The question of whether twins are generally smaller than babies born singly is common, and the answer is definitively yes. Twin pregnancies carry a higher biological load, which directly affects the growth and gestational timeline for both babies. This difference is consistently reflected in clinical and statistical data worldwide. Understanding the reasons behind this size difference requires looking at both the limits of the womb and the shared resources available to the developing fetuses.
The Statistical Difference in Weight
The most significant difference between twin and singleton pregnancies is the average length of gestation. A typical singleton pregnancy lasts approximately 40 weeks, resulting in an average birth weight of just over seven pounds. By contrast, the average length of a twin pregnancy is about 35 weeks, which is four to five weeks shorter than a full-term singleton birth.
This shortened gestation means twins are much more likely to be born prematurely (before 37 weeks). Consequently, the average birth weight for a twin is notably lower, usually around 5.1 pounds each. A birth weight below 5 pounds, 8 ounces is defined as Low Birth Weight (LBW), a classification that applies to a majority of twins. The mean birth weight of a twin is nearly 28% smaller than that of a singleton.
Primary Factors Influencing Twin Size
The reduced size of twins stems from two primary mechanisms: the shortened duration of the pregnancy and the physical constraints on fetal growth inside the uterus. Prematurity accounts for the largest portion of the size difference, as the last few weeks of gestation are when babies gain weight most rapidly.
The second mechanism is Intrauterine Growth Restriction (IUGR), which is a slowdown in the baby’s growth rate while in the womb. Fetal growth in twin pregnancies begins to diverge from that of singletons around 26 weeks, as shared space and placental capacity become limiting factors. The uterus simply runs out of room, and the total nutrient supply struggles to support two full-sized babies.
The way twins share resources is highly dependent on their chorionicity, or whether they share a placenta. Dichorionic (DC) twins, who have separate placentas, generally have more independent growth but are still affected by the stress of a multiple gestation. Monochorionic (MC) twins, who share a single placenta, face a greater risk of Selective IUGR (sIUGR). This condition occurs when one twin receives a disproportionately smaller share of the placenta, sometimes causing a weight difference exceeding 25% between the two babies.
Health Implications of Smaller Birth Size
Being born smaller and earlier presents several immediate challenges for twins in the hours and days following birth. The most common short-term issue is respiratory distress syndrome, as the lungs of a premature baby may not be fully developed and lack sufficient surfactant to keep the air sacs open.
Twins often struggle with regulating their body temperature because they have less body fat and a larger surface area relative to their weight. They may experience difficulties with feeding and gaining weight, requiring specialized care to ensure adequate nourishment. Due to the combined effects of prematurity and low birth weight, nearly half of all twins require a stay in a Neonatal Intensive Care Unit (NICU) or Special Care Baby Unit (SCBU) for monitoring and support.
The specialized care in the NICU focuses on stabilizing these functions, such as breathing assistance, maintaining warmth in an incubator, and establishing feeding through tubes or specialized bottles. While most twins overcome these immediate hurdles, the need for intensive care is a frequent reality associated with their smaller birth size and earlier arrival.