What this means is that, in the entire cycle of about twenty-eight days (for most women), there is only a small window when conception can take place. Male gametes (spermatozoa) do survive considerably longer, even though most die within twenty-four hours. In theory, sexual intercourse tonight could lead to fertilization tomorrow in a case where spermatozoa are deposited in the woman's genital tract. If, by coincidence, ovulation takes place within the critical 24-36 hour period when some healthy spermatozoa may be still lurking about, conception will occur.
This, however, is an over-simplification of a very complex process. There are a lot of other factors that come into play in order for fertilization to lead to an established pregnancy. Any of these, if not right, could thwart the conception.
It should not surprise anybody that people don't always end up conceiving as soon as they plan, just because they are having regular intercourse, minus contraception. It is estimated that regular intercourse for a healthy couple has a 25 per cent chance of leading to pregnancy each "month" or menstrual cycle. Surprisingly, the term "regular intercourse" seems to mean different things to different men and women. It should be regarded to mean having sexual intercourse on average three or four times a week, evenly spaced out.. Clustered acts of intercourse, followed by several days of abstinence, cannot be regarded as regular, whatever the total number might say.
It has happened
Once the "winning" male sperm fuses
with the female egg, a 'zygote" is formed. It is effectively one cell formed from the two gametes. It has forty-six chromosomes, like each of the resultant billions of body cells that will result from its divisions. In effect, the nine-month journey to form a new human being bas started.
Cell division starts in about thirty hours, when the first division of the zygote takes place to give two cells and, within four to five days, this will have reached a sixty-cell stage. All the time, the zygote is making- its way through the tube towards the womb, and is now called a "morula".
Implantation will start taking place towards the end of the week and, within two weeks, this too is complete. Up until them, the woman is completely unaware she is pregnant. In fact, this is about the time she will be expecting her period. When it does not arrive, then her suspicions that something may be afoot start to surface.
Two things could delay the discovery of the pregnancy at this stage- A woman who has irregular periods may regard this as just another late one and file it away at the back of her mind. Her suspicions may not surface again for weeks, until she asses yet another period or tell-tale symptoms start.
Another thing that may mislead a newly pregnant (but still unaware) woman is light vaginal bleeding that may occur at the time of implantation. Since this is about the time she is expecting her period, the bleeding may be dismissed as just a fight period. Again, she may remain oblivious of the momentous process taking place in her body until the next 'period" fails to arrive or she starts getting nauseous for no apparent reason.