Tubal Ligation

I am 27 years old and pregnant with my second child. I have a soon to be 4 year old son and now I am expecting a precious little girl. When I was pregnant with my son everything seemed so easy it was like the perfect pregnancy. With this pregnancy it has been a trial to say the least. One I wouldn't wish on my worst enemy. Parts of me feel like since I have been blessed with one of each that maybe that's good enough for me to get a permanent type of birth control but then again I don't want to be selfish either being that I'm not married yet. Is there anyone who is experiencing this or even thought about getting their tubes tied after giving birth. I just want to make sure I'm doing the right thing and thinking straight because of the pregnancy. Should I wait until after I have the baby to make that decision? I don't want to make a mistake.