sweetchecks I really hope you come back to this thread. I have read your other thread about your b/f hopping from his ex g/f to you and back again, and his ex g/f is now pg.
I don't know if you have other threads I have not read yet, but from what I have read on these 2 threads, your circumstances resonated with me so much.
5 years ago, I was with a man who left me for his ex g/f. Their relationship had lasted 3 years and had finished a few months before I met him.
It seemed to me that as soon as I was with him, she wanted him back. She was unable to have anymore DC, apart from the DC she had with her exDH not my XP.
In that way, we had different circs. I wanted him in my life, as I feel you want your b/f in your life. I felt there must be something wrong with me for him to keep returning to his ex g/f. I wondered what she had that I apparently didn't.
He upped and left with little or no warning 5 times for his ex g/f.
He eventually stayed with me.
It was a very hollow victory.
I wish I had kicked his sorry backside out of my life the first time he left. During the time he was with me, I would often think of his ex g/f and realise that she actually had the better life, without him in it. I wished I was in her position without XP more than I liked to admit.
I have never yet heard of or met a man, who repeatedly dumps his girlfriend for another woman, who was a good father. 
I may well be alone in my experience.
I truly understand that you want a happy family life, but if you stay with this man, you will look back one day and regret wasting any of your precious time with him.
If you think there's a possibility a baby might make him 'wake up' and be a good partner/father, please reconsider.
How do your parents feel about your pregnancy?
Have you told the father yet about the pregnancy?
What do you want to do?