[He Married a Woman and She Gave Birth After Five Months]

The answer—and Allah is the One who grants success: Here interests and harms (maṣāliḥ and mafāsid) are in conflict. In remaining silent there are interests and harms. Among the interests: concealing the wife’s matter—which the Lawgiver emphatically exhorted—maintaining and continuing the marriage between the spouses, blocking the door to discord and strife between the husband and the wife’s family, and concealing the newborn from being ascribed to other than his father.
As for the harms: they include bequeathing to one who is not an heir, and the boy—upon reaching maturity—entering upon unrelated women, along with other expected harms that come from children of fornication.
What appears stronger to me is: that the husband should be alerted, while finding a way to do so that does not expose or disgrace the woman or her family. Thus, the husband must be alerted, and the husband must conceal (the matter of) the woman.
We say this because sincere counsel (naṣīḥah) is obligatory for all Muslims in general and specific cases, and because the cohabitation of a believing, chaste man with a fornicatress is forbidden by the explicit text of the Qurʾān: "The [unmarried] male fornicator does not marry except a [unmarried] female fornicator or polytheist, and the [unmarried] female fornicator—none marries her except a [unmarried] male fornicator or polytheist; and that has been forbidden to the believers." [Al-Nūr:3].
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1