Science Explains Why One Twin May Be Smaller Than the Other

Twins aren't always the exact same size—and now, we have a better understanding of why one may be smaller than the other at birth.

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When we think about twin births, we tend to envision two babies who are the exact same size—a matched set, two peas in a pod, babies who look so similar, even their parents have a hard time telling them apart. 

But the reality is, twins aren't always the same size at birth. In fact, if you've ever seen a set of newborn twins, this may have taken you by surprise. After all, twins are born from the same mother, at the same gestational age—how could they possibly come in at different birth weights?

Well, we may finally have an answer to that question: According to new research from Boston Children's Hospital, the reason one identical twin could end up smaller than the other has everything to do with the placenta. 

When oxygen is transported slowly from mother to baby via the placenta, it leads to slower fetal growth, according to the study, which was published in Scientific Reports. Researchers observed pregnancies involving identical twins: While the placenta is split between both babies in these scenarios, it's split into separate compartments, which might explain why each twin may receive placenta at a different rate. 

The researchers performed MRIs on women between 29 and 34 weeks' gestation. They had the women inhale pure oxygen, then observed the amount of time it took for it to reach maximum concentration within the placenta, move through the umbilical cord, and ultimately, reach the fetus. Researchers then cross-referenced these findings against birth weight data from the babies. And their findings indicate that slower movement of oxygen through the placenta was associated with lower birth weight.

But this research's implications aren't just relevant to twin births: The researchers believe this finding may lead to additional understanding of pregnancy risk factors, and may help medical professionals alter prenatal care to ensure healthier pregnancies. “Our next goal is to figure out what causes variation in oxygen transport in the placenta and identify a cutoff value that would be of concern in a pregnancy,” researcher P. Ellen Grant, MD, said, according to the hospital's release. “Then, we can think about potential treatments to improve placental oxygen transport, and use our methods to immediately assess the success of these treatments.”

The takeaway? Your baby's size at birth may not be something under your control—yet. But in the future, treatments could help ensure your baby isn't underweight.