Selective Fetal Growth Restriction in Twin Pregnancies

Selective fetal growth restriction (sFGR) is a condition in which one twin in a monochorionic (identical-twin) pregnancy grows significantly less than its sibling, typically because the two fetuses share a single placenta unevenly. It complicates roughly 10 to 15 percent of monochorionic diamniotic twin pregnancies and is classified into three types based on blood-flow patterns in the smaller twin’s umbilical artery. The condition creates a tangled clinical picture because the two fetuses’ circulations are connected through shared blood vessels in the placenta, meaning what happens to one twin can directly affect the other.

Why One Twin Falls Behind

In a monochorionic pregnancy, both twins draw nutrients from the same placenta. How much territory each twin commands on that placenta varies, and when the split is lopsided, the twin with the smaller share receives less blood flow, less oxygen, and fewer nutrients. A study of 100 monochorionic placentas found that birthweight discordance rose in step with how unevenly the placental territory was divided.¹ A larger retrospective cohort of 449 cases confirmed this relationship, showing a strong positive correlation between placental share discordance and birthweight discordance.²

Unequal territory is not the only factor. How the umbilical cord inserts into the placenta matters too. In a normal insertion, the cord attaches near the center of the twin’s placental share, giving the blood vessels a short, efficient path to the tissue. In a velamentous insertion, the cord attaches at the edge of the membranes, and the vessels have to travel unprotected across the membrane surface before reaching the placenta itself. This arrangement is far more common in growth-discordant twins: about 22 percent of discordant placentas showed velamentous cord insertion compared to 8 percent of concordant ones.³

The Role of Shared Blood Vessels

Monochorionic placentas contain anastomoses, which are connections between the blood vessels serving each twin. These connections allow blood to flow between the twins, and their size and direction shape what happens in sFGR. Counterintuitively, larger anastomoses can actually help reduce birthweight discordance. In the study of 100 placentas mentioned above, when the connections between the two circulations were larger, the weight gap between the twins shrank for any given level of placental territory imbalance.⁴ In other words, large anastomoses act as a safety valve, allowing blood to redistribute and partially compensate for the unequal placental share. But these same connections are a double-edged sword: they also create the pathway through which one twin’s problems can spill over to the other.

Three Types, Three Different Outlooks

Clinicians classify sFGR based on blood-flow patterns seen on Doppler ultrasound of the smaller twin’s umbilical artery. The system, developed from a study of 134 monochorionic twins diagnosed between 18 and 26 weeks, divides cases into three categories.⁵

Type I: The smaller twin’s umbilical artery shows normal forward blood flow throughout the cardiac cycle. This is the mildest form and usually has the best prognosis. Many Type I cases can be monitored without intervention, and pregnancies often continue well into the third trimester.
Type II: The smaller twin’s umbilical artery consistently shows absent or reversed blood flow during the relaxation phase of each heartbeat. This pattern signals serious placental resistance and is associated with a higher risk of fetal deterioration and the need for early delivery.
Type III: The blood flow pattern is intermittent, cycling between normal forward flow and absent or reversed flow. This unpredictable pattern is linked to large artery-to-artery anastomoses in the placenta and carries a particular risk: the sudden swings in flow can cause acute shifts of blood between the twins, making the clinical course harder to predict.

The distinction between types matters because management decisions hinge on it. Type I pregnancies are generally watched with regular Doppler monitoring. Types II and III require closer surveillance and more frequent conversations about whether and when to intervene.

Catching It Early

Identifying sFGR before it becomes severe improves the options available. One approach looks at first-trimester ultrasound measurements. A case-control study found that the difference in crown-rump length (the head-to-bottom measurement taken early in pregnancy) was significantly larger in pregnancies that later developed sFGR, about 9.4 percent compared with 3.3 percent in unaffected pregnancies.⁶ That early size gap predicted sFGR reasonably well, though it is not definitive on its own. Most diagnoses are still made later, typically when second-trimester scans reveal that one twin has fallen below the tenth percentile for weight with a weight discordance of 25 percent or more between the pair.

Beyond conventional ultrasound, researchers have explored placental MRI. An imaging study using a technique called DECIDE MRI found that placental oxygen levels were lower on the side serving the smaller twin in sFGR pregnancies compared with uncomplicated monochorionic pairs. The average relaxation time, a proxy for tissue oxygenation, was also significantly lower for the smaller twin’s placental share. These findings suggest MRI could eventually help distinguish truly pathological placental function from a twin that is simply constitutionally smaller, though the technology is not yet routine clinical practice.

Maternal blood markers are another area of active research. A systematic review found that a ratio of two proteins in the mother’s blood, sFlt-1 and PlGF, was consistently higher in pregnancies with true fetal growth restriction compared with babies that were simply small. The difference was most pronounced in early-onset cases.⁷ A separate study found that when this ratio climbed above a specific threshold, it predicted serious adverse outcomes with high accuracy and correlated with a shorter interval between testing and delivery.⁸ Most of the biomarker research has been conducted in singleton pregnancies, so its direct application to sFGR in twins is still being worked out, but the principle is the same: the failing placenta leaks detectable signals into the mother’s bloodstream.

Cardiac Effects on Both Twins

One of the more surprising findings in sFGR research is that the larger twin’s heart can be hit harder than the smaller one’s. Because the twins’ circulations are connected through placental anastomoses, the larger twin may end up handling extra blood volume flowing from the struggling smaller twin’s territory. A study examining heart structure and function in monochorionic pairs with Type II and III sFGR found that the normally grown twin was significantly more likely to show signs of cardiac strain. About 42 percent of the larger twins had right ventricular thickening compared to 9 percent of the smaller twins, and roughly 37 percent of the larger twins had tricuspid valve leakage compared to 9 percent of their growth-restricted siblings.⁹ Markers of how efficiently the heart squeezed and relaxed were also more abnormal in the larger twin. Some of these cardiac changes persisted after birth, with over half of the larger twins who had abnormal prenatal heart scans still showing thickened right ventricles and valve leakage at delivery.¹⁰

This finding matters practically because clinical attention understandably focuses on the smaller, sicker-looking twin. The data suggest that the larger twin also deserves cardiac monitoring, both before and after birth.

Brain Development and the Threat of Single Twin Demise

The smaller twin’s brain develops differently in sFGR. An imaging study of 58 twin pairs found that the growth-restricted twin had significantly smaller brain structures, including the corpus callosum, the cerebellum, and the vermis, along with less white matter and deep gray matter and a smaller overall brain volume. The gap in brain volume was real but smaller than the gap in body weight: brain volume discordance averaged about 19 percent while birthweight discordance averaged about 32 percent, suggesting the brain is partially spared even when body growth is not.

The most feared complication in sFGR is the death of one twin in the womb. When one monochorionic twin dies, blood can rush through the shared anastomoses from the surviving twin into the low-pressure circulation of the dead twin. This sudden loss of blood volume can cause severe brain injury in the survivor. Type III sFGR carries a particular risk for this because its hallmark is large artery-to-artery anastomoses that allow rapid, large-volume blood shifts.¹¹ While the statistical trend toward increased neurological damage in sFGR pregnancies after a single twin death has not reached certainty in available data, the mechanism is well understood and drives much of the urgency around monitoring and intervention decisions.

Management Strategies

There is no single right approach to managing sFGR, and the best strategy depends on the type, the gestational age at diagnosis, and how both twins are doing. The three main paths are expectant management (close monitoring with planned delivery when the risks of staying pregnant outweigh prematurity risks), laser ablation of the shared placental vessels, and selective reduction of the affected twin.

For Type I, expectant management is usually the default. These pregnancies often reach the third trimester with good outcomes for both twins, though regular Doppler surveillance is essential because a fraction of Type I cases worsen over time.

For Types II and III, the decisions get harder. A study comparing expectant management with laser ablation of placental vessels in these more severe types found that the rate of at least one twin surviving to hospital discharge was roughly 83 to 91 percent with expectant management and 78 to 90 percent with laser treatment, depending on the type.¹² The median delivery with expectant management fell around 32 weeks for both types, while laser-treated Type III pregnancies delivered somewhat earlier, around 30 weeks. These numbers illustrate the core trade-off: laser ablation separates the twins’ circulations and eliminates the risk of acute blood shifts if one twin dies, but it also eliminates the blood flow that the smaller twin may depend on and can trigger earlier delivery.

Selective reduction, where the severely affected twin’s circulation is deliberately stopped to protect the surviving twin, is considered when the smaller twin’s condition is deteriorating and threatens the larger twin. Radiofrequency ablation is one technique used for this. A study of outcomes following radiofrequency ablation for complicated monochorionic twins reported a pregnancy success rate above 70 percent, with the highest success rates in the sFGR group.¹³ Separate data on radiofrequency ablation specifically in Type II and III sFGR showed that it allowed pregnancies to continue significantly longer than expectant management alone, which averaged about 29.6 weeks at delivery for these severe types.¹⁴

The optimal timing of delivery in sFGR remains genuinely unsettled. A systematic review of the literature found substantial variation in delivery timing across studies, particularly for Types II and III.¹⁵ The difficulty is balancing the risks of continuing the pregnancy (sudden deterioration, single twin demise) against the risks of very early delivery (prematurity-related lung, brain, and gut problems). There is no universally agreed-upon gestational age cutoff, and decisions are made case by case based on Doppler trends, fetal wellbeing indicators, and the type of sFGR.

How sFGR in Identical Twins Differs from Growth Restriction in Fraternal Twins

Growth restriction can happen in fraternal (dichorionic) twin pregnancies too, but the underlying problem is fundamentally different. Fraternal twins each have their own placenta, so there are no shared blood vessels and no risk of acute blood shifts between the twins. Growth restriction in a fraternal twin is essentially the same condition as growth restriction in a singleton: it reflects a problem with that individual baby’s placenta, not a competition for shared resources.¹⁶ Management follows the standard approach for singleton growth restriction, with the added consideration of not delivering the healthy twin too early on account of its sibling.

In monochorionic sFGR, every management decision must account for the interconnected circulations. Delivering the smaller twin too early may expose the larger twin to unnecessary prematurity. Waiting too long risks a catastrophic event that injures or kills the larger twin through the shared vessels. This constant balancing act is what makes sFGR in identical twins a distinct clinical challenge.

Epigenetic Signatures in the Placenta

Because monochorionic twins share the same DNA, sFGR offers a natural experiment for studying how the environment in the womb can alter gene activity without changing the genetic code itself. Researchers have found widespread differences in chemical tags on the DNA (known as methylation) between the placental tissue serving the growth-restricted twin and the tissue serving the normally grown twin. One study identified over 6,000 sites where methylation differed, with the overwhelming pattern being reduced methylation on the restricted side. The affected pathways involved steroid hormone production, cell signaling, and immune responses.¹⁷ One gene in particular, CYP11A1, which encodes a key enzyme in the production of steroid hormones from cholesterol, showed both reduced methylation and higher activity in the growth-restricted twin’s placental share. The same pattern was confirmed in growth-restricted singletons, suggesting a shared mechanism across different types of pregnancies.

More recent work using multiple techniques simultaneously found that the restricted twin’s placental tissue had reduced overall accessibility of its DNA to the cellular machinery that reads genes, alongside changes in methylation concentrated in pathways related to low oxygen and oxidative stress. Over 650 genes were expressed differently, with clusters involved in blood vessel formation and responses to oxygen deprivation.¹⁸ A scoping review of the broader epigenetic twin literature confirmed that sFGR placentas consistently show more down-regulated than up-regulated genes, with one study finding over 600 down-regulated genes compared to about 80 up-regulated ones.¹⁹

These epigenetic findings are still primarily research tools rather than clinical ones, but they are filling in the picture of how unequal placental territory translates into unequal growth at a molecular level. The fact that the same genes and pathways keep appearing across independent studies suggests these are real biological signals, not statistical noise.

Catch-Up Growth After Birth

Parents understandably want to know whether the smaller twin will stay small. The evidence is somewhat reassuring: a study tracking discordant identical twins through childhood found that the smaller twins showed rapid catch-up growth in the first two years of life and typically reached their genetic target height range between ages 8 and 11.²⁰ The word “despite” in the study’s own framing is telling: fetal growth restriction does leave a measurable imprint on childhood growth trajectories, but catch-up narrows the gap substantially over time. Because these are genetically identical twins, the persistent (though diminishing) size difference through childhood is direct evidence that the prenatal environment has lasting effects even when the genome is the same.

The Emotional Toll on Parents

sFGR forces parents into decisions that most people never imagine facing. When the condition is severe, families may be asked to consider selective reduction of the sicker twin to protect the healthier one, a choice that can feel impossibly cruel even when the medical rationale is clear. A qualitative study exploring the experiences of parents and clinicians involved in sFGR care found that discussions about selective termination appeared to cause lasting psychological harm, regardless of which choice the parents ultimately made.²¹ The study was part of efforts to design a clinical trial comparing intervention with expectant management, and one of its findings was that the very structure of decision-making in this condition needs attention, not just the medical outcomes it produces.

Part of what makes sFGR emotionally distinct from other pregnancy complications is its relational quality: the condition is defined by a comparison between two siblings, and any intervention that helps one twin may come at a cost to the other. Parents are not choosing between treatment and no treatment; they are navigating trade-offs between two children they are carrying at the same time. That framing shapes the grief, the guilt, and the long process of making sense of whatever happens next.