All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
If this marriage fulfilled the conditions for the validity of the marriage contract – the most important of which are the bride's Wali (guardian) and witnesses – then it is a valid marriage. Please refer to fatwa 83629 about the conditions for the validity of the marriage contract.
The fact that this woman vomited on the wedding day does not prove that she was pregnant! The same can be said about the medical scans; they cannot be a decisive proof that she was pregnant before the marriage. So the basic principle is that she was not pregnant then.
As for the fetus, if she delivers it six months after the marriage contract is conducted and the consummation of marriage was possible, then it is attributed to her husband. If she delivers it before six months, it is not attributed to the husband; rather, it is attributed to the mother. It is impermissible to accuse her of fornication without evidence, because it is possible that the pregnancy was conceived due to sexual intercourse while she was sleeping, for instance. The Muslim's honor is protected by the Shariah, and it is impermissible to accuse a Muslim of committing fornication without valid evidence.
Scholars held different opinions regarding marrying a woman who is pregnant from fornication. The Hanafis and Shaafiʻis held that marriage to her is valid, and their view is strong and has authoritative weight; however, the illegitimate child is not attributed to the adulterous father.
For more benefit, please refer to fatwa 122443.
Allah knows best.