All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
A pregnant woman is permitted to breastfeed her child if this breastfeeding does not cause her, her child, or the foetus she has in her womb any harm. This harm could be known by experience or by being informed about it by a trustworthy doctor. The Prophet said: "I was about to forbid Al-Gheelah (having intercourse with one's wife while she is breastfeeding), and then I remembered that the Romans and Persians did so, and it did not harm their children.” [Muslim]
The scholars differed in opinion when interpreting the meaning of Al-Gheelah in the above narration. Some interpreted it by a husband having sexual intercourse with his wife while she is breastfeeding. Some others interpreted it by a woman breastfeeding her child while pregnant. Anyway, in both cases, this is not forbidden if there is no real harm or when one is pretty sure that this does not cause any harm. For more benefit, please refer to Fatwa 87785.
As regards the narration which is mentioned in the question, it is reported by Ahmad and Abu Daawood and it is a weak narration.
Allaah Knows best.