Midrash for Bekhorot 91:23

נתגיירה מעוברת נשתחררה מעוברת

IF ONE HAD CHILDREN ALREADY AND MARRIED A WOMAN WHO HAD NEVER GIVEN BIRTH PREVIOUSLY OR IF SHE BECAME A PROSELYTE<span class="x" onmousemove="('comment',' Together with her husband and she gave birth, her offspring is regarded as a first-born to be redeemed by the priest, since the opening of the womb was of Israel, after the parents came under the influence of the law of Israel, but not as a first-born in respect of inheritance, since the conception of the infant was not in holiness and it is not therefore eligible for inheritance.');">¹⁸</span>

Sifrei Devarim