starting a family or starting a career

My boyfriend of 5 going on 6 years is beginning to talk about settling down and starting a family... btw we are 25... when  this topic is brought up I get really nervous because I will be finishing a post bachelors program soon and I feel like I wouldn't be able to balance the life of being a mother and truely pursuing my career. I don't want us to divide because of this issue... it is not as if I do not want a family but not so quickly in my life.