterine lining and your period begins. A new cycle starts.

If you would like to learn more about the complex and fascinating processes that enable women to conceive, we warmly recommend Toni Weschler’s bestseller Taking Charge Of Your Fertility.