All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
There is no objection for the woman to stop pregnancy momentarily even without necessity if two conditions are met:
Firstly, the consent of the husband; secondly, she should not use forbidden means for this operation. For more benefit, please refer to Fatwa 84436.
However, undergoing an operation to stop pregnancy permanently is not lawful unless pregnancy causes hardship to the woman beyond that of normal pregnancy and that this is resulting from a chronic disease that cannot be cured.
In your case, we believe that if this asthma cannot be cured and if trustworthy doctors attest to that, then there is no harm that this woman stops pregnancy permanently after giving birth to what she has now in her womb, as it is forbidden for her to abort it.
For more benefit on family planning, please refer to Fatawa 99184, 97251, 84436, 82154 and 87155.
For more benefit on permanent birth control, please refer to Fatawa 119261, 83837, 134895, 87304, 87155, 89857 and 84086.
Allah knows best.