Ordinarily a child is placed in foster care only if (1) the minor's welfare/safety is in danger (being a pregnant, unmarried minor doesn't usually suffice, absent additional factors); (2) there are no reasonable adults who can take care of the minor; and/or (3) there are other circumstances that may reasonably justify foster care (e.g., until the parent(s) can get back on their feet financially, health-wise, maybe there are legal matters that have to be dealt with and the parent can't adequately care for the child, etc.). If you are living in a reasonable, clean, safe environment, you have plenty of food, good shelter, adequate clothing, your safety isn't at issue, your health needs are adequately taken care of by your mother, etc., you may be fine.
While you don't want to talk about it, you and your mother need to consider having your bf (if he is the father of your possible unborn child) put on child support and paternity established after the child is born (if you really are pregnant). Also, he may be able to be charged with one of a number of possible sex offenses because you are under the age of consent if the two of you have had any type of sexual contact (not even necessarily going "all the way").
I suggest that you and your mother speak with a family law attorney to confidentially discuss your situation, and they should hopefully be able to at least advise you as to a good course of action, even if nothing necessarily has to occur right now.