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Accelerated Epigenetic Gestational Aging in Monochorionic Compared with Dichorionic Twins: A Cord Blood DNA Methylation Study

This study demonstrates that among full-term twins, monochorionic (MCDA) neonates exhibit significantly higher epigenetic gestational age acceleration compared to dichorionic (DCDA) twins, suggesting that the hemodynamic environment associated with placental sharing independently influences biological maturation beyond gestational age.

Original authors: Leejae Cho, Ji Hoi Kim, Jong Kwan Jun, Soong Deok Lee, Jong-Lyul Park, So-hee Kim, Joong Shin Park, Hwan Young Lee, Seung Mi Lee

Published 2026-08-25

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CC BY 4.0

Original authors: Leejae Cho, Ji Hoi Kim, Jong Kwan Jun, Soong Deok Lee, Jong-Lyul Park, So-hee Kim, Joong Shin Park, Hwan Young Lee, Seung Mi Lee

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Every baby arrives with a biological age that may not match the calendar age. A doctor can count the weeks a pregnancy has lasted, but that number does not always tell the whole story of how mature the infant's body truly is. Some babies born at the same time are biologically more developed than others, carrying different levels of readiness for the world outside. Scientists have found a way to measure this hidden maturity by looking at chemical tags attached to DNA in a baby's blood. These tags, known as methylation, change in a predictable pattern as a fetus grows. By reading these patterns, researchers can estimate a "biological gestational age" and compare it to the actual time the baby spent in the womb. The difference between the two is called gestational age acceleration. A positive difference means the baby is biologically older than the calendar suggests, while a negative difference implies they are younger. Understanding what drives this difference is crucial, because it reveals how the environment inside the womb shapes a baby's development long before birth.

For years, scientists have studied these patterns in single births, but twins offer a unique window into this process. Twins share the same mother and often the same genetic code, yet they can develop in very different environments depending on how they are connected to the placenta. In some twin pregnancies, each baby has its own separate placenta and blood supply. In others, the twins share a single placenta, with blood vessels connecting their two circulations. This shared connection creates a complex hemodynamic environment where blood flow can shift between the fetuses. Researchers wondered if this difference in how twins share a placenta might leave a mark on their biological maturity. Specifically, they asked whether twins who share a single placenta age differently at the cellular level compared to those with separate placentas, even if they are born at the exact same time.

To answer this, a team of researchers in South Korea analyzed cord blood from forty-nine pairs of identical twins. They focused on two groups: those with separate placentas and those sharing one. Using advanced technology to read the chemical tags on the DNA, they applied two different established methods to calculate each baby's biological age. The results showed that the twins' biological ages were closely linked within each pair, confirming that the shared environment and genetics create a strong similarity. However, a clear difference emerged when comparing the two types of twin pregnancies. Among the babies born at full term, those who shared a single placenta showed signs of accelerated aging. Their biological maturity was higher than that of their counterparts with separate placentas, despite being born at the same gestational age.

The study found that full-term twins sharing a placenta had a gestational age acceleration of about 0.34 weeks according to one measurement method, compared to a slight deceleration of 0.27 weeks for twins with separate placentas. This difference was statistically significant, meaning it was unlikely to be a random occurrence. When the researchers adjusted for other factors like the baby's sex, birth weight, and the mother's health, the link between sharing a placenta and higher biological maturity remained strong. The second measurement method showed the same trend, though the difference was not quite large enough to be statistically certain on its own. The researchers noted that being born early, or preterm, did not significantly change this pattern of acceleration, suggesting that the type of placental connection is a more powerful driver of this specific biological difference than the length of the pregnancy alone.

These findings suggest that the physical environment of the womb, particularly the shared blood flow in a single placenta, may push fetal development forward at a faster pace. The shared circulation in these pregnancies involves complex connections between the twins' blood vessels, which can alter how blood and nutrients are distributed. While the study did not measure these blood flows directly, the higher biological maturity observed in the shared-placenta group aligns with the idea that these unique hemodynamic conditions influence how the fetus matures. The researchers caution that this is an association, not a proof of cause and effect, and that more work is needed to understand exactly which parts of the shared circulation drive this change. Nevertheless, the study highlights that chorionicity, or the type of placental connection, is an independent factor in how twins develop. It adds a new layer to our understanding of neonatal health, suggesting that the way twins share a placenta might be a key variable in assessing their biological readiness at birth.

Technical Summary: Accelerated Epigenetic Gestational Aging in Monochorionic Compared with Dichorionic Twins

Problem Statement
While gestational age (GA) at birth is a primary determinant of neonatal health, biological maturity often diverges from chronological age. This discordance is quantified as gestational age acceleration (GAA) using DNA methylation-based epigenetic clocks. Although twin pregnancies offer a unique model for studying intrauterine environmental effects due to shared genetics, the specific influence of chorionicity (placental architecture) on epigenetic maturation remains unexamined. Monochorionic diamniotic (MCDA) twins share a single placenta with vascular anastomoses, creating a distinct hemodynamic environment compared to dichorionic diamniotic (DCDA) twins, who possess separate placental circulations. The study addresses the gap in knowledge regarding whether these hemodynamic differences drive variations in epigenetic gestational aging, independent of gestational age at birth.

Methodology
The study utilized cord blood DNA methylation data from a cohort of 49 Korean monozygotic twin pairs (26 DCDA and 23 MCDA).

Data Processing: Raw IDAT files were processed using the minfi package in R with NOOB normalization. Probes with detection $P > 0.05$ in $\geq 25\%$ of samples or missing beta values were excluded. Probe identifiers were mapped to legacy 27K and 450K nomenclatures to ensure compatibility with established clock coefficients.
Epigenetic Clocks: Two established clocks were applied to estimate DNA methylation-based gestational age (DNAm GA):
1. Knight's Clock: Based on 148 CpG sites from 27K and 450K arrays.
2. Bohlin's Clock: Based on 96 CpG sites from 450K arrays.
  Missing CpG values were imputed using reference medians.
GAA Calculation: GAA was defined as the residual from a linear regression of DNAm GA on clinically recorded GA. Positive residuals indicate accelerated aging; negative residuals indicate deceleration.
Statistical Analysis:
- Within-pair concordance: Assessed using the Intraclass Correlation Coefficient (ICC) via a one-way random-effects model.
- Group Comparisons: Welch's $t$ -test or Mann–Whitney $U$ test were used for exploratory comparisons between preterm/full-term and DCDA/MCDA groups.
- Regression Modeling: Ordinary Least Squares (OLS) regression with cluster-robust standard errors (Huber–White sandwich estimator) at the twin-pair level was used to assess the association between chorionicity and GAA, using full-term DCDA twins as the reference. Models were adjusted for sex.
- Sensitivity Analysis: 64 model specifications were tested by adding combinations of covariates (birth weight, SGA, maternal diabetes, hypertension, age, and mode of delivery) to evaluate robustness.

Key Results

Clock Performance: Both clocks correlated strongly with clinical GA (Knight: $R=0.85$ , MAE=1.64 weeks; Bohlin: $R=0.94$ , MAE=1.25 weeks).
Within-Pair Concordance: Both clocks demonstrated good within-pair concordance (Knight ICC: 0.760; Bohlin ICC: 0.879). Concordance was generally higher in MCDA pairs than DCDA pairs.
Chorionicity and GAA:
- Among full-term neonates, MCDA twins exhibited significantly higher GAA than DCDA twins using Knight's clock ( $0.34 \pm 0.96$ vs. $-0.27 \pm 0.80$ weeks; $p=0.010$ ).
- Bohlin's clock showed a similar directional trend (MCDA: $0.12 \pm 0.49$ vs. DCDA: $-0.09 \pm 0.48$ weeks) but did not reach statistical significance ( $p=0.11$ ).
- Regression analysis confirmed that full-term MCDA status was independently associated with higher GAA relative to full-term DCDA twins (Knight: $\beta=0.64$ weeks; 95% CI 0.02–1.26; $p=0.044$ ).
Sensitivity and Preterm Birth: The association between MCDA status and higher GAA remained robust in 52 of 64 sensitivity models. Preterm birth was not significantly associated with GAA in either clock.

Significance and Claims
The study concludes that chorionicity is independently associated with epigenetic gestational aging in full-term monozygotic twins. Specifically, MCDA twins exhibit higher epigenetic maturity than DCDA twins despite sharing the same gestational age at birth. The authors posit that the hemodynamic environment associated with placental sharing—specifically the vascular anastomoses in monochorionic placentas—may contribute to epigenetic maturation, distinct from the effects of gestational age alone.

The paper suggests that chorionicity should be investigated further as a biological variable relevant to neonatal risk stratification. However, the authors maintain a modest stance, noting that the proposed role of placental hemodynamics is currently inferential. They emphasize that the study does not establish causality or determine whether these GAA differences persist postnatally or impact long-term developmental outcomes. The findings highlight the need for future research linking GAA to quantitative features of monochorionic placentation and longitudinal developmental trajectories.

Technical Summary: Accelerated Epigenetic Gestational Aging in Monochorionic Compared with Dichorionic Twins

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