Assuming you didn't have sex with her before she was 16 you are legal, the age of consent in PA being 16. As you are 18 you are not required to live in your parent's house and follow their rules. Of course if you choose to stay there you will have to arrive at some kind of understanding with them. As father of the child to be you do have rights that are enforceable in Family Court.
Since you were a minor at the time that you had sex with your girlfriend I don't see anyway that you can be charged with a crime.
In Michigan, (where I work), you can be prosecuted as an adult at the age of 17; however, in Michigan your girlfriend would have the ability to legally consent to have sex at the age of 16. With that said I would add that our local prosecutor has on a regular and consistent basis charged by way of petition juvenile males who have had otherwise consenting sex with juvenile underage girls; (even though it would readily appear that the consenting underage girl is equally breaking the law). Since I am not aware of the legal age of consent in PA or the age at which a person can be charged as an adult I can't answer your question with more specificity. Additionally, I would point out that even if you are a juvenile for purposes of PA's law many jurisidictions have the ability to take otherwise juveniles into adult court depending on the severity of the offense and/or the prior criminal record of the particular juvenile offender. Finally, you need to tread lightly; in the final analysis your girlfriend is still a minor living with, and subject to her parents rules. The only way that that situation is going to change, or you could possibly have her move in with you is if she gets a lawyer and seeks legal emancipation; in otherwords, a judicial declaration that she can now live her life on her terms. This is a decision to be made by a family court judge. Also, whether her parents can force her to have an abortion is problematic; if she doesn't want one she probably isn't going to have one. However, you actually pretty much have no rights as the father. She can do what she wants as far as abortion is concerned. You are not able to prohibit that from occurring. If and when the child is born you can get an attorney and try to get your parental rights established which would result in establishing custody, visitation, child support, etc. The simplest thing that you can do at the moment is follow their rules since you and she do not really have a choice. Once the child is born go to court to get custody, parenting issues resolved.
Provided that it is consensual, the other person was not unconscious and not mentally disabled and she was not drugged to where she was not aware of what was going on, the answer is no for the past conduct. In the state of Pennsylvania, the charge of statutory sexual assault is defined as follows:
Except as provided in section 3121 (relating to rape), a person commits a felony of the second degree when that person engages in sexual intercourse with a complainant under the age of 16 years and that person is four or more years older than the complainant and the complainant and the person are not married to each other.
The only other possible charge for future conduct is Corruption of Minors which is as follows:
Whoever, being of the age of 18 years and upwards, by any act corrupts or tends to corrupt the morals of any minor less than 18 years of age, or who aids, abets, entices or encourages any such minor in the commission of any crime, or who knowingly assists or encourages such minor in violating his or her parole or any order of court, commits a misdemeanor of the first degree.
Finally, there is also interference with custody
A person commits an offense if he knowingly or recklessly takes or entices any child under the age of 18 years from the custody of its parent, guardian or other lawful custodian, when he has no privilege to do so.
(b) Defenses.--It is a defense that:
(1) the actor believed that his action was necessary to preserve the child from danger to its welfare; or
(2) the child, being at the time not less than 14 years old, was taken away at its own instigation without enticement and without purpose to commit a criminal offense with or against the child; or
(3) the actor is the child's parent or guardian or other lawful custodian and is not acting contrary to an order entered by a court of competent jurisdiction.
I cannot tell you more and advise you properly without more information. Some rural counties are more adamant in prosecuting such a series of events. Other more urbane ones are not.
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