The birth weight of twins averages between 2.3 and 2.7 kg per child, which is 500 to 1,000 grams less than a baby from a singleton pregnancy. This difference, measurable from the third trimester, does not necessarily indicate a health problem. It reflects biological constraints specific to twin pregnancies, and its interpretation largely depends on the type of growth curve used.
Singleton or twin growth curves: a choice that changes the diagnosis
Applying reference curves designed for singleton pregnancies to twins leads to classifying more than 50% of twins as growth restricted, according to French hospital data covering 770 births. This rate drops significantly when using curves specific to twin pregnancies.
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The weight discrepancy between twins and singletons becomes visible from 32 weeks of gestational age. In contrast, head circumference and height do not significantly diverge until 39 weeks, indicating a usually disharmonious growth restriction in the twin: weight is affected first, not head growth.
Several scientific societies now recommend not to systematically use singleton pregnancy curves to interpret the estimated fetal weight of twins. The reason: the overdiagnosis of growth restriction leads to unnecessary inductions or cesarean sections. A Cochrane review published in 2021 confirms this trend, while emphasizing that the available studies remain of limited quality. Trials are underway to validate dedicated curves.
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To better understand the weight curve of twins at birth, it is essential first to verify which reference has been used by the medical team, as the same weight can be classified as normal or concerning depending on the chosen curve.
Weight discordance between twins: monochorionic and dichorionic do not compare
The birth weight discordance between the two children of the same pair is sometimes a more revealing indicator than the absolute weight of each. A difference greater than 20% between the two twins is a threshold frequently monitored during ultrasound.
| Type of pregnancy | Main cause of discordance | Prenatal detection |
|---|---|---|
| Monochorionic (monozygotic, one placenta) | Unequal sharing of the placenta | 80% of severe discordances detected before birth |
| Dichorionic (dizygotic, two placentas) | Maternal factors (smoking, BMI, pathologies) | Later detection, often after 30 weeks |
In monochorionic twins, the twin-to-twin transfusion syndrome or unequal vascular sharing of the placenta explains most weight discrepancies. In dichorionic twins, the discordance is more related to the maternal environment. The prognosis of the same weight discrepancy differs according to the type of placentation, which is rarely explained in popular articles.
The average gestational age at delivery for twin pregnancies with discordance is around 34 weeks and 3 days. For severe discordances, 64% of deliveries occur before 34 weeks, which increases the risks associated with prematurity.
Maternal factors that amplify discordance
- A BMI over 25 kg/m², found in about 30% of patients monitored for twin discordance in a French hospital study
- Gestational diabetes, present in more than 13% of patients in the same series
- Pregnancy-related vascular pathologies (gestational hypertension, pre-eclampsia, HELLP syndrome), affecting about 30% of twin pregnancies with severe discordance
Weight catch-up after birth: what longitudinal data shows
The weight deficit observed at birth does not persist indefinitely. A longitudinal study conducted in Budapest on cohorts of twins followed for ten years shows that the weight of twins catches up to that of singletons by the age of 2 years, and height by 3 years.
This catch-up depends on the maturation status at birth. Researchers classified newborns into subgroups based on their weight and weight/height ratio. Twins in the lower quartile catch up more slowly, but the majority reach normal percentiles before entering kindergarten.
Postnatal follow-up: what benchmarks to use
Postnatally, standard growth curves become relevant again, as uterine constraints no longer exist. The French health booklet provides BMI, weight, and head circumference curves adapted for monitoring all children. The question of specific curves mainly arises during pregnancy and the first weeks of life.
Postnatal follow-up of premature twins, born before 37 weeks, requires an adjustment of age (corrected age) to correctly interpret their position on the curves. Without this adjustment, a twin born at 34 weeks will systematically appear delayed compared to the norms.
Overdiagnosis and overmedicalization: a documented risk in twin pregnancies
The use of inappropriate reference standards is not limited to a problem of statistical classification. It has direct clinical consequences. Classifying a twin as “small for gestational age” based on a singleton curve can trigger a cascade of increased monitoring, additional consultations, and sometimes avoidable obstetric interventions.
The Cochrane review of 2021 highlights this phenomenon without making a definitive conclusion, as randomized trials comparing the two types of curves are still ongoing. The challenge is to avoid unnecessary cesareans and inductions while maintaining reliable detection of true growth restrictions.
Assessing the weight of a pair of twins remains incomplete without evaluating the discordance between the two children, which constitutes an independent risk factor. One twin may have a normal weight in absolute terms but present a concerning discrepancy compared to their co-twin, justifying specific monitoring even in the absence of isolated low weight.