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.

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.

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.