mean they get the eggs out and the sperm and create embryo's. Most of the time very decent embryo's. So I feel like so much of the hard part is done. So when they implant it back in your uterus, why is there still such a chance it won't work.
Well, getting egg to meet sperm and create an embryo isn't really "half" the work. It's a tiny fraction of the work. There's a lot that needs to happen between the creation of an embryo and live birth of a baby.
You have to think about why most people are doing IVF in the first place. If you're not successfully getting pregnant through well-timed intercourse, there's a variety of things that could be wrong, some of which IVF can't fix. An older woman going through IVF is still going to face the issue of older eggs, for example; IVF doesn't solve the fact that egg or sperm may be chromosomally abnormal. Those embryos may develop for a time until the chromosome problems finally bring a halt to development.
IVF also introduces another level of uncertainty when you talk about handling and manipulating these tiny cells. Are they handling the embryos correctly? Is it possible that some minute bit of damage is introduced to embryos by handling them during the IVF process? I don't think we have the answers to that yet, though labs are getting better and better at their procedures.
There's also been some thought that all the drugs that women take during IVF make have some negative effect. I've read that some doctors recommend FET a month after retrieval to give those drugs a chance to leave your system.
There are so many details that go into conception that we are only beginning to learn about. Doctors can control a handful of those details, but lots of stuff is not really up to a doctor's control.
And it's the same during regular conception too -- a fertile young couple's chance of getting pregnant in any given month is somewhere in the 20-25 percent range. If IVF can boost your chances to the point where you're doing as well or better than as a fertile young couple, that's not bad.