The Ruling on One Who Married Another to What Is in His Wife’s Womb, If It Is a Female]

Answer—and Allah grants success: The marriage is valid according to the madhhab, and the condition included in the contract does not invalidate it, because it is not in fact a future non-existent.
We say it is valid because the contract of marriage was concluded from the guardian to the husband in the presence of the witnesses over the guardian’s daughter, and she existed at the time of the contract; her being in her mother’s womb does not prevent the validity of the contract, due to the absence of a proof to that effect.
However, something may arise that prevents the validity of the contract—namely, if her father married her off for purposes that do not serve her interests; such a marriage would not be valid, because the guardian’s authority over a minor is confined to those dispositions that serve the minor’s interests—whether pertaining to her person or her property—and any other dispositions in which there is no benefit to the minor, neither to her person nor to her wealth, are not lawful or valid. Allah Most High said concerning guardianship over the orphan: “And they ask you concerning the orphans. Say, ‘Improvement for them is best.’” [al-Baqarah:220]
The father’s guardianship over his child and others’ guardianship are the same in this: for the guardian’s act to be valid, the benefit returning to the minor must be observed; otherwise, the act is invalid.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.1