Ok. Here's how things work. It's harder to concive a child that it might seem. All those movies and TV shows have created over time an erroneous ideea inside peoples head , making them believe (including myself, until quite recently) that making babies is a very easy thing to do.
This is how things work in RL ( and no, I won't take in consideration the other aspects like dwarf fertility, magic et cetera)
Inside the ovaries of a woman there are some special structures called follicles. Out of them, around 300-400 are able to produce egg cells, one at a time (usually) . So a woman has around 300-400 menstrual cycles from their first one until their menopause.
The menstrual cicle of a woman lasts approximately 28 days. It is divided in two parts : the pre and postovulation period.
The first one lasts until ~ day 14. During this time, one of the follics goes through some transformations that will ultimately lead to it creating the egg cell.
In the 14th day the follic breaks and the egg cells is sent in the abdominal cavity, where it's absorbed by the uterin tubes...in other words the ovulation takes place. It remains viable only 24 hours after it was expelled from the ovary.
Now let's take in consideration the sperm cells. The average sperm quantity ejaculated during a sexual act is around 3,5 ml ( Sorry for those of you that use the imperial system, but let's face it...it's just plain awful). In 1 ml of sperm there are around 120000000 sperm cells. If the number drops below 20000000 then the guy is most likely infertile ( sorry Grey Wardens)
Most of the sperm cells don't last more than 24 hours inside the genital tract of a woman, but some of them can last up to 72 hours.
Than been said, the sexual act must take place somewhere between 1 day prior to the ovulation up to 1 day after it to ferlitize the egg cell. If the sexual act takes place in a different day that doesn't meet this criteria then there will be no baby, even if the love birds don't use protection.
(Note: this parameters might slightly change from case to case, so if you want to...aham...use this information for...aham...scientific purposes I advice you to add some extra days to the waiting just to be sure)
If the egg cell is fetilized, then you get a baby. If not, in the 19-20 day of the cycle the inside layer of the uterus goes through some vasculary transformations that lead to its necrosis, some hemoragy and the elimination of the egg cell. Don't worry, it rebuilds itself
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Questions?