A girl has eggs in her. She becomes pregnant when an egg is released and a sperm (cum is full of sperm) manages to make it to the egg.
Some times of the month are more likely for pregnancy, as eggs are only released once a month. Likewise, a guy's semen/cum might have problems.
Any time a guy and girl have sex without a condom, there is a chance of pregnancy. It is not automatic. There is also a chance of giving someone a sexually transmitted infection (if one person has one). It is possible to have an STI/STD and not know it.
Even when an egg is fertilized by a sperm, it still needs to attach to the wall of the womb. Many women have miscarriages (lose the baby) within the first month of pregnancy and do not even realise they were pregnant.
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