First, you do not press charges. You report the incident to the police and then the district attorney will press charges if there is evidence to support the crime.
Because the age difference is less than 4 years and the victim is over 13, the crime would be Sexual Assount, a misdemeanor
632-A:4 Sexual Assault. –
I. A person is guilty of a class A misdemeanor under any of the following circumstances:
(a) When the actor subjects another person who is 13 years of age or older to sexual contact under any of the circumstances named in RSA 632-A:2.
(b) When the actor subjects another person, other than the actor's legal spouse, who is 13 years of age or older and under 16 years of age to sexual contact where the age difference between the actor and the other person is 5 years or more.
(c) In the absence of any of the circumstances set forth in RSA 632-A:2, when the actor engages in sexual penetration with a person, other than the actor's legal spouse, who is 13 years of age or older and under 16 years of age where the age difference between the actor and the other person is 4 years or less.
RSA 632-A:2 refers to agravated sexual assault and you have not specified any details that would match that charge.
As for the time limit for prosecution:
508:4-g Actions Based on Sexual Assault and Related Offenses.
A person, alleging to have been subjected to any offense under RSA 632-A or an offense under RSA 639:2, who was under 18 years of age when the alleged offense occurred, may commence a personal action based on the incident within the later of:
I. Seven years of the person's eighteenth birthday; or
II. Three years of the time the plaintiff discovers, or in the exercise of reasonable diligence should have discovered, the injury and its causal relationship to the act or omission complained of.
Since the pregnancy would be a reasonable indicator of sexual activity, the limit is when the victim turns 25.
