However, some have attempted to justify this loss of human life by saying that even when conception occurs in the normal way through a sexual union, some human life can be lost. Embryos that are naturally conceived may fail to implant in the uterus, and the woman's cycle will continue normally without her even being aware that conception had occurred. In other cases, miscarriages can take place through no fault of the woman or anyone else. Why should the loss of life that occurs in the process of IVF be considered to be any different?
The answer, very simply put, is that the above examples occur within the natural order that God created. They are not usually attributable to something we've done.* What happens, then, is in God's hands. It is for Him to give and sustain life, or not, as He sees fit within the one flesh relationship of man and woman that He created. It's His decision, His doing.
IVF on the other hand makes the loss of life our decision and our doing. It's something we've willfully decided to pursue. That makes us responsible and accountable for the loss of life that occurs as a result. In IVF the process both of conception and of implantation is not natural but artificial; it goes well beyond medicine's proper role of healing and helping the body to function properly. It is outside the natural order that God created. Therefore the loss of life is on us.
Especially from a Christian perspective, this is a matter we can't ignore. For at the heart of the Christian faith is the reality that our Lord Jesus went through all the stages of human existence and made them holy. The Son of God Himself was once an embryo. Therefore, every embryo is of the greatest worth, not only because human life is created in the image of God, but also and especially because God Himself shared fully in our humanity from the very moment of conception. To say that some embryonic human life can be manipulated and sacrificed in this way is ultimately to undermine the truth of the incarnation of our Lord and the very Gospel itself.
At the end of the day, every justification of the process of IVF is just that--a justification, a rationalization. Christians who are facing issues of infertility should consider this honestly as they seek to follow and do God's will.
*Although see here for how hormonal contraceptives and IUDs can work against implantation of an embryo in the uterus