There are two main types of twins. Fraternal twins come from two eggs fertilized by two sperm. Identical twins come from one fertilized egg that splits in two. Everything else you will hear is either a second way of sorting the same babies or a rare exception. The second sorting is by placentas and sacs: di-di, mono-di and mono-mono. That one is read off an ultrasound, and it is the one that sets your appointment calendar. The rare types, such as conjoined and semi-identical twins, are real, and you are very unlikely to meet them.
One frame first. This is a vocabulary page, not medical advice, and nothing here says anything about your own pregnancy. Your OB or maternal-fetal medicine (MFM) team are the source of truth for that.
The two questions people mix up
“What type of twins are they?” is really two separate questions, and they have separate answers.
| Question | The word for it | Possible answers | How you find out |
|---|---|---|---|
| How many eggs? | Zygosity | Fraternal (two) or identical (one) | Sometimes obvious, otherwise a DNA test after birth |
| How many placentas and sacs? | Chorionicity | Di-di, mono-di, mono-mono | Ultrasound, during pregnancy |
Most confusion about twin types comes from answering one question with the other’s vocabulary. “Di-di” is not a synonym for fraternal. “One placenta on the scan” is not a synonym for identical.
Fraternal twins: two eggs, two sperm
The National Human Genome Research Institute defines fraternal twins, also called dizygotic twins, as the result of “the fertilization of two separate eggs with two different sperm during the same pregnancy” (genome.gov). Three things follow from that:
- They are siblings who share a birthday. In the institute’s words, they “share half their genomes, just like any other siblings.”
- They can be any combination of sexes. Two boys, two girls, or one of each. Mine are a boy and a girl, so the question was settled at the anatomy scan.
- They may look alike or nothing alike. The same as any brother and sister.
Fraternal twins are the more common kind. Cleveland Clinic’s explainer, which interviews an OB/GYN who is a twin himself, says that “fraternal twins are more common than identical twins” and that fraternal twinning tends to run in families while identical twinning does not (Cleveland Clinic).
Each fraternal twin builds their own placenta. Those two placentas can grow into each other and look like one on a scan, which is one reason a placenta count is not a reliable way to tell the types apart.
Identical twins: one egg that splits
Identical twins, or monozygotic twins, “result from the fertilization of a single egg by a single sperm, with the fertilized egg then splitting into two” (genome.gov). They share the same genome and are nearly always the same sex. Nobody knows what makes the egg split; Cleveland Clinic says so plainly.
Cleveland Clinic puts identical twins at “about 3 to 5 out of every 1,000 births”. They are also less identical than the name suggests. Small differences can show up from birth, such as a birthmark or a size difference, and more arrive with age. If you want the exceptions to the same-sex rule, they are covered in can identical twins be different genders.
Mirror twins are not a separate type. They are identical twins in whom some features turn up on opposite sides: a birthmark on the left for one and the right for the other. There is no test for it, and the figures quoted for how common it is are shaky. The evidence is laid out in mirror twins explained.
The other sorting: placentas and sacs
This is the sorting your care team uses, because it is the one visible on ultrasound and the one that decides how closely a twin pregnancy is watched. The three labels count two things: chorions (placentas) and amnions (sacs).
| Label | Placentas | Sacs | Which twins |
|---|---|---|---|
| Di-di | Two | Two | All fraternal twins, and some identical twins |
| Mono-di | One, shared | Two | Almost always identical |
| Mono-mono | One, shared | One, shared | Identical |
Two lines per type is all this page will give, because each has its own post:
- Di-di. Two placentas, two sacs, and the most common setup. What it means for your scans is in di-di twins explained.
- Mono-di. One shared placenta, a sac each, and a closer scan schedule. The reasons are in mono-di twins explained.
- Mono-mono. One placenta and one sac, rare, and specialist care from the day it is identified.
How do identical twins end up in different rows? Cleveland Clinic’s explanation is timing. If the egg splits early, “around three or four days after fertilization”, the twins make two separate placentas. If it splits later, they may share one.
Why you often cannot tell which type you have
Put the two sortings together and you get the situation many twin parents are actually in:
- Boy and girl: fraternal. Done.
- One shared placenta, confirmed on an early scan: almost certainly identical.
- Same sex, two placentas: could be either. Di-di twins of the same sex may be fraternal, or identical twins whose egg split early.
For that third group, looks will not settle it and neither will the delivery-room placenta count. Cleveland Clinic’s OB is direct about it: “The only way you know for sure whether twins are identical or fraternal is through DNA tests after delivery.” If you are in that group, it is a fair thing to ask your pediatrician about once the babies are home. Knowing is optional. Plenty of families never test.
The rare types
These are the ones that make the news. Cleveland Clinic describes them as “isolated cases”, and the counts it gives show how isolated.
| Type | What it means | How rare, per Cleveland Clinic |
|---|---|---|
| Conjoined twins | One egg begins to split and does not fully separate | Estimated once in every 50,000 pregnancies |
| Parasitic twin | One twin develops fully, the other does not | Fewer than 1 in 1 million births |
| Superfetation | A second egg is released and fertilized during an existing pregnancy | About 10 confirmed cases |
| Twins with different fathers | Two eggs fertilized by sperm from two partners | Described as possible, no count given |
| One egg, two sperm | Twins who share all of their mother’s DNA and part of their father’s | Two cases on record |
The names for that last row are a mess. Cleveland Clinic calls the two recorded cases “half-identical” and keeps “semi-identical” for a different, purely theoretical idea called polar body twinning, for which it says no case has been identified. Other sources call the recorded cases semi-identical, or sesquizygotic. If you read about any of them, check which mechanism the article means.
None of these is something to look for on your own scan. If one were ever in question, a specialist team would be the one raising it.
FAQ: types of twins
What are the three types of twins?
People usually mean one of two lists. By eggs there are two: fraternal and identical. By placentas and sacs there are three: di-di, mono-di and mono-mono. Some lists add the rare types as a third group.
Are di-di twins always fraternal?
No. All fraternal twins are di-di, but some identical twins are di-di too, when the egg splits early enough for each to build a placenta.
Which type of twins is most common?
Fraternal. Cleveland Clinic states that fraternal twins are more common than identical twins, and that the identical rate stays much the same around the world while the fraternal rate varies.
Can a scan tell me if my twins are identical?
Sometimes. A boy/girl pair is fraternal, and a single shared placenta points strongly to identical. For same-sex twins with two placentas, a scan cannot say, and a DNA test after birth is the way to know.
