Learning about pregnancy
Adrian Whyte and Joanna Johnson, Contributor

Athletes pose on day four of the Boys and Girls Athletic Championships at the National Stadium on April 1. - Ricardo Makyn Photo

THE MENSTRUAL CYCLE

Between age 10-15, enough follicle-stimulating hormone is being produced by the pituitary gland into the bloodstream of the female. This stimulates the development of the ovum (egg) in the follicle. Oestrogen, produced in the follicle, initiates the development of secondary sexual characteristics such as breast development, hair underarms, pubic areas and broadening of the hips.

Every 28 days an ovum matures and is released from the follicle within the ovary. This is called ovulation. The egg is sent to the funnel-shaped part of the fallopian tube. Immediately, fine hair [cilia] sweep the egg down toward the uterus. The remains of the follicle cells are now called the
yellow body [corpus luteum].

This yellow body is stimulated to produce a hormone called progesterone which causes the lining of the wall of the uterus to further prepare for a fertilised egg. Blood sinuses are formed and the uterus wall becomes thick and spongy.

If the egg released by the follicle is not fertilised, the wall of the uterus breaks down and blood, mucus and the disintegrated unfertilised egg are all discharged through the vagina. This is the menstrual flow and it lasts for about five days. A new follicle begins to develop and about 14 days after the beginning of the menstrual flow, ovulation occurs as the cycle is repeated.

If the egg is fertilised, it is implanted in the now-thickened wall of the uterus by about the 21st day.

PREPARATION OF THE UTERUS FOR THE FERTILISED EGG

The hormone oestrogen produced in the developing egg follicle stimulates the enrichment of the wall of the uterus with blood vessels.

After the release of the mature egg from the bubble [follicle] in the ovary, the follicle is stimulated by a hormone from the pituitary gland to secrete a 'pregnancy hormone, progesterone, which will further prepare the wall of the uterus for the arrival of a developing embryo. The inner lining of the uterus wall thickens, forms little pockets and acquires a rich supply of blood. The uterus is now like a spongy carpet of well-nourished tissue.

The pregnancy hormone, progesterone, is essential for the development of the connection between the embryo and the uterus wall of the mother, so that a low progesterone level results in an early miscarriage.

About seven days after fertilisation, the developing embryo enters the well-prepared uterus and a nutritive and metabolic connection is established between the embryo and the mother. Up to the time when the embryo enters the uterus, it gets nutrients and energy from a small amount of the egg yolk from the egg. The nutritive and metabolic connecting structure is called the placenta.

At the time when the young, developing embryo attaches itself to the wall of the uterus, it is composed of two sets of cells. One set will develop into the young offspring, and the other set develops into four sets of membranes.

One of these membranes, called the amnion [water bag], which surrounds the embryo forming a space. This space is filled with a lymph-like fluid called the amniotic fluid. This forms a private pond which acts as a shock absorber for the developing embryo. Also, as the foetus develops and is cramped into this small space, the fluid between the developing parts helps prevent them sticking together.

The other important membranes become closely associated with the wall of the uterus. The lining of the uterus wall develops glands and blood spaces. Finger-like projections containing many blood capillaries fit into the blood spaces. They fit so that the mother's blood and the foetus' blood do not mix. They are in close contact, being separated only by the thin membrane which keeps the mother's blood in the blood spaces. This specialised area of the wall of the uterus and the membranes form the placenta.

* Adrian Whyte and Joanna Johnson teach Biology at Ardenne High School
masterbio@gmail.com.