It's common to have some light spotting in the early weeks of pregnancy when your next period would have been due. This is usually nothing to worry about, and it happens when the fertilised egg attaches itself to the lining of the womb. But you should always report it to your GP or midwife anyway.
However, if you have any heavy, fresh bleeding in any trimester, always contact your GP or midwife straight away.
As soon as any bleeding starts, pop a sanitary towel in your knickers to help your midwife assess how much blood you've lost.