Natural Conception or Natural Pregnancy is a complicated affair. It is the result of a perfect orchestra of ovulation, egg pick by the fallopian tube, sperm reaching the egg at the right time and the fertilized egg or embryo reaching the uterus and embedding into the wall of the uterus at the right time.
For understanding that we need to understand the reproductive system under the headings:
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The female reproductive system consists of the uterus, two fallopian tubes and two ovaries.
The lowest part of the uterus is called the cervix, which opens into the vagina where sexual intercourse occurs.
The ends of the tubes have finger-like projections called fimbriae, which does the essential function of picking up the eggs from the ovaries.
The ovary has follicles within which are the eggs. Every follicle is filled with fluid and has one egg in each.
The male reproductive system has two testes stored in scrotal sacs and ducts which open to the outside via the male organ that is the penis along with the urethra.
In a man, every 72days, fresh sperm are produced at the age of puberty, and the process continues throughout their life.
So at any point, the sperms are not more than 72 days old. However, the testes are placed outside the body and are maintained 2 degrees less than body temperature.
However, in women, it is different.
A woman is born with a fixed number of eggs after which there is no fresh production of eggs.
To add to this, every month, there is natural destruction of a cohort of eggs.
So a girl born with millions of eggs will have only a few lakhs of eggs at puberty and will be left with a few thousand after 35 years.
As women age, the eggs are also old. Her age is equal to the age of her eggs, and the best eggs are recruited in the early part of reproductive life. So the remaining eggs will be inferior in quality.
Every month 15 to 20 follicles are recruited per ovary, of which we can see only 5 or 6 per ovary. Follicles are not eggs; it is the shell that contains egg.
Out of this only one follicle which is the most sensitive to the hormones will grow. Once it reaches 20mm, it can rupture or release the egg, which is picked up by the fallopian tube. It remains alive in the tube for 24-48 hrs before which the sperm has to unite with it to form an embryo or else it degenerates.
Once natural intercourse occurs, sperm are deposited in the vagina, and most of them get trapped in the cervix which acts as a natural filter against microorganisms and so ends up in holding back a lot of sperms. Only those sperms which have good motility will be able to cross the cervix and reach the uterus and eventually the fallopian tubes, the fastest sperm which reaches the tube is the one which fertilizes the egg. Only if the sperm is healthy enough to fertilise the egg, it will be able to do so.
Once the embryo is formed, the cilia inside the tube push the embryo to the uterus after three days, where it is absorbed into the inner layer of the uterus on the 5th or 6th day of embryo formation. This process is called implantation.
This process starts after seven days of ovulation. When the embryo is implanting Beta HCG hormone is secreted, which is detected in the urine or through the blood.
This is the hormone detected by a pregnancy test.
As the pregnancy proceeds, these hormone levels also increase. When the levels are above 1000, a gestational sac or pregnancy sac can be seen in the uterus through transvaginal ultrasound.
I hope this helps you in understanding how Spontaneous Pregnancy happens.