The menstrual cycle
From menarche (the first period) to the menopause, the female reproductive system repeats the same sequence of changes month after month. This is the menstrual cycle: its purpose is to prepare an egg and a possible place for it to implant.
The phases of the cycle
Although only a few days of bleeding each month are visible from the outside, simultaneous and continuous changes are taking place inside the body in two organs at once: what stages do the ovary and the uterus go through in parallel?
The first day of menstrual bleeding is day 1. The ovarian cycle comprises the follicular phase, ovulation and the luteal phase; the uterine cycle comprises menstruation, proliferation of the endometrium and the secretory phase. The two are coordinated, but they describe different organs.
Name the phases of the menstrual cycle and state, in one sentence, what happens in each of them.
Name the phases of the menstrual cycle and state, in one sentence, what happens in each of them.
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.
Explain what the menarche is, at what stage of life it occurs, and what it indicates about the maturation of the reproductive system.
Explain what the menarche is, at what stage of life it occurs, and what it indicates about the maturation of the reproductive system.
Ovulation and the endometrium
Ovulation releases an oocyte, commonly called an egg, from the ovary. It can be fertilised for approximately 12–24 hours; sperm can survive for up to about five days in the reproductive system. For this reason, sexual intercourse before ovulation can also result in pregnancy.
If pregnancy does not occur, the corpus luteum degenerates, progesterone and oestrogen levels fall and part of the endometrium is shed: this is menstruation. Menstrual blood comes mainly from the lining of the uterus, not from the egg.
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 the endometrium is, in which organ it is found, and why its thickness changes throughout the cycle.
Explain what the endometrium is, in which organ it is found, and why its thickness changes throughout the 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.
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.
The hormones that regulate the cycle
The ovary and the uterus are physically separate and carry out different tasks, yet they synchronise their changes perfectly month after month: how do these organs communicate at a distance to coordinate their cycles?
The pituitary gland releases FSH, which promotes the maturation of the follicle, and LH, whose sudden surge triggers ovulation. The follicle produces oestrogen, which promotes the proliferation of the endometrium. After ovulation, the corpus luteum mainly produces progesterone, which prepares and maintains the secretory endometrium.
Explain what oestrogens and progesterone are, which organ produces them, and which organs have their function altered by them.
Explain what oestrogens and progesterone are, which organ produces them, and which organs have their function altered by them.
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.
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.
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.
The fertile window and the limits of the calendar
The fertile window covers approximately the five days before ovulation and the day of ovulation; the survival of the oocyte can extend the possibility of fertilisation to about one day afterwards. It is not possible to identify days with no risk of pregnancy using a calendar alone.
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.
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…
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.
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.