Down syndrome and twin births—two things people don’t always connect. But when they do, a flood of questions pops up.
- Can twins both have Down syndrome?
- Can one twin have it and the other not?
- Is it more common or less common in twins?
- How does it affect pregnancy and life after birth?
Here’s what we know (and what we don’t).
How Common Is Down Syndrome In Twins?
For single pregnancies, the chance of a baby being born with Down syndrome is about 1 in 700. For twins, it drops to around 1 in 1,000 twin pregnancies involving at least one baby with Down syndrome.
It’s less common in twins than in single pregnancies. Weird, right? Scientists aren’t totally sure why.
Why Is Down Syndrome Rarer In Twins?
A few theories:
- More miscarriages – Some researchers think pregnancies with a twin who has Down syndrome are more likely to end early, often before the parents even know.
- Egg division differences – The way embryos split in twin pregnancies might lower the odds of the extra chromosome 21 sticking around.
- Something else we don’t know yet – Science is still catching up.
Identical vs. Fraternal Twins: Who Gets It?
The biggest factor? Whether the twins are identical or fraternal.
Identical Twins (Monozygotic)
- Start from one fertilized egg that splits into two.
- Same DNA, so if one has Down syndrome, both will.
- This is rare but does happen.
Fraternal Twins (Dizygotic)
- Two separate eggs, two separate sperm.
- Genetically no different than regular siblings.
- One twin can have Down syndrome while the other does not.
Think of it like flipping two different coins. One might land on heads, the other on tails.
How Down Syndrome Happens In Twins
Down syndrome comes from an extra chromosome 21. Usually, people have 46 chromosomes (23 from each parent). In Down syndrome, there’s an extra copy of chromosome 21, making it 47 total.
Three Types Of Down Syndrome:
- Trisomy 21 (95% of cases)
- Every cell in the body has three copies of chromosome 21.
- If twins are identical, both will have it.
- If twins are fraternal, one could have it while the other does not.
- Mosaic Down Syndrome (1% of cases, super rare in twins)
- Only some cells have the extra chromosome.
- Can happen in one twin and not the other, even if they’re identical.
- Translocation Down Syndrome (4% of cases)
- A piece of chromosome 21 attaches to another chromosome.
- Can be passed down from a parent.
- Happens at the same rate in single and twin pregnancies.
So, if you’re wondering why one twin has Down syndrome and the other doesn’t, it’s usually because they’re fraternal. If they’re identical, it’s either Trisomy 21 or an extremely rare mosaic case.
Pregnancy With Twins & Down Syndrome
Having twins is already intense. Add Down syndrome into the mix, and it’s a whole new level of monitoring, doctor visits, and planning.
Higher Risks In Twin Pregnancies
- Preterm birth – Most twins are born early. Babies with Down syndrome also tend to be born early. That means doctors are extra cautious.
- Heart defects – About 50% of babies with Down syndrome have heart issues. Twins already face a higher risk of congenital heart problems, so it’s double the reason for close monitoring.
- Growth differences – One twin might grow slower than the other, especially if they have Down syndrome. Doctors call this growth discordance.
More Tests, More Monitoring
Doctors watch twin pregnancies more closely than single ones. If Down syndrome is suspected, parents usually go through extra screening and diagnostic tests:
- Blood Tests – Check hormone levels to see if there’s an increased risk.
- Ultrasounds – Can show markers for Down syndrome (like a thickened neck fold).
- **Chorionic Villus Sampling (CVS) – Early test, done in the first trimester.
- Amniocentesis – More accurate, done in the second trimester.
The tricky part? Testing twins is harder. A blood test might say “high risk”, but it doesn’t tell you which twin has it (or if both do). That’s why ultrasounds and diagnostic tests are extra important.
Can One Twin Have Down Syndrome While The Other Doesn’t?
Short answer: Yes—if they’re fraternal twins.
If they’re identical, the odds of only one having it are almost zero (unless it’s an extremely rare mosaic case).
Parents of fraternal twins often say they get a lot of questions from strangers:
- “Are they both the same?”
- “How does one have Down syndrome but the other doesn’t?”
- “Are you sure they’re twins?”
Yes, yes, and yes. It’s just like how one twin can have blue eyes and the other brown—their genetics are different.
Real-Life Twin Stories
Nothing makes this more real than hearing from parents who’ve been through it.
The Identical Twin Surprise
A mom in Texas had identical twin boys. Everything looked normal on ultrasounds. Then, at birth, doctors realized both babies had Down syndrome. She had no clue during pregnancy.
Her reaction? “Shocked at first. Now? Wouldn’t change a thing.”
The Fraternal Twin Mix
Another mom had boy-girl twins. At 12 weeks, doctors told her one twin had Down syndrome. The other? No signs of it.
She says people always ask:
- “Which one has it?”
- “Do they get along?”
- “Is it harder having one with Down syndrome?”
Her answer? “It’s just life. Some days are easy, some are tough. Just like any other family.”
Raising Twins With Down Syndrome
Having twins is already double the diapers, double the chaos. Add Down syndrome, and it’s a unique experience.
The Challenges
- Different Milestone Timelines – One twin might walk, talk, and hit milestones later.
- More Medical Appointments – Therapies, heart checks, specialists—lots of appointments.
- Questions From Strangers – Some well-meaning, some just… weird.
The Wins
- Built-In Best Friend – The twin bond is next level.
- Natural Learning – The twin without Down syndrome often helps teach and encourage their sibling.
- Unmatched Love – Parents say their kids bring out the best in each other.
One mom put it best: “It’s not twice the work, it’s twice the love.”
What Science Still Doesn’t Know
Down syndrome in twins is still a mystery in some ways. Researchers are still looking for answers to big questions like:
- Why is Down syndrome less common in twins?
- Do twin pregnancies lower or raise the risk?
- Does being a twin affect Down syndrome symptoms?
Science isn’t done yet.
Final Thoughts
Twins and Down syndrome together? Rare, but possible.
- Identical twins? Both will have it.
- Fraternal twins? One might, one might not.
- Pregnancy? More tests, more monitoring.
- Life after birth? Challenges, but also incredible moments.
If you or someone you know is expecting twins and Down syndrome is on the table, know this—you’re not alone. Plenty of parents have been in the same spot, and they’ll tell you:
It’s a wild ride, but one filled with love.
