The Gemara on Amud Aleph Mentions that we save a life, and even prolong a life, despite it being temporary, as the person is going to die anyhow.

Aside from the obvious moral lesson here, that every moment of life is sacred, there is an interesting lomdishe discussion found in the Gur Aryeh (Bereshis 4:24).  Rashi quotes a Midrash Tanchuma that after Lemech’s wives had a boy and a girl, fulfilling the commandment to be fruitful and multiply, they distanced themselves from him.  They figured, “If the sons of Cain are cursed anyhow and will not live long, why bother having more children?”

Gur Aryeh asks the obvious question: If Lemech’s wives’ logic was correct, why did they wait until after fulfilling the commandment of Peru Urevu? And, if you would argue that even moments of life are valuable, what gave them the right to make that argument after they fulfilled their procreative duties?

Gur Aryeh answers that the requirement to have children after having the minimum male and female offspring is a hedge and insurance policy that you will remain with righteous and healthy children.  The Talmud (Yevamos 62b) directs,that one should continue to have children and also continue to teach students, no matter how many you have already established because, as it states in Ecclesiastes (11:6):  Sow your seed in the morning, and don’t hold back your hand in the evening, since you don’t know which is going to succeed, the one or the other, or if both are equally good.

Gur Aryeh argues then, if so, once the commandment to have children is fulfilled, and there is a legitimate concern that additional children may not live well anyhow, the additional hedge commandment does not apply.  Thus, Lemech’s wives were justified in their argument.

A potential  halakhic outcome of this concept is, that it could be argued, in the situation of parents who have a recessive gene and many of their offspring will live shortened ill lives.  In such a case, where they already fulfilled the mitzvah of Peru Urevu, even if there are no other mental health considerations of the anguish (which of course there are!), this shows there may be no moral or ethical compunction to have additional children. The Talmud’s requirement to have more children beyond Peru Urevu would only seem to apply when there is a reasonable assumption of healthy quality of life.