When two babies are delivered in the delivery room and one weighs significantly less than the other, the first reaction is often concern. However, in maternity wards, teams do not look at the gross weight displayed on the scale: they cross-reference this figure with gestational age, the twin growth curve, and the difference between the two children. Understanding the weight and size of twins at birth requires a different approach than that of a singleton pregnancy.
Weight discordance between twins: the criterion that teams monitor first
Before even comparing one twin to a reference curve, caregivers calculate the weight discordance between the two babies. This percentage, obtained by relating the weight difference to the weight of the larger twin, often determines the level of neonatal monitoring.
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Studies conducted in French level 2 and 3 maternity wards show that discordance can be detected as early as the first trimester through certain ultrasound measurements (crown-rump length, nuchal translucency). When this early gap is significant, the likelihood of a notable weight difference at birth increases significantly.
In practical terms, moderate discordance does not trigger heavy management, but it leads to more frequent growth ultrasounds and, sometimes, to an earlier scheduled delivery. Discussing the weight and size of twins at birth without mentioning this gap amounts to ignoring the most decisive parameter in twin monitoring.
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Twin growth curves: why singleton benchmarks can be misleading
Parents who compare their twins’ weight to that of a full-term singleton baby almost always encounter an anxiety-inducing discrepancy. The majority of twins are born weighing less than a singleton, and this is largely physiological.
Growth curves specific to twin pregnancies are starting to become more common in clinical tools. They allow each baby to be positioned relative to percentiles calculated on twin populations, not on singleton pregnancies. A twin at the 30th twin percentile can fall below the 10th singleton percentile, without indicating a growth delay.
The distinction that matters in practice is based on the intersection of weight and gestational age. Perinatal scientific societies now classify a twin born close to term but below the 10th percentile for their gestational age as having intrauterine growth restriction (IUGR), even if their gross weight seems acceptable to parents. Conversely, a modestly weighted twin but well-positioned on the twin curve does not fall under this diagnosis.
Birth weight and gestational age of twins: some concrete benchmarks
It is observed that most pairs of twins born close to term have an individual weight significantly below the singleton average, which hovers around 3.3 kg. Feedback from maternity wards converges towards a lower range, often between 2.3 and 2.7 kg per baby.
Height follows the same logic: twins often measure a few centimeters less than a full-term singleton, without this affecting their subsequent growth. A newborn typically loses up to 10% of their weight in the days following birth, and this phenomenon is also observed in twins, sometimes asymmetrically between the two.
Skin-to-skin contact and early nutrition: protocols that change the game in neonatology
In level 2 and 3 maternity wards, there is a gradual normalization of the management of low-weight twins thanks to dedicated neonatal protocols. Three elements consistently emerge from field feedback:
- Early skin-to-skin contact, offered as soon as the delivery room allows, including for moderately premature twins. This contact promotes thermal regulation and the initiation of breastfeeding.
- Progressive nutrition protocols tailored to each twin’s weight, with different milk volume thresholds if one baby is smaller than the other.
- Individualized monitoring of weight gain: each twin has their own curve, and the team does not compare the two to decide on discharge.
These practices explain why twins with modest birth weights often catch up to the standard growth curve in the first months.
Factors influencing the weight of twins before birth
The birth weight of twins depends on parameters that parents do not always control, but which are useful to know to understand the outcomes in the delivery room.
- Chorionicity (mono or dichorionic): twins sharing a single placenta have a higher risk of weight discordance, as blood flow distribution may be uneven.
- The duration of the pregnancy: each additional week after the threshold of viability counts. Twin pregnancies often result in delivery before the expected term for a singleton pregnancy.
- Maternal health: the mother’s weight gain, gestational hypertension, or gestational diabetes directly affect fetal growth, sometimes asymmetrically between the two babies.
Feedback varies on this point among teams, but most agree that the type of placentation remains the most predictive factor of a significant weight discrepancy at birth.
Post-birth monitoring of twins: when the growth curve normalizes
Upon leaving the maternity ward, parents receive two separate health records, and this is intentional. Each twin follows their own growth trajectory. Systematically comparing the two babies generates unnecessary anxiety, especially when the initial discordance was moderate.
In consultations, the healthcare professional checks that each child is progressing on their curve, not that they are reaching the same figures as their co-twin. Weight catch-up is generally observed in the first six months, provided that feeding is well-managed and monitoring is regular.
Sleep, often staggered between the two babies at first, gradually synchronizes in the majority of pairs. This adjustment phase is normal and does not reflect a developmental issue related to birth weight.
Keeping in mind that the weight recorded on the birth bracelet does not predict a child’s future body composition is probably the most useful guideline for navigating the first weeks without excessive stress.