Twins are fascinating — and the question of whether they're fraternal or identical is one of the first things everyone asks when twins are announced. But beyond satisfying curiosity, the distinction matters for medical reasons too: it affects HLA matching for stem cell transplants, cord blood banking logistics, and what we understand about genetic inheritance. Here's the complete picture.
The Core Difference: How They Form
The fundamental difference between fraternal and identical twins comes down to a single moment at conception. Fraternal twins (dizygotic) are formed when two separate eggs are fertilised by two separate sperm during the same cycle. This results in two genetically distinct individuals who happen to share a womb. Fraternal twins can be the same sex or different sexes, and they are no more genetically similar to each other than any other pair of siblings — sharing on average around 50% of their DNA.
Identical twins (monozygotic) are formed when a single fertilised egg (zygote) splits into two, producing two embryos with identical (or near-identical) genetic material. They are always the same biological sex, and they share approximately 99.9% of their DNA.
💡 In the US, twins occur in approximately 1 in 30 births — a rate that has risen sharply over the past three decades, largely due to increased IVF use and maternal age at conception. Around one-third of twins are identical; two-thirds are fraternal.
The Genetics in Depth
Fraternal twins are genetically siblings, not clones. Each twin has a unique genome — a unique combination of chromosomes inherited from both parents. They may resemble each other closely or look quite different, and can have different blood types, eye colours, and disease risks.
Identical twins share the same genome at the point of the embryonic split — but as they develop, epigenetic differences accumulate. A 2021 study in Nature Genetics analysed the genomes of 381 pairs of identical twins and found that, on average, identical twins differ by about 5.2 early developmental mutations — confirming they are not perfectly genetically identical, but extraordinarily close. This distinction matters clinically: identical twins are not guaranteed to be perfect stem cell donors for each other, though they are the closest possible match outside of the individual themselves.
Placenta and Chorionicity: Why It Matters for Cord Blood
Beyond genetics, the type of twins affects placentation — and this has direct relevance for cord blood banking.
Fraternal twins always have two separate placentas (dichorionic-diamniotic, or DCDA). This means each twin has their own cord blood supply — two separate cord blood collections are possible at birth, one per baby.
Identical twins may share a placenta or have separate ones depending on when the embryo split: splitting at days 1–3 produces two placentas and two amniotic sacs (same logistics as fraternal, two collections possible); splitting at days 4–8 produces one shared placenta and two amniotic sacs (monochorionic-diamniotic — collection more complex, lower volumes per baby); splitting at days 8–12 produces one placenta and one amniotic sac (rarest, highest risk, requires careful planning).
A 2016 study in Twin research and Human Genetics examined cord blood collection volumes in twin pregnancies and found that monochorionic twins (shared placenta) produced significantly lower per-baby collection volumes than dichorionic twins — underscoring the importance of discussing banking logistics with your provider early in a twin pregnancy.
What Determines Whether Twins Are Fraternal or Identical?
Fraternal twins are influenced by genetics, maternal age, and fertility treatment: family history on the mother's side increases likelihood, older maternal age increases the chance of releasing multiple eggs, and IVF with multiple embryo transfer directly causes fraternal twinning.
Identical twins are largely random — a spontaneous split of the early embryo that occurs at a consistent rate of about 3–4 per 1,000 pregnancies worldwide, relatively unaffected by genetics, age, or fertility treatment. If you conceived via IVF with a single embryo transfer and are carrying twins, they are almost certainly identical.
HLA Matching and Stem Cell Transplants
One of the most clinically important aspects of twin genetics is HLA (human leukocyte antigen) matching — the compatibility system used for stem cell and organ transplants. Identical twins are, for all practical purposes, perfect HLA matches for each other. A stem cell transplant from an identical twin donor (a syngeneic transplant) carries virtually no risk of graft-versus-host disease — the closest thing medicine has to a perfect transplant.
Fraternal twins share around a 25% chance of being a full HLA match — the same probability as any two siblings. This distinction matters enormously for families banking cord blood: if you are expecting fraternal twins, banking both samples doubles your family's stem cell resource and maximises the likelihood that at least one baby's cord blood will be a match for the other — or for a future sibling.
Cord Blood Banking with Twins: What AlphaCord Parents Need to Know
If you are expecting twins — fraternal or identical — cord blood banking is absolutely worth discussing early. For dichorionic twin pregnancies, two separate cord blood collections are possible. AlphaCord can accommodate this — contact the team before your due date to ensure two kits are dispatched and your delivery team is briefed. If your identical twins share a placenta, discuss collection logistics with your OB in advance, as lower volumes are possible. Each baby's sample is stored separately under their own account — both available as individual resources for the family.
To understand the full science of what cord blood contains and what conditions it can treat, read our complete guide: What Are Stem Cells? A Complete Guide for Parents
Frequently Asked Questions
Can you tell if twins are identical or fraternal before birth? Yes, usually. Ultrasound can determine chorionicity most accurately in the first trimester. Dichorionic twins are almost always fraternal (though a small number of identical twins also split early enough to have separate placentas). DNA testing after birth gives definitive confirmation.
Do identical twins have the same blood type? Yes — identical twins originate from the same zygote, so they have the same blood type. Fraternal twins may or may not share a blood type, just like any two siblings.
Can fraternal twins be a stem cell match for each other? There is approximately a 25% chance that any two siblings — including fraternal twins — are a full HLA match, making sibling cord blood transplant possible. Banking both samples is the best way to preserve this option.
Are identical twins always the same sex? Almost always — since they originate from a single zygote with one sex chromosome combination. Very rarely, chromosomal events during the split can result in identical twins with different sexes, but this is exceptionally uncommon.
If I'm having twins via IVF, are they identical or fraternal? It depends on how many embryos were transferred. If two embryos were transferred and both implanted, the twins are fraternal. If one embryo was transferred and it split, the twins are identical. Your fertility clinic will confirm this at your early ultrasound.
Two Babies, Two Opportunities
If you're expecting twins, you have a rare and remarkable opportunity: two cord blood collections from a single pregnancy. Don't let either go to waste. AlphaCord's team will help you plan for both. Get in touch before your due date — and make sure your delivery team is briefed and ready.
