Fertilizing just one egg from an IVF cycle is certainly technically possible, but would be preposterously expensive due to the low success rate per embryo. Even in natural conception in perfectly healthy women with no fertility problems, most fertilized eggs will never develop into a baby. Per-embryo success rates are actually higher with IVF, due to control over what day the embryo lands in the uterus, as well as quite a lot of control over the woman's hormone levels at the time of the transfer; but much of the success lies in fertilizing a large number of eggs, and then waiting to see which ones survive long enough to be transferred back into the woman. Even those that make it to that stage have less than a 50% chance of turning into a baby; and the ones that didn't last that long had no chance at all if they had been transferred earlier.
The cost of a cycle would be the same if only one egg were fertilized after whatever number were retrieved (since the same amount of drugs would be used, and the same procedures). Telling a couple they can only fertilize one would result in the need for around 10 times as many cycles, putting the cost of actually having a baby this way out of the reach of all but the wealthiest couples (since obviously no insurance company going to finance this approach).
Embryos at this stage cannot rationally be considered "babies", for the simple reasons that 1) very few would ever have become real babies under any circumstances, and 2) they are still at a stage where they can be split into two or more developing embryos, or can merge into one developing embryo (these processes can happen naturally or artificially, though artificial splitting is not currently legal).
Interesting. In my thoughts today, I had considered the point that one needs to (in IVF) usually "try" multiple fertilized eggs in order to achieve pregnancy, which is why I arrived at the suggestion to only try one egg at a time. I wasn't aware however, that in IVF, multiple eggs are inserted at one time. Is this true? Is this what you were saying? (I'm not entirely sure).
If that's the case, then I still don't see any problem with that, since they're being given a chance to develop in the uterus. My only concern would be that there would be "left over embryos". In other words, I would seek out a protocol where each egg is given a chance to fertilize (no matter how many there are) and the whole mixture of sperm and egg be simply dropped into the uterus, hoping some of the eggs were fertilized.
So, I would also ask: In common protocols, are more eggs fertilized than are implanted? Do you know?
As for whether or not a fertilized egg can reasonably be called a "baby", I've decided long ago it's not my place to decide that, as that starts the slippery slope down to abortion. I've reasoned that a nice cut off is a fertilized egg. Before that, there is clearly a difference between a non fertilized egg/sperm, and a fertilized egg, as the latter can, under optimum conditions, develop into an infant, but an egg or sperm never can. We can disagree on that I guess; I'm just clarifying why I believe what I believe.
So, it would seem there aren't any moral impediments for anyone who's pro life for IVF. The embryos are given just as much (and even more really, as you pointed out) a chance to develop as "natural" embryos.
Anyway, thanks for the original post, it was very informative.