In vitro fertilization no longer leads to catastrophically large fetal counts, or to the less known fused embryos that cause chimera and hermaphrodites. That's because the success rate per embryo has improved significantly so there is no longer need to implant a large number of embryos. Typical now is only two.
It turns out that most fertilized eggs do not survive,* so about-to-die embryos were inadvertently being implanted. Modern fertility clinics avoid this by growing IVF embryos in a petri dish for as long as possible, five days, and then microscope inspecting to discard the bad ones and pick out only the best for implantation.
And perhaps just as important to understand is that about half the time the reason for early die off is deficiency of eggs at the time of ovulation. The hard part to understand here is interpretation. Is there such a thing as a single unique time or event for the beginning of human life?
This web site attempts to avoid the interpretation problems and theology and just stick to observable facts and science. Start with the pull-down menu to "What Makes An Embryo".
*The statistics are for an average over all ages and health of couples.
About half of couples have difficulty achieving pregnancy easily.