My boyfriend slept with his ex-girlfriend

You have obviously been pushed out of the picture and you should feel relieved. You should count your lucky stars. Now that that ex has gotten someone pregnant he might come back on all fours to beg forgiveness and ask to be taken back. Be strong and have nothing to do with him. You will probably find someone who is faithful and with whom you will want to be part of your life. 

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As you state: "You heard - claim on heard" that he slept with his ex, obvious that you have not discussed this with your boyfriend. Please do yourself the favour and make him sit down and demand the truth, if he did and confess then you have the choice to either leave him and stick with your decision or both of you try very hard and make it work even if she is pregnant does not mean that he loves the mother of his child,and I promise you if a guy ask for a second change he actually means it, and 100% sure he regret it the minute he realised what he was busy doing. Also remember you as woman knows when something is wrong, you know when your guy is cheating on you and when there is another woman involved - it is called INTUITION, always listen to that voice inside of you (its never wrong trust me - I know what I am talking about)