Suppose a 17-year-old girl is with a 20-year-old guy and she got pregnant. Can her mom put him in jail?
That depends entirely on where you live. In the United States, age of consent laws (the laws that govern how old you can be to lawfully engage in sex) are set by each state. In other places, the age of consent is nationwide.
To find out the age of consent in your area, check out this table at AVERT.org. Bear in mind laws change periodically, and this list is not a real substitute for knowing the laws in your area.
If you are not legally old enough to consent to sexual activity, your partner can be imprisoned for committing statutory rape. That's not just jail, that's prison. There is a difference between the two: Jails in the US are operated at the municipal and county level and are for incarcerating people convicted of major misdemeanors and minor felonies. Prisons are for state and federal felons. If he somehow manages to avoid jail or prison, there is still a good chance he could be put on a sex offender registry. If you ever wondered how bad that is, try looking through the real estate ads and find a house that wasn't more than 2000 feet from a park or school. I guarantee you it ain't easy. Have you ever looked at a sex offender registry? You can check out the Dru Sjodin National Sex Offender Public Registry, named after Dru Sjodin, a college student who was abducted, raped and murdered by a paroled repeat violent sex offender. You can type in your zip code (if you're an American, anyway) and see if there are sex offenders living in your area.
Can your mom press charges? Sometimes your mom doesn't even need to press charges. If the law enforcement authorities believe you were abused or raped (not just statutory rape, but violent rape), they can press charges without consulting you or your family. But if that's not the case and sex is prohibited between 17-year-olds and adults in your area, then your mother could press charges (not that this is a good idea, since it helps to have a father around to help raise the baby in question).
Penalties for violating age of consent laws will vary by region. Some place are very strict about the age, and others will offer some leeway for couples who are close in age, even if one is underage. You just have to look it up online or in a law book, or you can ask a friendly police officer in your area.