If your period doesn’t arrive when expected (amenorrhoea) you may be pregnant. There are some other things that could cause you to miss your period, such as illness, stress, shock or strenuous exercise, but the most common cause, if you are sexually active, is pregnancy.
All of the changes that we have described happen because of pregnancy hormones. About seven days after the embryo is implanted in your uterus, the amount of the hormones progesterone and human chorionic gonadotrophin (HCG) greatly increase in your body.
Some women can be pregnant and still have a small amount of bleeding at the time their period is normally due during the first few months of their pregnancy.