Discordant twins
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Discordant Twins
Definition
Discordant twins are twin pairs that differ in one or more traits - a phenotype, disease, growth parameter, or genotype. The concept applies broadly to both types of twin pairs:
- Dizygotic (DZ) / fraternal twins - share ~50% of genes; discordance is expected and common
- Monozygotic (MZ) / identical twins - arise from one zygote; discordance here is scientifically significant because it implies non-genetic (epigenetic, environmental, or somatic) causes
1. Growth Discordance (Obstetric Context)
This is the most clinically common use of the term.
Definition and Measurement
Growth discordance is calculated as:
(Weight of larger twin - Weight of smaller twin) / Weight of larger twin × 100
- Significant discordance: generally defined as ≥20-25% difference in estimated fetal weight (EFW) or birth weight
- The term implies or includes severe IUGR of one twin with EFW below the 10th percentile
- Creasy & Resnik's Maternal-Fetal Medicine, Ch. 5
Causes by Chorionicity
| Mechanism | Dichorionic Twins | Monochorionic Twins |
|---|---|---|
| Unequal placental sharing | Primary cause | Contributes |
| Peripheral/velamentous cord insertion | Yes (smaller twin almost always has peripheral insertion) | Yes |
| Uteroplacental perfusion differences | Yes | Yes |
| Structural/chromosomal anomaly in one twin | Yes | Yes |
| Twin-twin transfusion syndrome (TTTS) | No | Major cause |
| TAPS (Twin Anemia-Polycythemia Sequence) | No | Yes |
| Sex difference alone | <10% discordance only | <10% discordance only |
Placental finding: In placentas with uneven sharing, the smaller share is virtually always the smaller twin's territory. Similarly, when cord insertions differ, the smaller twin typically has the marginal or velamentous insertion.
Clinical Significance
- Growth-discordant twins carry 3-7x higher perinatal mortality and morbidity than singletons
- In monochorionic twins, growth discordance >20% is a risk factor for adverse perinatal outcomes; this is not consistently seen in dichorionic twins
- Discordant nuchal translucency >20% in first trimester predicts risk of TTTS development
Surveillance Criteria Triggering Intervention (Monochorionic)
- EFW of one twin <10th percentile
- EFW discordance ≥25%
- Umbilical artery pulsatility index >95th percentile in the smaller twin
- EFW <3rd percentile regardless of discordance
2. Twin-Twin Transfusion Syndrome (TTTS) - A Cause of Extreme Discordance
TTTS occurs in 9-15% of monochorionic twin pregnancies due to unbalanced arteriovenous anastomoses in the shared placenta. It produces:
- Donor twin: anemic, oliguric, stuck (oligohydramnios)
- Recipient twin: plethoric, polyuric (polyhydramnios), at risk for cardiac failure and hydrops
This is distinguished from simple discordant growth because management is fundamentally different - TTTS usually requires laser ablation of placental anastomoses, whereas isolated growth discordance is often managed expectantly.
3. Genetic and Epigenetic Discordance (MZ Twins)
This is where discordant MZ twins become a powerful research model.
Why MZ Twins Become Discordant
Monozygotic twins were long assumed to be genetically identical. This is now known to be incomplete. Several mechanisms cause divergence:
Postzygotic genetic events:
- Unequal blastomere allocation during cleavage
- Postzygotic nondisjunction (confined twin mosaicism) - can produce heterokaryotypia (e.g., one twin with Turner syndrome 45,X, the other 46,XY)
- Single-gene point mutations in one twin
- X-inactivation differences
- Differences in telomere length
Epigenetic divergence:
- Discordant DNA methylation
- Discordant histone acetylation
- Age-dependent epigenetic drift - approximately one-third of MZ twin pairs show significant epigenetic differences, increasing with age
- A classic example: Beckwith-Wiedemann syndrome - relatively high frequency of discordance in female MZ twin pairs, mediated by epigenetic imprinting errors
- Creasy & Resnik's, Ch. 5; Genome Medicine 2014
Research Value of Discordant MZ Twins
Because MZ twins share genetics, age, sex, gestational environment, and often early-life exposures, discordant MZ pairs let researchers isolate environmental and epigenetic contributions to disease, controlling for confounders that plague population studies. This has been used in:
| Disease/Trait | What Discordant MZ Twins Revealed |
|---|---|
| Psychiatric disorders (schizophrenia, bipolar) | Environmental triggers, de novo somatic variants, epigenetic differences |
| Autoimmune diseases | Role of epigenetic changes in gene expression |
| Type 2 diabetes | Epigenetic regulation of insulin signaling |
| Cancer | Promoter hypermethylation of tumor suppressor genes in one twin |
| Kwashiorkor | Fecal microbiome transplant studies showed the kwashiorkor-associated microbiome causally drives poor weight gain, independent of diet (Harrison's, 2025) |
| Rett syndrome | Clinical severity variation despite identical MECP2 mutations (PMID 40898246, 2025) |
| Psychosis | Whole-genome sequencing found no consistent somatic variants, supporting epigenetic mechanisms (PMID 39080272, 2024) |
4. Discordance for Structural Anomalies
Structural or chromosomal anomalies can affect only one twin:
- One fetus with anencephaly, cardiac defect, or gastroschisis while the co-twin is normal
- This drives decisions about selective fetal reduction in monochorionic pregnancies, which carries risks to the co-twin through shared vasculature
The Harriet Lane Handbook lists discordant twins (>25% weight difference) as one of the indications triggering immediate neonatal resuscitation planning at delivery.
Key Takeaway
| Context | "Discordant" Means | Significance |
|---|---|---|
| Obstetrics/MFM | ≥20-25% birth weight difference | IUGR, TTTS, placental pathology |
| Genetics/epigenetics (MZ twins) | One affected, one unaffected for disease/trait | Proof that environment/epigenetics drive phenotype |
| Research | MZ twins differ in any measurable trait | Controls for genotype; isolates non-genetic factors |
Discordant MZ twin studies remain one of the most elegant natural experiments in medicine - they provide in-vivo models for dissecting why genetically identical individuals develop different diseases.
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