it means that in the babies cells, the chromosome 15 is present in three copies instead of two. we all should have only two copies of each chromosome, one coming from mothers' egg and the other coming from fathers' sperm.
when a chromosome is present in three copies, it's called triploidy and the number 15 means that it is the chromosome 15 that has an extra copy.
what causes this are either errors in the division of the embryo at very very early stages (like 1-4 cells); but that is more rare as a case.
usually, it can either be due to an extra chromosome 15 present in your eggs or an extra chromosome 15 present in your hubby's sperm (each egg and each sperm should have only one copy of each chromosome) long before the conception occurs. This doesn't mean that ALL your eggs or ALL of his sperm have this extra chromosome - some do, some don't but you can't know that beforehand when you are trying. ie. your two living children were conceived with a healthy egg and a healthy sperm.
since you had multiple losses, the fact that they now found a T15 triploidy means that also your other losses could have been due to the same cause - meaning that it's something that's morel likely to happen also in the future pregnancies.
Since your previous angels weren't scanned for this condition, the doctors can't really tell if this was an isolated case or it is something that was undetected so far.
the thing is, the pregnancies with trisomies of larger chromosomes (like 15) shouldn't be viable and should be arrested in very early stages, often even before the implantation occurs and you ever get your BFP.
but in some people, due to how their eggs and sperms are made, they somehow manage to "compensate" for the extra chromosome or fail to detect its presence until the pregnancy has already progressed, and was viable - meaning viable enough to get detected, have a heartbeat and reach a certain developmental stage.
the genetic report on both your OH and you will explain you if the tendency to trisomy T15 is on your or his side - as it's either your eggs that have an extra chromosome 15, or it is his sperms that have it.
this should then give you some indication on how to proceed in future - in case they find either one of you has the tendency to trisomic gametic cells (eggs and sperms), you could try the IVF route as the embryos can then get tested beforehand for it, before being implanted. if the trisomy is on your hubby's side, if i'm not wrong, there is a possibility to go the IUI route, and select the sperms that don't contain the extra chromosome (as they have a different weight compared to those that have an extra copy - they are lighter as they carry less DNA).
if they find no tendency on both your and OH's side, then your previous losses that haven't been tested for T15 have probably been unrelated to it; yet this can't be proven for sure.
i am so sorry that you and your OH have to go through all of this, but i hope this diagnosis may shed come light at least on your conditions and give you some answers and some hopes for the future without further losses as well.
I know well that the new babies don't substitute the angels we've lost and don't heal the old wounds - people are not replaceable, nor are our angels. what i hope here is that this diagnosis and correct medical help might help you avoid the further losses and make your rainbow baby arrive safe in your arms as soon as possible.
i hope this helped, if you need to talk feel free to message me
how are you feeling 
