Imagine discovering that identical twins, born from the same egg and sperm, could have completely different genetic profiles. Sounds impossible, right? But here's where it gets controversial: this rare phenomenon, known as heterokaryotypic monozygotic twinning, does exist—and it challenges everything we thought we knew about identical twins. In this case report, we delve into a fascinating yet complex instance of a monochorionic twin pregnancy where the twins shared a placenta but had strikingly different genetic makeup. And this is the part most people miss: even though they started as one fertilized egg, postzygotic errors led to one twin developing Turner syndrome while the other remained genetically normal—a condition so rare it’s barely documented in medical literature.
- Introduction
Heterokaryotypic monozygotic twinning is a medical rarity where identical twins, despite originating from a single egg and sperm, exhibit genetic differences. Typically, identical twins are expected to share the same genetic material, but exceptions arise due to mechanisms like asymmetric X-chromosome inactivation, unequal gene imprinting, or postzygotic mitotic errors such as nondisjunction. These errors can occur during the early stages of embryonic development, leading to one twin having a normal genetic profile while the other carries abnormalities. The rise of assisted reproductive technology (ART) has also been linked to increased epigenetic discrepancies in monozygotic twins. Here, we present a detailed account of diagnosing, managing, and understanding the outcome of such a rare case, previously showcased at the 2024 RCOG World Congress.
- Case Description
A 28-year-old woman, already a mother of one, conceived monochorionic diamniotic twins naturally. At 11 weeks, the shared placenta was confirmed, and by 14 weeks, one fetus showed signs of nuchal edema, which progressed to a massive cystic hygroma. By 22 weeks, the affected twin’s hygroma measured 16.7 × 10.6 cm, leading to fetal hydrops, while the co-twin appeared structurally normal. Amniocentesis at 17 weeks revealed the affected twin had pure Turner syndrome (45,XO), and the normal twin had a variant Turner syndrome (46Xdel(X)(p11)). Despite discussing selective fetal reduction, the couple opted for conservative management. Unfortunately, the mother went into premature labor at 23 weeks, resulting in the loss of both fetuses. The hydropic twin weighed 1900g, while the co-twin was only 350g. Post-delivery examination confirmed the monochorionic placenta.
- Discussion
Monochorionic twin pregnancies are notorious for complications like twin-to-twin transfusion syndrome (TTTS), selective fetal growth restriction (sFGR), and increased mortality risks. Perinatal morbidity and mortality in these cases are 3-6 times higher than in dichorionic twins. Structural abnormalities are also more common in monozygotic twins compared to dizygotic twins or singletons. While monozygotic twins are often labeled 'identical,' genetic discordance is increasingly reported, challenging this assumption. In our case, postzygotic mitotic errors likely caused the heterokaryotypia, similar to other reported Turner syndrome cases.
But here's the controversial part: should we still call monozygotic twins 'identical' when genetic discrepancies can occur? This case underscores the importance of dual sac amniocentesis in monochorionic twins with discordant anomalies, even in natural conceptions. While procedures like CVS and cordocentesis are less accurate, dual sac amniocentesis ensures precise diagnosis and timely intervention. Selective feticide, though ethically complex, can improve the survival of the normal twin. What do you think? Is it fair to prioritize one twin’s survival over the other in such cases?
- Conclusions
Monochorionic pregnancies are inherently risky, and heterokaryotypia, though rare, significantly impacts prenatal and postnatal outcomes. Detailed sonographic evaluation, dual sac amniocentesis, and zygosity determination are critical for management. This case adds valuable insights to the limited literature on spontaneously conceived monochorionic twins with genetic discordance.
Acknowledgement
We extend our gratitude to the patient and her spouse for their cooperation and consent to publish this case.
