Of course a teenage pregnancy means that there has been serious violations of the Law of Chastity and there will be consequences depending on the situation, up to and including excommunication if the disciplinary council that looks into the matter deems that is the proper response.  
 
Regardless of the actions of a formal disciplinary council the membership of the Ward should still welcome the family with open arms. We all fall short of the glory of God and while this sin is a very serious sin, with only murder and blasphemy against the Holy Ghost begin greater transgressions. All can repent and receive the forgiveness of the Lord.

I assume you are referring to an out-of-wedlock pregnancy here. Having a child out of wedlock naturally brings many challenges and difficulties to those that place themselves in that situation. It affects many people. The individual(s) themselves, their families, the child, to name a few. The natural consequences of this can be drastic. It can affect your education and employment opportunities, you future relationships and most importantly, your standing with your Heavenly Father. 
 
Heavenly Father knows that many people will make poor choices and has provided means for those people to overcome them. The consequences of these choices are real but they are not insurmountable. Through the atonement of Jesus Christ, one can become completely clean and worthy again through faith and sincere repentance. The way is not always easy, but it is within our capacity. 
 
The Church provides support for those who find themselves in this situation. By counseling with the Bishop, an individual can begin the process of repentance and putting their life back together. The Church is not there to condemn or give out punishment. The natural consequences of the situation do this enough. Rather, the church is there to provide guidance, counseling, support and comfort. The whole purpose being to bring the person back into the loving arms of the Savior. 
 
The main program the church has for these kind of situations is LDS Family Services. You can visit their website at http://ItsAboutLove.org 
 
I recommend you look around that site. I think you will be hard pressed to find a better support network or a richer source of information and resources than this.