All the phases of your monthly cycle, explained
As most women are already aware long before they try for a baby, a normal menstrual cycle can be anything from 24 to 36 days, not necessarily the textbook 28-day cycle. Your menstrual cycles can be even longer or shorter than this, and even the most regular of women can experience unusually long cycles, usually when stress or a similar issue has delayed ovulation.
Your menstrual cycle is split into two main parts:
While the second, or luteal, phase is around the same length each cycle (usually 12-16 days), the length of the first phase of your cycle can vary. That’s why there can be misunderstandings around ‘late’ periods and suspected pregnancy.
In the first phase of your menstrual cycle, several different reproductive hormones act together to ensure the maturing and release of an egg from one of your two ovaries.
Each cycle, the Gonadotrophin-releasing hormone produced in your brain prompts your pituitary gland to release Follicle Stimulating Hormone, or FSH, into your bloodstream.
The FSH stimulates your ovaries to start maturing eggs, around 15-20 eggs in each ovary, in individual follicles that produce oestrogen.
As well as encouraging the eggs within the follicles to mature, oestrogen stimulates the lining of the womb, or endometrium, to thicken in preparation for a potential conception.
It usually takes about two weeks for the eggs to mature. However, it can be as little as eight days, as long as a month, or sometimes even longer.
As the oestrogen level reaches a threshold it causes your pituitary gland to release a flood of luteinising hormone (LH), sometimes called the LH surge. This then triggers the release of an egg from the most mature follicle. The egg bursts out of your ovary and into your pelvic cavity – and this is ovulation!
Occasionally two eggs are released within hours of each other. If both eggs are fertilised, you’ll have conceived fraternal twins.
Once the egg is released into your pelvic cavity it’s caught by the finger-like fimbria at the ends of your fallopian tube and drawn into the tube. The egg is driven along the fallopian tube by the vibrations of tiny, hair-like cilia on its walls.
The follicle that released the egg collapses, becoming a ‘corpus luteum’, or yellow body, and starts releasing progesterone. Within 24 hours, the progesterone will stop any further eggs being released during that cycle.
The progesterone also encourages the thickening of the lining of your womb in preparation for conception. Progesterone is responsible for physical changes that you look out for when charting, such as:
The corpus luteum lives for between 12 and 16 days, determining the length of the luteal phase.
The egg will live for 24 hours at the most if it isn’t fertilised. It will then get reabsorbed by your body or will be expelled when you menstruate.
If the egg isn’t fertilised, the corpus luteum will die at the end of its normal lifespan (12 and 16 days) and won’t longer release progesterone any more.
The levels of progesterone and oestrogen then drop, your womb contracts (the cause of period cramps) the lining of your womb starts to disintegrate, and your menstrual flow starts.
Then the whole menstrual cycle process can start again!
If the egg that’s been released is fertilised it means conception has happened. Conception will usually be within a few hours of ovulation and in the upper part of your fallopian tube.
Sperm swim up through your cervix and womb and into your fallopian tubes. Of the millions of sperm that are released during ejaculation, only one will penetrate the walls of the egg, or ovum, and fuse with it to form the zygote.
An average 28-day menstrual cycle will mean you are most fertile between days 12 and 15; a 23-day cycle will mean you’re most fertile between days 7-10; a 35-day cycle will mean you’re most fertile between days 19 and 22.
The bundle of cells then burrows its way into the thickened lining of your uterus and starts to produce the pregnancy hormone human chorionic gonadotrophin, or HCG. This alerts the corpus luteum to prolong its usual lifespan and to continue releasing progesterone so that the lining of your womb won’t disintegrate and can provide a nourishing environment for the embryo.
An anovulatory cycle is when your body is unable to reach the oestrogen threshold necessary to trigger ovulation, so ovulation doesn’t occur.
You can still have what appears to be a period when you’re not ovulating. This can happen for either of two reasons:
If you’re not charting your temperature then you might not realise that you haven’t actually ovulated.
It’s more obvious that you're not ovulating when your periods stop altogether for a period of time, resulting on one very long cycle.
When you’re breastfeeding you’re also unlikely to ovulate – nature’s way of helping your body keep a healthy gap between pregnancies. That said, there’s no guarantee you won’t get pregnant whilst breastfeeding – many women have, thinking they didn’t need to consider contraception.
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