HOW DOES PREGNANCY
HAPPEN NATURALLY?

Usually a woman desiring pregnancy has to have the following system in place.

As per the drawing above, the woman needs to have a uterus which shall hold the baby during pregnancy. This uterus is a hollow organ with a muscular wall. The hollow of the uterus is the womb or the cavity which is where the baby resides and grows. The womb is lined by a layer which is called endometrium which is shed off every month as menses and regenerated again.

There are two tubes which come out of the uterus at its two angles and are known as fallopian tubes. They are the ones which catch the egg when it is released from the ovary. These tubes are open at two ends. At the distant end it is open to the abdominal cavity and at the near end it opens into the uterine cavity through openings known as ostia. The tubes are lined internally by fine hair like structures (cilia) which actively push the egg after fertilisation towards the uterine cavity. The distant end of the tubes have finger like projections which help in guiding the egg into the tubes. These are known as fimbriae.

The lower end of the uterus is a tunnel like structure called the cervix. It is the cervix which acts point of entry into the uterus. For the whole of the month the cervix is filled with a gum like liquid called mucus which does not allow any sort of infection to enter the uterus. At the time of ovulation when the egg is to be released from the ovary, this mucus becomes watery and allows the sperms to easily pass through (sort of a dynamic gate). Such is the wonder of nature!

The woman is born with a fixed number of eggs in her ovaries. We estimate it to be around 1- 2 million at time of birth which reduces to about 400,000 by the time her menses start. These eggs are then used up @ 30 per day so that she has exhausted all her eggs by the time she is about 47-48 which is when she hits menopause. Of the 800-900 eggs she uses up every month( @ 30 per day), only about 30 odd are available to grow up into mature eggs at the start of any menstrual cycle. Of these 30 immature eggs called antral follicles only one grows up to full maturity and rest are discarded. This process of selecting one and rejecting the rest of the eggs is mediated via two important hormones called FSH ( follicle stimulating hormone) and LH ( luteinising Hormone). They are manufactured in the brain. These two hormones have to be released systematically and rhythmically by the brain to be able to select any one of the eggs available in the ovaries. The area in brain that makes FSH and LH is adjacent to one making TSH (thyroid hormone) and Prolactin (breast hormone). So if there is some trouble with these hormones, then there may be some trouble with FSH and LH release as well.

FSH is released in higher amounts than LH initially and it acts as the selecting hormone working on choosing any one of these eggs. By the 7th day of onset of menses, the chosen one has been identified and now it is LH that matures this egg alone. By the 13th day, this egg is ripe and we know this for a fact as by now the LH hormone is so high in the body that it leaks in urine as well (LH surge).

24-32 hours later, the egg is released from the ovary and it finds its way inside the fallopian tube, which is a cylinder like structure connecting to the uterus at one end while the other end is free. Let's call the place where the egg settles down as point A. This egg is going to stay alive for the next 12 hours or so. All this time while the egg was maturing, it has been releasing another hormone called Estradiol, which is preparing the womb lining (endometrium) to receive the baby.

Vagina leads on to the mouth of uterus called cervix. As discussed earlier, it is clogged with mucus which acts as a barrier for entry of anything including sperms into the uterus. But at the time of release of the egg this mucus becomes watery allowing for easy entry of sperms into the uterine cavity. Sex around this time allows the partner to deposit his semen in the vaginal canal at point B. Because vagina is shaped like an inverted glass, most of this semen flows out. This is normal. What matters is what remains inside sort of lubricating the inside walls of vagina. This semen holds millions of sperms. A man ideally places about 50-80 million sperms during sex at point B. So even if most of the semen has flown out, there is still enough left fromwhich the sperms start what is arguably the toughest race of their lifetimes. Only about 1500 to 5000 fittest of these millions of sperms would eventually end up near the egg nestled nicely in the fallopian tube at point A.

The egg just as any egg in the animal kingdom has a tough shell around itself. These sperms then secrete acids, which dissolve the shell unevenly, which allows any one sperm to enter the egg (fertilization). Once that happens the shell immediately shuts out other sperms.

This fertilized egg is now the embryo that is going to grow for the next 4-6 days there itself. The embryo is what is then going to further divide into the fetus and the placenta and other stuff that makes up the pregnancy..

Then with the aid of hair like structures inside the fallopian tube called cilia, which behave like the windshield wiper of your car, the embryo is slowly pushed to its final point of rest i.e. the womb. By this time the womb is lined with the endometrium built up by the hormones secreted by the egg. If this so called bed is free of any disease, receptive to the embryo then it sticks to endometrium and starts growing transforming into the fetus first and then the baby one was craving for all along!