This is how I understood it as it was explained by my RE:
A Day 5 gives the blastocyst more time to grow and develop. Sometimes, if they're growing slowly or if you have past implantation issues, I think they tend to go the 5-day to see what happens. I didn't know this before I went through IVF, but the blastocyst actually "hatches" at day 4 or 5 or so, and starts coming out of its little insular shell to implant in the wall of the uterus. Sometimes eggs have problems hatching, so they wait until they see it start to to transfer.

For some reason (which I never could fully understand), if you have fewer than 4 good embryos, they tend to transfer at the cell stage (Day 3), and if you have more, they wait. Perhaps that's because having more means they can afford to wait and pick the very best growers and still have something to transfer at day 5? I'm not sure. We only had two, one A grade and one B grade blast. We had 2 Cs that fizzled out by Day 4, so they couldn't be frozen. The RE transferred the two good ones on day 3, and we had one implant.

I hope that helps!

My RE felt we had a better chance of it working if we got to Day 5. So I was put on a Day 3 to 5 flex. They would have only transferred on Day 3 if I had less than 3 embies, but would push to Day 5 if I had more than that. This cycle I had 8 on Day 3 so I was pushed to Day 5 and 5 became blastocysts. One was put in and the other 4 were frozen. The blastocyst they put in was Grade A and already hatching, they were happy to see that it was hatching. My clinic will not freeze unless they are blastocysts or early blastocysts. My understanding is blastocysts unfreeze better so you are less likely to lose them from unfreezing.

My RE felt really strongly about trying to get my to Day 5 since when I went on Day 3 I miscarried. This is not to say that Day 3's are bad because they are not and absolutely can work. This is just the protocol of my clinic. And one other thing, my RE does not want to put in more than one blastocyst as the possibility of it splitting and getting multiples is higher. Again every RE is different. Check with your RE on their protocols and what is best for you since you have already been through a cycle with your RE, he/she will know best. Best of luck for lots of embies!

A 5 day transfer has better odds, but the odds of the egg surviving to day 5 goes down. So they usually only do 5 day blastocysts if you have a bunch of eggs, otherwise your odds of pregnancy goes down.

Not all clinics do 5 day blasts, and they will also make decisions based on how things look with your eggs. Hope this helps!

PS the reason they don't all survive to day 5 is the blast actually needs nutrients by day 4. Before that, they can survive on their own. So docs made this synthetic gu which is supposed to mimic our uterus and provide nutrients, but it is not fool proof.

So it is riskier, but if you have a lot of eggs, it shows which eggs are the strongest.

Yep. Just what they said: the blastocysts are supposed to be the best quality. I cant remember exactly. Mine was done a while ago but I can tell you that my first cycle we transferred fresh and I believe 2 were blasts but still none took. Then a maybe 6 mths later we transferred the frozens which I think one was a blast and 2 stuck!!! Of course I miscarried but I was surprised the frozens took but the fresh didnt.

This has great info:
http://www.advancedfertility.com/blastocy.htm

At my clinic they will only push to a day 5 transfer if you have many embryos. I believe over 10 but I am not sure of the exact count. If not, they are afraid of taking the risk of having all the embryos die before day 5 and having nothing to transfer.

There are mixed feelings on what is best day 3 or 5, some clinics feel that it's better to put back the embies earlier so they will grow within the uterus, while others feel it's better to push to day 5 to see which ones are the strongest and have survived the longest.

My RE believes a bit differently than the others. Certainly, if an embie makes it to day 5, it's got to be strong. However, I have only done day 3 freezes and transfers, with my doctor's advice. He's experience and personal views are that no matter what- an embryo is safest in the womb and he believes that transferring on day 3 is best. This may not be popular belief but I believe it, too. I may be yelled at about this but my honest feelings are that some clinics that push 5 day or exclusively do 5 day may be benefiting by those who have no embies make it to day 5. Think about it- then that couple would want to do another IVF- more money for the clinic. I don't think this is common but at a business standpoint- this may occur on a small scale. When it comes down to it- they are your embryos- you should choose what you want to do with them. Me, I chose 3 day transfer and freezes. If I had a ton of eggs and only wanted one child, I would definitely go out to 5 days if my fertilization was very successful. I just think that I want my embryos to grow and develop inside of me and I didn't have any assist in hatching- so that wasn't a factor. Best of luck!!!

My clinic does day 3 vs day 5 depending on the number of embryos you have. I've never had more than 4 so we've always done 3 day transfers. I've gotten pregnant twice, with one birth. However, I think for freezing clinics may prefer day 5.

Day 5 = blastocyst which is what they embryo is once it arrives in your womb from your tubes. Day 3 it should technically still be in your tubes and not in the womb.

As courage said, my RE believes in they are safer inside. Some clinics that have more advanced labs and can do culture growth etc may push for day 5.

Well - my clinic want to inject every egg collected, icsi ( they are hope to get more that 15 eggs to play with so who knows how many embies we will get to blastocyst) then will then put them back in at day 4 ? ODD coz i keep reading about day 3 and 5