I've tried to read up on it, but I find it all confusing. Here is what I think I know (please correct me!):
It can be mono-mono, mono-di, di-di and that has to do with one vs two zygotes and one vs two sacks
Dizygotic are definitely fraternal and monozygotic are definitely identical?
And how can they tell via u/s? Do you need an early u/s to determine it or will they be able to tell me at my 20 week u/s?
Anything else I should know?
I appreciate your knowledge!
Ellie, partner (12.2.08), mommy to E (7.5.10) & my Baby Belugas H & L (2.25.13)
Yes, dizygotic are fraternal and monozygotic are identical. But that is different than when we're using the terms di/di or mono/di, etc. The zygote is referring to the egg - di = there were two eggs, mono = there was one that split. The term di/di is short for dichorionic, diamniotic. It's referring to the placenta (chorion) and the sack (amnion): di/di = two sacks, two placentas.
All fraternal twins are di/di (although sometimes placentas fuse and look like they're mono/di). 20-30% of identicals are also di/di. A mono/mono is definitely identical, and a mono/di could be either, although I think a skilled tech can tell if the placentas are really single or two that fused, and if they're really single it's definitely identical.
Why do we bother with knowing di/di, mono/di or mono/mono? There are different risks associated with each of the three options. di/di twins are in the lowest risk category. mono/di twins are at risk for TTTS (twin-twin transfusion syndrome), and mono-mono twins are very high risk, since they share one sack they can get tangled in each other's umbilical cords, and they're also at risk for TTTS. di/di twins can still have discordant growth - it's not as serious as TTTS, but that's why they want to monitor your twin's growth with ultrasound regularly.
Yes, they'll be able to tell you at your 20 week u/s. I'm not sure if it's easier to tell if the placentas are fused or single earlier on or not.
Good luck at your u/s! keep us posted.
Work at home, homesteading mom sharing child care 50/50 with my wonderful WAH DH. DD1 born Jan. 2010. March '12. DD2 & DD3 (twins) born Feb. 2013
there is a chance that a later U/S like a 20 week one will not be able to see the membrane between the babies well any more and that things are too crowded to see the T or the "Lamba" where the innr and out sacks relate to eachother, the tell tell signs of separation that is the difference between di/di and Mo/di, so that is the reason that is usually better to find out earlier. and because finding out earlier lets you understand and be educated on the issues of each type.
So what if I don't fit cleanly into a defined parenting style, my kids don't fit into a personality archetype either!
This is a good breakdown of mono/di and all the terms!
FYI, it was during my NT scan which was at 13 weeks, I think, that they were able to fully determine chorionicity. The sacs have been clearly visible on each u/s so far; I go every three weeks for a quick check-in that is a low-res u/s to listen for heartbeats.