While twin weight discordance measured by ultrasound can signal potential issues, recent studies emphasise that tracking changes over time is crucial for accurate assessment and management, especially in high-risk pregnancies.
A growth scan can be reassuring, but for parents expecting twins it can also bring a new number to worry over: the percentage difference between the babies’ estimated weights. That figure, known as twin weight discordance, is simply a way of describing how far apart the twins are in size at a given point in pregnancy. A difference does not automatically mean something is wrong. Twins are rarely identical in size, and many pregnancies show a modest gap that never leads to trouble. NICE guidance on twin and triplet pregnancies makes clear that clinicians use this information as part of wider monitoring, not as a diagnosis on its own.
The number is calculated from ultrasound estimates, not from an actual weighing. Sonographers measure each baby’s head, abdomen and thigh bone, then use those measurements to estimate fetal weight. That estimate is compared between the two babies, with the larger baby used as the reference point. Because it is based on scan measurements, the figure can only ever be an approximation. Evidence reviews for twin and triplet pregnancies have noted that screening for growth problems is complicated by variation in definitions and the limits of ultrasound estimates, which is why a single scan rarely settles the question.
A small difference in size is often entirely normal. Twins may have different genetic make-ups, may receive blood flow differently through the placenta, or may simply grow at slightly different rates. This is particularly common in fraternal twins, who do not share identical genes. Even so, doctors watch the trend carefully. A stable gap over time is generally less concerning than a widening one, and the overall picture matters more than one isolated percentage. In practice, clinicians look at each baby’s growth pattern, the fluid around them and blood flow in the umbilical cord, often called Doppler assessment.
That is why the commonly discussed thresholds matter. In UK practice, a discordance of 20% or more often prompts closer follow-up, while a gap of 25% or more, especially if one baby is also small for gestational age, may lead to referral to a fetal medicine specialist. The exact cut-offs vary between guidelines and hospitals, which reflects the fact that these are practical trigger points rather than hard lines. NICE recommends different scan schedules depending on whether twins share a placenta, because monochorionic pregnancies carry higher risks and need more frequent surveillance.
When the size gap is larger, doctors also consider whether it forms part of selective fetal growth restriction, or sFGR. That is a separate diagnosis in which one twin is not growing properly. Expert definitions usually look for a combination of factors, such as a very small baby, a discordance of around 25% or more, or abnormal blood flow, rather than relying on size difference alone. This distinction matters because a sizeable gap can exist without sFGR, and not every twin pregnancy with discordance needs the same management.
Recent research underlines why clinicians pay such close attention to the trend rather than to one measurement alone. A study from Prague published in Scientific Reports in October 2026 followed 434 twin pregnancies in which the babies were delivered very early or were very small. The researchers found that gestational age at birth and antenatal discordance were major predictors of neonatal outcome. Twins with a discordance of 25% or more on an early scan were less likely to go home without serious complications, and breathing problems were more common. At the same time, some pairs that looked less discordant at the scan were more different by birth, which shows why repeat monitoring remains essential.
For parents, the most useful question is not simply what the percentage is, but whether it is changing and how each baby is doing on its own. A gap that remains steady may be less concerning than one that widens from scan to scan. Blood flow, fluid levels, chorionicity and overall growth all help shape the plan, especially in pregnancies where the babies share a placenta. Twin weight discordance is therefore best understood as an alert to look more closely, not as a verdict on the pregnancy.
Disclaimer: This content is for informational purposes only and is not intended to be a substitute for professional medical judgment, advice, diagnosis, or treatment.
