Exercises: The menstrual cycle

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.

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.

Name the phases of the menstrual cycle and state, in one sentence, what happens in each of them.

Explain what the endometrium is, in which organ it is found, and why its thickness changes throughout the cycle.

Explain what oestrogens and progesterone are, which organ produces them, and which organs have their function altered by them.

State whether these statements are true or false and justify your answer: a) all women have cycles of exactly $28$ days; b) menstruation is part of the menstrual cycle; c) during pregnancy the menstrual cycles continue.

Explain what the menarche is, at what stage of life it occurs, and what it indicates about the maturation of the reproductive system.

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 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.

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.

Explain the relationship between ovulation and the fertile days: why, if the cycle lasts about four weeks, the probability of pregnancy is concentrated in just a few days.

A girl has regular $32$-day cycles. Knowing that about $14$ days pass from ovulation to the next menstruation, estimate around which day of the cycle she ovulates and explain why it does not happen exactly in the middle.

During pregnancy there is no menstruation. Explain why, relating it to the function of the endometrium and to the hormones that maintain it during gestation.

Intense stress, long journeys or eating disorders can make the period temporarily disappear. Reason what the body needs to maintain regular cycles, and why doctors consider the period a "sign of health".

Explain what information a girl can record about her cycle (dates, duration, symptoms) and what use knowing the regularity of her own cycle has for her health.

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."

1. 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".
2. 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.

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).

1. a) Predict what will happen in the ovaries of a person taking the pill continuously and justify why ovulation does not occur.
2. 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.

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.

1. a) Calculate between which days of her cycle (counting day 1 as the first day of bleeding) ovulation can actually occur in Lucía.
2. 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.

There is a popular belief that a mother who continuously breastfeeds her baby cannot become pregnant as long as her period has not returned.

1. a) Justify, from an evolutionary and offspring-survival perspective, the biological advantage for mammals of intensive breastfeeding temporarily inhibiting ovulation.
2. b) Refute the described popular belief with biological arguments: explain the temporal relationship between ovulation and menstruation, and reason why a breastfeeding woman can conceive without having had her first period after childbirth yet.

Toward the end of the luteal phase (days 25 to 28 of a typical cycle), if fertilization has not occurred, the corpus luteum degenerates and progesterone and estrogen levels plummet.

1. a) Knowing that progesterone is the hormone responsible for keeping the endometrium thickened and vascularized, predict the consequence that this sharp hormonal drop will have on the inner wall of the uterus and explain how menstruation originates.
2. b) Someone argues that "menstruation occurs because at that moment sex hormones reach their highest level to push the tissues outward". Justify with reasons whether this statement is true or false based on the actual hormone levels.

A couple wishes to have a baby. The woman has regular 32-day cycles and her last period started on March 2. Her partner checks a website and suggests: "Since ovulation happens around the middle of the cycle, our most fertile day will be day 17 (March 18), so we will only have intercourse on March 18 and 19."

1. a) Knowing that the luteal phase lasts a fixed 14 days (from ovulation to the next period), calculate on which day of the cycle and on what exact date in March ovulation will actually occur.
2. b) Considering that sperm can live up to 4 days in the female genital tract and the ovum about 24 hours, determine the woman's fertile window in March and explain whether her partner's proposed plan has a high probability of success or if they should modify it.

In elite female athletes with excessively low body fat percentages and very high energy expenditure, the hypothalamus often halts the menstrual cycle (athletic amenorrhoea), which causes a drastic drop in oestrogen secretion.

1. a) From the perspective of energy balance and survival, explain why the body "shuts down" reproductive function under conditions of severe energy scarcity to prioritise vital functions.
2. b) Knowing that oestrogens promote calcium fixation in the bone tissue matrix, predict what effect this prolonged situation will have on the bones of a young female athlete and explain what type of skeletal injury she will be at greater risk of suffering during her training sessions.

For centuries, there has been a myth that menstrual blood is a "toxic" substance or a periodic waste product through which the female body "purifies itself and eliminates accumulated impurities."

1. a) Refute this myth by detailing what tissues, cells, and fluids truly make up the menstrual flow and what the exact physiological cause is for its shedding.
2. b) During the nine months of pregnancy, menstruation does not occur. Use this fact to explain why the hypothesis of "periodic toxin purification" lacks any biological foundation and explain what function the endometrium performs if embryo implantation occurs.