Over the course of this pregnancy we’ve been given different guesses at whether our twins are going to be identical or fraternal. We thought it would be good to explain what each of these types of twins is and why it may be difficult to tell if our twins are identical or fraternal.

Identical twins start from a single egg and a single sperm and the fertilized egg splits into two early in the development process. Since they started from a single fertilized egg, they share all of the same genetic material. This means that they will be the same sex, have the same blood type, have the same hair color, etc.

Fraternal twins start from two separate eggs, which are fertilized by two separate sperm. Normally, only one egg is released during each cycle, but there are different factors that can make it more likely that two eggs will be produced. Fertility drugs, the mother’s age, or the appearance of other fraternal twins in the mother’s family are all factors which make fraternal twins more likely. Because there are two separate eggs and two separate sperm, fraternal twins are no more closely related than any other two siblings.

At this point in our pregnancy, there are no certain tests that can tell us whether our twins are identical or fraternal — there are simply signs that indicate which is more likely. The case for fraternal twins seemed to make sense at first. Agnes’ father has fraternal twin sisters, which was one of the factors that makes fraternal twins more likely. Fraternal twins are also more common, comprising about 2/3 of all twin births. Finally, we saw separate gestational sacs in the early ultrasound images. Fraternal twins always have separate gestational sacs. Identical twins only have separate sacs if they separated in the first 3-5 days after the egg was fertilized, which happens only 1/3 of the time. Overall, twins with separate gestational sacs are fraternal about 86% of the time.

On the other hand, the perinatologist did a detailed ultrasound and found other signs that the twins may be identical. The membrane separating the twins tends to be thinner with identical twins than with fraternal twins, and the junction where that membrane meets the uterine wall often has a characteristic shape when the twins are identical. Based on these reasons, he was guessing with about an 80% certainty that our twins would be identical.

We may not know for sure for years. If our twins are the same sex (and right now we believe that they are both girls), then there won’t be any clear sign at birth if they are identical or fraternal. They will both have the same blood type, O positive, because both Agnes and I have O positive blood and O is recessive. The fact that O is recessive means that neither of us carry a different gene that would cause any of our children to produce a different blood type.

We may be able to look for other characteristics that tell us if the twins are genetically different. For instance, we could check to see if they have attached or detached earlobes. Both of us have detached earlobes and detached earlobes are the dominant trait. We could both be carrying the recessive trait for attached earlobes, and if one twin had detached earlobes and the other’s were attached, then we would know that they are genetically different.

Ultimately, it probably doesn’t matter. It’s just to satisfy our own curiosity that we would want to know if they are identical or fraternal twins. Right now, if you ask us if they’re identical, we’ll have to hedge our bets and just say that we think they are.