How do twins occur?
There are a couple different ways that twins can occur in pregnancy. The type of twins is determined by how conception happens and/or how the embryo (or zygote) responds soon after.
Identical Twins
The first is where, after conception, the zygote splits into two. This type is commonly called identical twins (or maternal twins) and is medically classified as monozygotic. This means the twins have the same genetic material because they came from the same sperm and egg. Interestingly, Dr. Nathan Fox says, "[the] event where one embryo splits into two, it's the same across all populations, the likelihood of it happening. It's not affected by family history, it's not affected by age. It's a completely random event, which is really interesting just from a scientific standpoint that it happens in the same frequency."
Non-Identical Twins
The second type of twin happens when the mother ovulates two eggs instead of one in a given month. These two eggs are fertilized by two sperm, which is why these types of twins are typically called fraternal twins (or dizygotic). Of non-identical twins, Dr. Jen Lam-Rachlin, a board-certified Obstetrician/Gynecologist and maternal-fetal medicine specialist, says "they act more like siblings, only obviously they're the same age, the same conception date."
How do fertility treatments affect the chances of twins?
Fertility treatments can significantly increase the chances of twins occurring. Dr. Fox says, "in nature, it's about 1 in 300, but with IVF, it could be 1 in 30 or 1 in 40. It could be 10 times higher. So, it's not that it commonly happens, but definitely much more frequent in IVF pregnancies." This is because, for IVF to be more effective, sometimes multiple embryos are placed within the uterus rather than a single one. Dr. Lam-Rachlin says that twins are also more likely for women who undergoing ovulation induction therapy"…when they go through hormone treatments, sometimes they ovulate more than one egg. Let's say, for instance, they ovulated twice and there's two eggs that get released and both of those eggs can get fertilized by different sperms and then resulting in twin pregnancies."
Placenta, sacs, and configuration inside the uterus
Most people classify twins based on their genetic makeup, or identical and non-identical twins. But, Dr. Lam-Rachlin says, "[in] our field, it's less important to know identical versus fraternal, more important to know how many placentas, how many sacs." Twins can be considered dichorionic diamniotic, meaning each twin has its own sac and placenta. This means they aren't in competition with each other for nutrition or blood supply. These configurations don't impact whether your twins are identical or non-identical, Dr. Lam-Rachlin says. "I think the most common misconception is, you know, when someone's told that they have di-di twins or two separate placentas, two separate sacs, that means that they're fraternal and that's not always the case. They could still be identical. But from a monitoring perspective, it doesn't really change how we do the monitoring."
Monochorionic twins, however, share the same placenta. They may be diamniotic where they still have separate sacs, or they may be monoamniotic where they share the same sac. Monochorionic twins occur from early splitting of one zygote/embryo therefore they are by definition identical twins. There are higher complications due to the sharing of placenta and/or amniotic sacs therefore monochorionic twin pregnancies are commonly co-managed with a maternal-fetal medicine specialist.
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