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.
You did not mention to us the age of the fetus in the womb of his mother, because knowing this affects the ruling. When the fetus reaches four months, the soul is breathed into it, and in this case, it is forbidden to abort it without a dire need, according to the agreement of the jurists. At this stage, it is undoubtedly killing a soul.
As regards before the soul is breathed into it, there is a difference of opinion among the jurists about its ruling. The most preponderant opinion in our view is that it is not permissible to abort it even at the Nutfah (sperm-drop) stage. Although it is not a real killing in this case, yet it is still considered a transgression (against the fetus).
Moreover, you did not clarify to us your wife's health problem. If it is a serious matter that threatens the mother's life if the pregnancy continues, and specialized doctors testify to this, then it is permissible to abort the fetus even after the soul has been breathed into it. However, if the disease does not reach this level of seriousness, but pregnancy can cause harm to her, then there is nothing wrong in acting according to the concession of the scholars who allow abortion before the soul is breathed into it for a valid excuse.
Nonetheless, if her health problem is simple, then it is not permissible for you to abort it. This is the best course of action in order to be on the safe side and acquitted from sin.
It should be noted that a Muslim should not refrain from having abundant children because, in this case, he deprives himself from much goodness of this world and the Hereafter. Having an abundant number of children is something that Allah stated to be a favor from Him upon some of His Slaves as He Says (what means): {….And We helped you with wealth and children and made you more numerous in man-power.} [Quran 17:6]
Additionally, the Prophet urged us to have an abundant number of children. Ma'qil ibn Yasaar, may Allaah be pleased with him, narrated that a man came to the Prophet and said: "I found a woman of honourable lineage and who is beautiful, but she does not give birth to children (infertile). Should I marry her?" He said: 'No'. The man later returned again (asking about the same issue), and the Prophet again prohibited him (from marrying her). The man returned again for the third time, and the Prophet said: 'Marry women who are loving and very prolific, for I shall outnumber the nations (on the Day of Resurrection) by you (your abundant number).'"
For more benefit on abortion, please refer to Fataawa 278191, 276748 and 87649.
Allah Knows best.