How exactly is pregnancy caused ?

If you want to understand how pregnancy happens, you need to know about the mesntrual cycleof a woman.
After menstruating for the first time , a girl has her period once every month. Often its every 28 days although for some women its less and some its more than 28 days.

The first day of the period is counted as the first day of menstrual cycle. After the bleeding stops one egg starts yo grow in the ovary. Also, the lining of tje uterus starts tp build up, in order to receive the fertilised egg.

Between the eleventh and fourteenth day, the the egg is released from the ovary and start to move through the fallopian tubes to the uterus. The woman gets pregnant , if the egg is fetrtalised by sperm in the fallopian tube. That is possible, if she has sex few days before before the egg is released or the same day the egg is released .

If the egg is fertilised while moving towards the uterus, it will ettle in the lining of the uterus. The baby is protected in the uterus while growing until the day its born

If the egg is not fertilised , it dies and together with the lining of the uterus it leaves the body of the woman as menstrual blood. That is, the woman has has period as usual and she will know that she is not pregnant.