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While a pre-natal DNA test is possible, she would have to consent to it. Without a DNA test, the DA would not know 100% if the baby is the father's or not, but they only need prima facie (on the face, at first glance) evidence to bring a criminal charge, so the simple suspicion may be enough.
Age of consent in TN is 18. There is a close age exception of a 13 year old who can consent to sex with someone who is less than 4 years older. However, if she was 15 and he was 20, then the close age exemption does not apply.
As such, if father impregnated mother, then by that very fact, he can be charged criminally. DHR will do an investigation on the alleged abuse of the female by the male.
If she turn 16, and can get married to the father (she would need her parents' consent), then it is much less likely that he would be criminally charged. There is really no other way to minimize his criminal culpability. He would technically still possibly be criminally liable, however, the DA is less likely to charge him if they are married.
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