The Menstrual Cycle
Ximena is 12 years old, and today she got her first period. She feels embarrassed to ask, but she has questions: Why is she bleeding? Will this happen every month? What is going on inside her body? From the first period (menarche) until menopause, the female reproductive system repeats the same sequence of changes month after month. This is the menstrual cycle: it prepares an egg and a place where an embryo could implant. It is a normal part of sexual maturation.
The Phases of the Cycle
From the outside, all you notice is a few days of bleeding, but inside, two organs change at the same time: the ovary and the uterus. What stages do they go through in parallel?
The first day of bleeding is day 1 of the cycle. The ovarian cycle has a follicular phase, ovulation, and a luteal phase. The uterine cycle has menstruation, proliferation of the endometrium, and a secretory phase. Both are coordinated, but they describe different organs.
The diagram represents the $28$ days of a menstrual cycle: the curve shows the thickness of the endometrium and four moments are marked with letters. Write down what…
The diagram represents the $28$ days of a menstrual cycle: the curve shows the thickness of the endometrium and four moments are marked with letters. Write down what happens or what each letter (A, B, C, and D) represents, and put these four moments in order throughout the cycle.
Ovulation and the Endometrium
Ovulation is the release of an oocyte (the egg) from the ovary. It can be fertilized for about 12 to 24 hours. Sperm, on the other hand, can survive up to about five days in the female reproductive system. That is why sexual intercourse before ovulation can also result in pregnancy.
The endometrium is the tissue that lines the inside of the uterus. During the first half of the cycle it thickens and fills with blood vessels, so it is ready to receive an embryo if fertilization occurred. If there is no pregnancy, the corpus luteum (what remains of the follicle) breaks down, progesterone and estrogen levels drop, and part of the endometrium sheds: this is menstruation. What is expelled is blood and tissue from the lining of the uterus, not the egg. The arrival of menstruation indicates that there was no pregnancy in that cycle.
Explain what ovulation is, in which organ it takes place, and at what point in the menstrual cycle it usually occurs.
Explain what ovulation is, in which organ it takes place, and at what point in the menstrual cycle it usually occurs.
Explain what menstruation is, what is shed during it, and what its arrival indicates regarding whether or not a pregnancy has occurred in that cycle.
Explain what menstruation is, what is shed during it, and what its arrival indicates regarding whether or not a pregnancy has occurred in that cycle.
In the menstrual cycle graph, the curve represents the thickness of the endometrium over the $28$ days. Describe how this thickness changes during the cycle, relate…
In the menstrual cycle graph, the curve represents the thickness of the endometrium over the $28$ days. Describe how this thickness changes during the cycle, relate these changes to the time of ovulation, and reason why the endometrium thickens before a possible fertilisation.
The Hormones That Regulate the Cycle
The ovary and the uterus are separate, yet they change in a coordinated way. How do they communicate from a distance? Through hormones: chemical messengers that travel through the blood (the endocrine system).
The pituitary gland, at the base of the brain, releases FSH, which promotes the maturation of the follicle, and LH, whose sudden surge triggers ovulation. The follicle produces estrogens, which make the endometrium grow. After ovulation, the corpus luteum mainly produces progesterone, which prepares and maintains the endometrium. Because these hormones repeat in every cycle over about forty years of life, the cycle is called "cyclical."
The menstrual cycle repeats time and time again for around forty years of a woman's life. Explain why it is said to be "cyclic", which hormones regulate it (oestrogens…
The menstrual cycle repeats time and time again for around forty years of a woman's life. Explain why it is said to be "cyclic", which hormones regulate it (oestrogens and progesterone) and which organs produce them, indicating in general terms what each one triggers in the ovary or in the uterus.
Combined contraceptive pills provide daily, continuous doses of hormones similar to estrogen and progesterone. In the natural ovarian cycle, when levels of these…
Combined contraceptive pills provide daily, continuous doses of hormones similar to estrogen and progesterone. In the natural ovarian cycle, when levels of these hormones are high, the pituitary gland slows down the production of FSH (the hormone that stimulates follicle maturation) and LH (the hormone that triggers ovulation).
- a) Predict what will happen in the ovaries of a person taking the pill continuously and justify why ovulation does not occur.
- b) A common misconception is believing that the pill acts by "killing or destroying already mature eggs". Refute this idea based on the role played by FSH.
The Fertile Window and the Limits of the Calendar
The fertile window spans roughly the five days before ovulation plus the day of ovulation itself; since the oocyte lives for up to a day, the possibility of fertilization can last until about one day afterward. In an ideal 28-day cycle, ovulation occurs about 14 days before the next period, and the most fertile days are the day of ovulation and the two days before it.
In a $28$-day cycle, ovulation occurs about $14$ days before the end of the cycle, and the most fertile days are the day of ovulation and the two preceding days. If a…
In a $28$-day cycle, ovulation occurs about $14$ days before the end of the cycle, and the most fertile days are the day of ovulation and the two preceding days. If a girl gets her period on the 6th of a month and her cycle lasts $28$ days, calculate around which day her ovulation will take place and which three days of the month would be, approximately, the most fertile.
An informational leaflet claims: "Since the human egg only survives about 24 hours after ovulation, a person with regular 28-day cycles can only become pregnant if they…
An informational leaflet claims: "Since the human egg only survives about 24 hours after ovulation, a person with regular 28-day cycles can only become pregnant if they have sexual intercourse on day 14 of the cycle."
- a) Refute this claim taking into account that sperm can remain viable in the female reproductive system for 3 to 5 days, and define the days of the cycle that make up the actual "fertile window".
- b) If this person's period began on May 4, predict within which date range of that month there is a high probability of fertilization if they have unprotected sex, and justify your answer.
Lucía has irregular menstrual cycles whose duration varies between 26 and 34 days. In humans, the luteal phase (the time elapsed between ovulation and the start of the…
Lucía has irregular menstrual cycles whose duration varies between 26 and 34 days. In humans, the luteal phase (the time elapsed between ovulation and the start of the next menstruation) is remarkably constant and always lasts about 14 days, whereas the phase prior to ovulation is responsible for the variations.
- a) Calculate between which days of her cycle (counting day 1 as the first day of bleeding) ovulation can actually occur in Lucía.
- b) Justify why the method of trying to predict fertile days by simply counting 14 days from the start of the last menstruation is very unreliable and carries a high risk of unintended pregnancy.
Go back to the beginning: what Ximena is experiencing is the shedding of the endometrium, which repeats every cycle when fertilization did not occur. It is normal, and understanding the cycle helps you take care of yourself.