Is selling an Umm Walad allowed?

Context of this hadith Sunan Ibn Majah 2517 >We used to sell our slave women and **the mothers of our children (Umahat Awaldina)** when the Prophet (ﷺ) was still living among us, and we did not see anything wrong with that. Why it’s say mother of our children i mean you can’t sell slave when she gets pregnant by master right?