Menstruation Defined

In some cultures, a girl’s first menstrual period, known as menarche, is a very big deal. They have special ceremonies and feasts, and they have good reason. When a girl has her first period, her body is mature enough that she can have children. Whole tribes celebrate because it means their numbers will grow.

Menstruation is the periodic shedding of the uterine lining when pregnancy does not occur in a woman. When a girl starts having periods, she is able to get pregnant. When she reaches menopause—the cessation of the menstrual cycle usually occurring around age fifty—she cannot get pregnant anymore.

When a female baby is born, she already has all the equipment she needs to have a baby. She has a uterus, two fallopian tubes, and two ovaries. These are the organs of her reproductive system. They enable her to produce another human being.

The Female Reproductive System

The female reproductive system. View Larger Image © Superstock, Inc.

The uterus, or womb, is where an egg grows into a baby for approximately nine months before it is born. The uterus, located in the lower abdomen, is shaped like an upside-down pear, with the narrow end joined to the top of the vagina. The uterus is hollow and made of muscles that are some of the strongest in the body. They have to be strong to push a baby out. The organ is small—not quite as big as a fist, and about 1 inch (2.5 cm) thick. It weighs only 1 to 1.5 ounces (30–40 g). It can grow large enough to hold a baby that is 9 pounds (4 kg) or bigger.

On each side of the uterus is a fallopian tube. These two tubes are attached to the uterus on one end and open on the other. They are thin tubes, like tiny straws, each about 4 inches (10 cm) long. The eggs travel in these tubes to the uterus. Very fine hairs inside the tubes move the eggs along.

The eggs begin in the ovaries. Girls have two ovaries, one on each side of the uterus next to the open ends of the fallopian tubes. They are shaped like, and are about the same size of, almonds. When a baby girl is born, her ovaries already have all the eggs she will need to have children. In fact, she has more than she will ever need—about one million to two million. These are not mature eggs. They do not even begin to mature until she is at least eight years old, and more often ten or twelve years of age or even older as she enters puberty.

When the first egg matures, a girl’s reproductive system goes into operation. The egg moves from the ovary in which it was formed into the fallopian tube next to that ovary. From there, it travels to the uterus. During the time the egg is developing and moving, the uterus is getting ready to receive it. The body sends blood to the tissues that line the walls of the uterus. The lining is called the endometrium. The endometrium grows new layers so it becomes thick and soft. This makes the uterus a safe place for the growing baby. It also supplies the nutrients the baby needs to grow. Fertilization occurs when a sperm from a man joins with the woman’s egg while it is traveling in the fallopian tube. It will then attach itself to the cushiony lining of the uterus.

If the egg is not fertilized, the uterus does not need the extra blood and added layers, so it gets rid of them. Along with the egg that has decomposed, blood and some of the tissue that makes up the endometrium flow out through the opening at the bottom of the uterus, the cervix, through the vagina and out the woman’s body. This is menstruation. Although most people call menstruation “bleeding,” the discharge—what comes out—is only partly blood. Most of it is the extra uterine lining. The flow lasts just a week, or even less. Sanitary pads or tampons are used during menstruation to absorb this flow.

If a fertilized egg plants itself in the uterus, this is the beginning of pregnancy. Menstruation stops until after a baby is born or the uterus is emptied. When no fertilized egg is in the uterus, the process repeats itself about once a month. Menstruation comes from the Latin word for “month.” Because the shedding happens regularly, we call it the menstrual cycle. When it happens, we say a girl has her period because the process happens periodically. What makes it happen? What makes an egg mature and start the process? How does the uterus know when to prepare?

Hormones

Hormones are chemicals that control how different parts of the body operate. The body makes more than forty kinds of hormones, each with a specific message for a specific organ. The organs of the female reproductive system are controlled by a chain of hormones that begins in the brain.

In the center of the brain are the hypothalamus (the command center for the entire body) and the pituitary gland (the master gland). They regulate growth, hunger, sleep, and many other activities, including reproduction. First, the hypothalamus sends a hormone to the pituitary gland. Then the pituitary gland sends hormones to the ovaries. Each egg is inside a pocket, or sac, called a follicle. When the ovaries get the message, several eggs start to develop. As the eggs mature, the ovaries release the hormone estrogen. This hormone is what makes the lining of the uterus, the endometrium, thicken. When the uterus is ready for an egg, the hypothalamus sends another hormone to the pituitary gland that starts the process of getting an egg to the waiting uterus. The pituitary gland sends a new type of hormone to the ovaries that makes the most mature follicle burst open and release its egg. The egg leaves the ovary, finds its way into a fallopian tube, and from there travels to the uterus.

Now the broken follicle begins to produce two important hormones: estrogen and progesterone. They are often called female hormones. An egg cannot grow properly without a good supply of these two hormones. The follicle produces these hormones the whole time the egg is in the uterus. But if the egg is not fertilized, it dies and the follicle stops making hormones. Without the hormones, the blood supply to the extra layers of the uterine lining dries up. The layers start to fall off.