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Progesterone is a female hormone and the principal progestational hormone that is made mainly by the corpus luteum in the ovary and by the placenta.
Progesterone prepares the lining (endometrium) of the uterus (the womb) to receive and sustain the fertilized egg and is vital to allow a pregnancy to remain successful.
Progesterone rises pre-menstrually, but continues to rise further after the period is missed. This is why pre-menstrual feelings can be confused with early pregnancy signs.
Most women don't get pregnancy signs until after their period is missed and the hormones increase to levels that their body is not used to.
A few women do get pregnancy signs earlier than five or six weeks of pregnancy, especially if they have had a baby before, because their body is more sensitive to the hormones and they know they 'feel different' to being pre-menstrual.