December 28, 2014

So You're Having Twins - Here's (Some of) What You Need To Know

In case you got excited, I will not be thrilling you with my knowledge of successfully raising twins today (please, if you do know how to do this, get in touch with me.) Instead I want to talk about types of twin pregnancies. 

My twins are Monochorionic Diamniotic aka Identical. It is generally considered to be of medium risk. As grateful as I was to not be considered "high" risk, I quickly became aware of the potential problems and protocol for a pregnancy of my kind. I hated it at first and to this day wonder what would have happened if I had only... (read more on that here)  but I do know that I was lucky to have been in the care of people who knew what was best for this type of pregnancy, as I myself certainly did not. And I'm glad I live in a place where people advocated for the safety of my babies rather than providing me with a false sense of security via uneducated guesses as so often happens with twins.

When we came home after finding out about the twins the only thing I (thought) I knew about twins in general was that there were two types: Identical and Fraternal. One type was in the same sac (identical) and the other were in separate sacs. Our original ultrasound tech said there were two sacs but didn't elaborate much more. I was ecstatic that there could possibly be a boy and a girl. Upon leaving Dr. Margono's that day I was shocked to learn that it wasn't as simple as that.

There are a few things to consider when figuring out what type of multiple pregnancy you have.

Zygosity refers to how many eggs were fertilized and implanted prior to any splitting that might occur.

Chorionicity refers to the number of placentas that attach - either one or two (or more). Amniocity refers to how many amniotic sacs there are.

The combination of these things determines your type of pregnancy although they may not all be evident at any given time. Placentas can fuse, dividing membranes between amniotic sacs can appear invisible and eggs can split before implantation. Our bodies are weird, why would multiple pregnancy be any different?

Here's a Breakdown of the Basic Types of Twin Pregnancy:

Dichorionic-Diamniotic Fraternal Twins (DiDi or DCDA)

The most common type of twins, they are no more related than ordinary siblings. They are created when the mother hyperovulates, or drops two eggs that are each fertilized by their own separate sperm. This is a trait carried and passed on only by the mother's father, so even if twins run in your husbands side that has no bearing on whether or not you will have twins, but your daughter might be predisposed to hyperovulation. 

They always have separate placentas (the chorionic sac) and always have separate sacs (the amnion). In rare cases the placentas may fuse shortly after implantation giving the appearance of one but an ultrasound can determine true chorionicity based on the size of the dividing membrane (thick and very clearly there) or it can be determined at birth.

Usually the Dr. can tell by the membrane thickness if it the babies share a chorionic sac or not. If it is not obvious with an ultrasound it should be upon delivery (there is an obvious "horn" or seam where the placentas fused) or you may have a DNA test performed. These twins can be conceived not only on different days but can even have different fathers or be different races entirely. It all depends on when the woman ovulates and the eggs are fertilized.

DiDi Fraternal twins are the ONLY twins that can be a healthy boy and girl (although frat girl/girl are 2nd most common followed by frat boy/boy) and they are never identical.

Dichorionic-Diamniotic Identical Twins

So there's this widely accepted misconception that di-di = fraternal. It does not. In fact, 30% of Di-Di pregnancies are actually identical twins. I know, I just blew your mind!  Turns out, if the eggs splits super duper early (during days 1-3 after fertilization) then you end up with DiDi Indenticals.

Since they will be the same sex usually a blood or DNA test is required to know for sure whether they are identical or fraternal. The children are identical twins although the pregnancy will likely not be cared for in the way that most identicals are as it is not as high risk since each baby has its own placenta. 

So many people have DiDi Identicals and don't even know it. I have my own speculation that this is the real case of the so-called fraternal twins, Mary-Kate and Ashley Olsen.

Monochorionic Diamniotic (MonoDi/MoDi or MCDA)

MoDi twins occur when the fertilized egg splits early, about 4-6 days after fertilization into two separate embryos that then implant on the uterus with a shared placenta.  They will have the same basic DNA and look very much alike although they rarely are "identical". 

Identical does NOT mean carbon copy. Your babies may look very different depending on weight discordance, placental sharing and positioning in the womb. The most common difference I have found is that one has a rounder, larger head and one has a more oblong, thinner head. I think this has something to do with the fact that Baby B is often stuck up under the ribs!

MoDi twins share a placenta (chorionic sac) but they are each in their own inner amniotic sac divided by a thin membrane. This is one of the most confusing type of twins to explain to others as most people only ever hear of sharing an amniotic sac. People think that determines whether the babies are identical or not when it really comes down to the chorionicity.  The doctor will look for the membrane separating the babies as well as the "horn" found when a placenta fuses to determine if they might be fraternal or identical.

MoDi pregnancies are usually treated as moderate risk as they may suffer from Twin to Twin Transfusion Syndrome (TTTS), Twin Anemia Polycythemia Sequence (TAPS), Selective Intra-Uterine Growth Restriction (SIUGR) and preterm placental degradation among other things. They should be monitored at least bi-weekly until week 30 and then weekly and should not be allowed to progress past 36 weeks (+/- 6 days) without daily monitoring due to the risks. Things can go very wrong very quickly and not only lead to a traumatic birthing experience but fetal demise as well.

As far as science knows identical twins DO NOT run in families - so it doesn't matter how many twins you or your husband have in your family. It is completely up to chance whether your egg will split.Although many modi dads will joke that their super sperm did it. Who knows? Thanks to this, identical twins are often called, affectionately of course, freaks of nature.

Monochorionic Monoamniotic (MonoMono/MoMo or MCMA)

MoMo twins occur when the egg splits later, usually 6-8 days after fertilization. These twins are separate but share everything and are always identical (again, mostly in DNA not just looks). They occur in less than 5% of twins pregnancies and are very high risk. Often Mom is on bedrest from as early as 20 weeks and usually will deliver by 34 weeks at the latest. This is because of the risk of the babies being able to touch each other, as well as proximity to each others' umbilical cord. This pregnancy is heavily monitored and may be induced around the 32nd week. Although it is very risky and requires special care that famous image of the twins born holding hands shows the beauty of a MoMo pregnancy.

Boy-Girl Identical Twins

This is a phenomenon that I simply have to address due to the fact that there is SO much confusion out there. There cannot, EVER, be a pair of HEALTHY boy girl identical twins (unless of course one of the pair has undergone gender reassignment surgery). 

There can be twins with a chromosomal abnormality in which one of the boy twins from a split egg develops as a girl when the body drops a chromosome. This phenomenon known as Turner's Syndrome is EXTREMELY rare, although plenty of twin parents will make sure you know it can occur.

It gets even more complicated when you throw in semi-identical, mirror, conjoined twins, there might even be a few that I don't know about, but then it's likely you won't either!

So, do twins really run in your family? What common misconceptions have you heard about twins?


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