The thing that struck me about having twins – particularly high-risk twins, since that comes with so many more scans and doctors appointments – is that the whole thing is very, well, mammalian. There are different types of twins, characterised by different placentas and amniotic sacs, and there is a surprising amount of confusion among the non-twin-bearing public about how they work.

So I’ve written a quick guide, and include the usual disclaimer about how I’m not a medical professional, just someone who is surprised by the degree to which people are confused about the biology of twins who’s done a lot of googling.

First, there are fraternal twins. Fraternal twins occur once in about every 80 pregnancies, although the number is going up as maternal age rises and fertility treatments increase the incidence of multiples. Vanishing twin syndrome, in which one pregnancy isn’t viable and just sort of…fizzles….only happens with fraternals.

In fraternal pregnancies, there are two separate placentas with two separate amniotic sacs. They are concurrent pregnancies, but there are no links between the babies in utero; they just happen to be two babies born on the same day. Fraternal twins can be girl-girl, boy-girl or boy-boy, and while many pairs of siblings may look alike, they are no more genetically similar than any other pairs of siblings. There is a genetic component, however: a woman’s likelihood of becoming pregnant with fraternal twins is passed down the maternal line (i.e. mother to daughter), because it relies on a woman’s likelihood of releasing two eggs in any given cycle (though, as mentioned above, there are a number of other relevant factors as well).

The natural world is full of fraternal twins. Every animal that has a litter is, in essence, having fraternal twins.

Identical twins occur once in about every 400 pregnancies, but are (I suspect) less prevalent in the general population because there are some common complications that make fewer identical-twin pregnancies viable. Within the umbrella of ‘identical twin’ there are three separate types of pregnancies:

1. Dichorionic-dizygotic: two placentas, two amniotic sacs. On an ultrasound, this pregnancy will look exactly like a fraternal pregnancy; in fact, some people do not know until they have genetic testing whether they have identical or fraternal twins even after birth (not all identical twins look alike. Mine don’t!)
2. Monochorionic,-dizygotic: one placenta, two amniotic sacs. The placenta is divided into two sections, serving each twin individually, but the line can only be distinguished post-partum. This is the most common type of twin pregnancy and occurs in roughly 92% of identical twin pregnancies (though I have also seen reputable websites that claim its more like 60%. Since not all di-di twins will be confirmed as identical there is probably some wiggle room here).
   There are also blood vessels connecting the twins across the sacs; despite the roughly hojillion ultrasounds we had, I was never 100% clear on how that works. In twin-to-twin transfusion syndrome (TTTS), one of the more common and serious complications, the babies’ blood vessels are connected, and one twin receives more blood than the other (donor) twin. Selective intrauterine growth restriction, which is what my girls had, is a similar condition in which one twin has a larger allotment of placenta than the other. Most identical twins are different sizes at birth; sIUGR is diagnosed when one of the fetuses is below a certain percentile (I believe its 10th percentile but I couldn’t confirm that with a quick Google so I’m just going to go with it)
   TTTS affects about 15% of identical twin pregnancies and sIUGR about 10%. If you’re really unlucky, you can have both simultaneously; one does not preclude the other, though the differences between the two disorders are only subtle. Both are not possible in pregnancies without a shared placenta, so do not affect fraternal or di-di pregnancies.
3. Monochorionic-monozygotic: there is a single placenta and a single sac. This occurs in about 2% of twin pregnancies and is super high-risk. Best practice as of 2017 calls for women with mo-mo pregnancies to be put on bed rest at 25 weeks and deliver at 32. The reason is that there is a risk of the umbilical blood vessels getting tangled within the single sac, which can have really awful, tragic outcomes. Mo-mo pregnancies are also at an even higher risk for TTTS and sIUGR, although I’m not sure why.

The type of twin pregnancy is determined by when the egg splits: if it happens early, you get a di-di pregnancy; if it happens late, mo-mo. Everything in between is mono-di. What surprised me is the definition of ‘early’ vs ‘late’: any egg that splits more than 13 days after conception will result in conjoined twins. Eight-13 days = mono-mono and a split between five and eight days results in mono-di.

One note about identicals: in very rare cases, they can be different sexes if one of the babies has Turner Syndrome and the other does not. Turner Syndrome occurs in 1 in every 2,500 births, but its incidence in twins is extremely small – like, five documented cases ever. Still, I was floored to find out there is any instance in which identical twins can have different sexes.

You may think you don’t know enough identical twins for the 1/400 number to sound right; this is because (I think) there have been dramatic advances in maternal/fetal care for twins in the last decade or so, as well as major improvements in neonatology, which is a fast-moving field. In most cases, the treatment for complications is premature delivery, which on average has much better outcomes now than it did thirty years ago (which is not to say there aren’t lots of healthy humans who were born prematurely out in the world, though there is not great data about how they fare in old age).

When we told my parents we were having identical twins, they said ‘but how do you know?’ and the answer is, there was only one placenta, so it was definitely identical. There is no way to increase your chance of having identicals; it just happens sometimes.