What causes some twins to be born with different birth weights?

With multiple gestations, it is not uncommon for one fetus to be smaller than the other, according to the American Congress of Obstetricians and GynecologistsOff Site Icon (ACOG). Multiple gestations are more likely to have growth problems than single gestations, but this doesn’t always become problematic, according to the ACOG.

In rare cases, the difference in birth weight can result from twin-to-twin transfusion, according to the National Institutes of HealthOff Site Icon (NIH). This occurs in about 15 percent of the twins that share a placenta (monochorionic) – identical twins – and happens when there is a net shift of fluid from one infant to the other, according to the Twin to Twin Transfusion Syndrome FoundationOff Site Icon.

These identical twins of different size are referred to as discordant twins; twin-to-twin transfusion is diagnosed by ultrasound and may be treated by advanced in-utero therapies, according to the NIH.

Ask your doctor about the varying birth weight of twins.

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