Conception Physiology

by J.C. Willke MD

Let's review fertilization physiology, or how does conception occur in the woman's body?

As you all know, the act of human intercourse deposits sperm inside the woman.  Each deposit contains tens of millions of sperm.  There may be as many as a hundred million in a single ejaculate or deposit of sperm.  Each sperm is a freely moving cell.  Under the microscope, each looks a lot like a tadpole.  By moving its tail, the sperm cell swims through the cervical canal (the mouth of the womb) and into the cavity of the womb.  Now, in order for this to happen easily, the mouth of the womb must be, as we say, "open."  What do I mean by that?

Well, through most of the woman's monthly cycle, this opening is plugged by a thick viscid mucus that is somewhat difficult for the sperm to get through.  In effect, by this thick mucus for most of the month, nature erects a natural barrier at the mouth of the womb.  But during her fertile time, this thick mucus melts away, and is replaced by a thin, watery mucus of egg-white consistency.  The sperm can swim through this mucus very easily as well as drawing strength from its nutrients.  People often use the phrase "potential life."  Well, here we have potential life.  It is millions of eager sperm seeking one ovum.

Let's follow the journey of human spermatozoa (or sperm) through the female genital organs and out through her tubes.  Once the sperm are inside the cavity of the womb, they swim deep into the womb itself, and many of them will find the openings to the fallopian tubes.  Continuing their journey, these sperm swim into the tubes.  A woman's tubes are not simple open pipes that the sperm would coast through like so many sleds sliding down a hill.  No, they are convoluted, have innumerable twists and turns, and the journey of the sperm, therefore, is difficult.  This is nature's way of assuring that only the strong and healthy win the race.

Finally the winners emerge out the other end of the tube into her abdomen.  Once through the tube, the sperm are guided to the ovary by the fringe, almost octopus-like fimbria, at the ends of the tubes.  Inside of the ovary are hundreds of immature ova.  Each month one or more of these ripen and migrate to the outside, just under the skin of the ovary.  And then in what we call "ovulation," one of these ova or eggs ruptures through its follicle, or shell, and breaks out of the ovary to float free in the abdominal cavity.  One of these ova is about the size of the head of an ordinary straight pin. 

The sperm are much smaller.  Think of a tiny ant crawling up on a basketball and you will have an idea of the contrast in size.    

Sperm swarm around the ovum and then one will dig its head right into the wall of the ovum.  Material shoots out from the wall, engulfs the sperm and sucks it into the ovum.  This is the "moment" of conception or fertilization.  Once one sperm enters the egg then we have an actual new human life.  A human life with vast potential. 

Instantaneously, a kind of electric or chemical change is set up around the entire shell of the ovum, and this prevents any other sperm from entering.  The 23 chromosomes of the ovum line up and join the 23 chromosomes of the sperm.  This uniting completes the process of fertilization, and, at this time, a brand new unique human being exists, even though this new male or female is still only a single cell.    

And then?  Well, now the journey must be retraced.  This new single celled human has much to do.  Within a few hours, those same friendly fimbriated ends of the tube begin to gently sweep it back into the tube.  At the end of the first day of life this fertilized egg or zygote, divides into two cells and is now called an embryo.  The cell division continues as this new human embryo floats freely back down her tube.  When one week old, this tiny human enters the womb.  He or she then seeks a spot on the wall of the womb, and burrows into this nutrient lining.  Once attached within the lining, he or she quickly sends a chemical, hormonal message out into the mother's blood stream and into her body, to a gland in her brain.  That message says to the mother's body, "don't menstruate, I'm here."    

We now know, and we didn't know until a few years ago, that it was each of us who stopped our mother's menstrual period.  And then?  Well then we each settled down and began to grow.