How Babies are Made - The Basics of Human Fertility and Reproduction

Every school child knows that you need eggs and sperm to make a baby. However, we need to examine the basics in greater detail, so let's start by taking a guided tour of the reproductive system.

How does a woman's reproductive system function ?
The Reproductive System of a Woman

The sexual and reproductive organs on the outside of the body are called the external genitals. There are three openings in the genital area. In front is the urethra, from where urine comes out; below this is the opening to the vagina which is called the introitus ; and the third is the anus from where a bowel movement leaves the body.

The outer genital area is called the vulva. The vulva includes the clitoris, the labia majora and the labia minora. The most sensitive part of the genital area is the clitoris. This is a pea shaped organ that's full of nerve endings since its only purpose is to provide sexual pleasure. The clitoris is protected by a hood of skin, and is the equivalent of the man's penis.

The labia majora, or outer lips, surround the opening to the vagina. They are made of fatty tissue that cushions and protects the vaginal opening. Between these outer lips are labia minora, or inner lips. These are sensitive to sexual pleasure. As they are stimulated, they get deeper in color and swell.

The vagina is a muscular tunnel that connects the uterus to the outside of the body. It provides an exit for the menstrual fluid; and an entrance for the semen. Normally flat, like a d balloon, the vagina can stretch to accommodate a tampon, a penis or a baby's head. The walls of the vagina are muscular, smooth and soft. The vagina is a closed space which ends at your cervix.

The uterus, or the womb, is the place where the fertilized egg grows and develops into a baby during pregnancy. The uterus lies deep in the lower abdomen - the pelvis - and is just behind the urinary bladder. It is a hollow organ shaped like a pear and is about the size of the fist. Inside the muscular walls of the uterus is a very rich lining - the endometrium, and it is in this lining that the fertilized egg implants. If pregnancy does not occur, the lining is shed along with blood as the menstrual flow.

The neck of the uterus is called the cervix. It connects the uterus to the vagina and contains special glands called crypts that make mucus which helps to keep bacteria out of the uterus. The cervical mucus also helps sperms to enter the uterus when the egg is ripe.

The two fallopian tubes (also known as oviducts) are attached to the upper part of the uterus on either side and are about 10 cm long. They are about as big as a piece of spaghetti . Each tube forms a narrow passageway that opens like a funnel into the abdominal cavity, near the ovaries. The ends of the fallopian tubes are draped over the two ovaries and they serve as a passageway for the egg to travel from the ovary into the uterus. The tube is lined by millions of tiny hairs called cilia, that beat rhythmically to propel the egg forward. Of course, the tube is not just a pathway - it performs other functions too, including nourishing the egg and the early embryo in its cavity. Also, the sperm fertilizes the egg in one of the fallopian tubes.

The two almond-sized ovaries are perched in the pelvis, one on each side, just within the fallopian tubes' grasp. The ovary serves two functions: the production of eggs and the secretion of hormones. Each month, at the time of ovulation, a mature egg is released by an ovary. This is "picked up " by the fimbria and drawn into the fallopian tubes.

The eggs in the ovary are stored in follicles (from folliculus, meaning sack in Latin). These cellular sacks contain the eggs; as well as granulosa cells and theca cells which nurture the egg , and produce the female hormones. The ovary has about 2 million eggs during fetal life. From that point onwards, the number of eggs progressively decreases, till only about 300,000 eggs are left at the time of birth - a lifetime's stock. During the fertile years fewer than 500 of these eggs will be released into the fallopian tubes - once in each menstrual cycle. Unlike the testis which is continually churning out billions of new sperm, the ovary never produces any new eggs. One of the existing eggs is matured for ovulation each month - and this limited supply runs out at the time of menopause.

Figure 1. Female external genitalia

Figure 2. The female reproductive system

Can you explain the menstrual cycle and its role in fertility ?
The Menstrual Cycle

The aspect of the reproductive system that women are most aware of is the menstrual period which they have every month. The menstrual cycle is the time from the beginning of one period to the beginning of the next one. Usually menstrual cycles last about 28- 35 days, though anywhere from 3 to 6 weeks is considered normal .

During the menstrual cycle, the uterus gets ready for pregnancy. Under the influence of the hormones estrogen and progesterone, its lining grows rich and thick to prepare for the fertilized egg. If pregnancy doesn't occur, the uterus must get rid of this lining so that it can grow a new one in the next cycle. The old lining passes out of the uterus through the vagina as the menstrual flow.

The menstrual flow thus consists of:

  1. the shed uterine lining
  2. blood (this comes from the blood vessels which are torn when the lining is shed)
  3. the degenerated unfertilized egg

If the menstrual flow is heavy, there may sometimes be clots in it. Sometimes the uterine lining is shed as large fragments - and these may sometimes looks like bits of pregnancy tissue to some women, who think they are miscarrying.

Many infertile women are obsessed with their menstrual periods, and they worry about every little variation - whether it's too dark, too light, too much or too little. However, remember that the menstrual flow has no connection to your fertility and you should not be too concerned about variations, which are quite common and of little significance.

Authored by : Dr Aniruddha Malpani, MD and reviewed by Dr Anjali Malpani.