For whatever cosmic reasons, specific souls are assigned to you. You are karmically responsible for giving those souls a physical body. (The only exception is when your life is endangered by the pregnancy.) But you may not be karmically responsible for raising a child. Adoption is a responsible alternative.
Often, when a pregnancy is unplanned, a woman may not be able to care for the child. Perhaps you're single, or you have a bad relationship with the father, or you're still in school and you don't want a child now, or your baby has physical or mental abnormalities that you can't deal with.
In fact, you may be responsible only for giving the soul a physical body and finding adoptive parents. The people who need to raise the child have a different karma with the soul. Their karma probably requires them to nurture the child and bring him or her to adulthood. That is why adoption is such a critical option to consider if you are faced with an unplanned pregnancy.
Once you understand reincarnation and karma, you don't need to feel guilty about placing your baby for adoption. In fact, you can feel proud that you had a responsibility to fulfill and you did it well.