First and foremost, I want to state that the crime of zina (illicit sexual relations) is proved by one of two means: the evidence of four reliable witnesses or the confession of the adulterer or the fornicator.
However, other things such as pregnancy and the like are merely signs or indications and are not qualified enough to serve as a legal proof. For instance, pregnancy does not necessarily denote fornication. Therefore, it should not be taken as evidence to prove the crime. The scholars state that the prescribed punishments (hudud) are to be fended off for the least occurrence of suspicion.
As far as DNA test is concerned, if it is proved by reliable physicians and experts that it certainty serves as a valid legal proof, then it is the role of Muslim scholars to decide how to deal with it.