Watch her, if she gives birth to a white-complexioned child having straight hair and sore eyes, he is the son of Hilāl ibn Umayyah, but if she gives birth to a child with eyes that appear to have antimony in them, curly hair, and thin shanks, then he is the son of Sharīk ibn Sahmā'

This Hadīth indicates that the Companion Hilāl ibn Umayyah (may Allah be pleased with him) accused his wife of committing adultery with Sharīk ibn Sahmā', and she was pregnant. Hilāl wanted to deny his fatherhood of the child through Li‘ān. In this process, the husband and wife make testimonies, which they confirm with an oath and curse invoked on the one of them who is not saying the truth.

Then the Prophet (may Allah's peace and blessings be upon him) mentioned signs whereby the child would be identified to belong to the husband or the adulterer. He said that if the child was in a perfect form and with straight hair, then he would belong to the husband, owing to the genetic similarity between them. And if the child had very dark eye lashes and curly hair, then he was the son of the adulterer, Sharīk ibn Sahmā'.

The point of this Hadīth is that it is permissible to make Li‘ān against a pregnant wife.