When two babies share the same pregnancy, their birth weight is generally well below that of a singleton. Most twins are born weighing between 2.3 and 2.7 kg each, compared to about 3.3 kg for a singleton. This difference, often a source of concern, does not automatically indicate a health problem.
Why the 2,500-gram threshold is not enough to assess twins
In medicine, a newborn is considered to weigh “little” if it is below 2,500 g. This threshold, defined by the World Health Organization, was designed for singleton pregnancies. When applied to twins, it loses much of its relevance.
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More than half of twins fall below this threshold at birth. If we relied solely on this raw figure, the majority of twin pairs would be classified as “low weight,” while many of these babies are doing very well.
To correctly assess the growth of twins, several parameters must be combined: gestational age at the time of delivery, type of placentation (a shared placenta or two distinct placentas), the sex of each baby, and the characteristics of the mother.
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Recent recommendations from scientific societies like ISUOG emphasize the use of personalized growth curves (GROW, INTERGROWTH-21st) rather than a fixed threshold in grams. These curves incorporate sex, parity, and maternal profile to place each twin on its own trajectory. Understanding the birth weight of twins according to standards therefore requires going beyond a simple reading of the baby scale in the delivery room.
Chorionicity and gestational age: the two variables that change everything
You may have heard of “monochorionic” and “dichorionic” twins without fully understanding the difference. In practice, this refers to the number of placentas.
Dichorionic twins: two placentas, two circuits
Each baby has its own placenta. Growth is often more consistent, and birth weights tend to be closer to each other. The weight discordance generally remains moderate in this configuration.
Monochorionic twins: a shared placenta
Both babies are nourished through the same placenta. The sharing is not always equitable: one twin may receive more blood and nutrients than the other. This imbalance can create significant weight discrepancies at birth, sometimes exceeding 20%.
Gestational age also plays a crucial role. A twin born at 36 weeks weighing 2.4 kg is not in the same situation as a twin of 2.4 kg born at 32 weeks. The former is often mature in terms of lung and metabolic development. The latter will likely need a stay in neonatology. Weight alone, without gestational age, tells us almost nothing about the actual health status of the newborn.
Weight discordance between twins: when should we be concerned?
In most pairs, the two babies do not weigh exactly the same. A difference of a few hundred grams is common and rarely problematic.
Medical teams monitor weight discordance, expressed as a percentage. Beyond 20%, the risk of intrauterine growth restriction in the smaller twin increases, and obstetric management is adjusted.
The criteria that alert practitioners include:
- An estimated weight discordance greater than 20% during third-trimester ultrasounds, especially in monochorionic twins
- An estimated weight of the smaller twin that drops percentiles over two consecutive examinations, a potential sign of growth restriction
- Abnormal blood flow in the umbilical Doppler, indicating placental insufficiency
A moderate discordance, less than 15-20%, does not predict major complications in the vast majority of cases. Regular ultrasound monitoring helps distinguish a physiological difference from a true growth problem.
Weight catch-up after birth: what long-term follow-ups show
Many parents worry about seeing their twins as “small” compared to the curves displayed in the health booklet. These curves are calibrated for children born alone, at term.
Follow-up studies published after 2019 provide reassuring insights. In twins born with moderately low weight but without major neonatal pathology, growth often catches up to singleton curves by 2 to 3 years. The initial gap gradually closes, provided that nutrition and pediatric follow-up are appropriate.
Another notable finding: weight discrepancies at birth within the same pair (moderate discordance) are found to be poor predictors of cognitive performance at 5-6 years, once the socio-economic context is taken into account. A lighter twin at birth will not necessarily struggle in school.
Postnatal follow-up of twins is therefore not limited to monitoring the scale. Evaluating psychomotor development, nutrition, and the parent-child bond weighs as much, if not more, than the grams displayed on the day of birth.
Reading the birth weight of twins: useful practical guidelines
To correctly interpret the birth weight of twins, a few concrete guidelines help clarify:
- A weight between 2.3 and 2.7 kg per twin at term (around 36-37 weeks) corresponds to the most common range and does not, in itself, constitute a warning signal
- The type of pregnancy (monochorionic or dichorionic) modifies expectations: a monochorionic twin weighing 2.2 kg may be perfectly suited to its placental situation
- The personalized GROW or INTERGROWTH-21st curves allow each baby to be situated relative to its own profile, not against a universal threshold
- The discordance between the two twins matters more than the absolute weight of each
Holding onto a single figure as a reference (“a twin should weigh X grams”) oversimplifies a clinical reality that depends on multiple intersecting factors. The birth weight of twins is always read in context: gestational age, chorionicity, maternal health, and fetal growth trajectory form an inseparable whole.