הלכה על יבמות 155:23

Not to distance an Edomite of the third generation from coming into the congregation: Not to distance the seed of Esav after they convert - which is to say that we should not prevent ourselves from marrying with them, as there is no prohibition, like there is with Ammon and Moav; but rather, they are permissible after two generations. And about this is it stated (Deuteronomy 23:8-9), "You shall not despise an Edomite, since he is your brother, etc. Children that will be born to them in the third generation shall come into the Congregation of the Lord." And the third generations is the grandson of the convert. And they, may their memory be blessed, said (Yevamot 78a) about a pregnant Egyptian woman that converted while she is still pregnant, that her son is called the second [generation], even though his conception was while the mother was still an Egyptian.

From the laws of the commandment is that which they, may their memory be blessed, said (Yevamot 78a) [that if] an Egyptian marries [another] Egyptian and she converts while she is still pregnant, the offspring is called the second [generation], as it states, "Children that will be born to them, etc." - and the verse makes it dependent on the birth; and [that if] an Ammonite convert marries an Egyptian woman, the offspring is an Ammonite, [but in the case of] an Egyptian convert that married an Ammonite woman, the offspring is an Egyptian. This is the general principle: With the [gentile] peoples alone, it goes after the males, and like we find that Scripture relates them to the males, as it is stated (II Kings 20:12), Baladan the son of Baladan. [But if] they [both] convert, it goes according to he less [advantageous]. And the rest of the details of the commandment are elucidated in the eighth chapter of Yevamot and at the end of Kiddushin (see Tur, Even HaEzer 4).