Twins—smaller at birth, on average, than singletons—start out smaller in pregnancy than was previously known, according to a study by researchers at the National Institutes of Health (NIH). The ...
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What to Know About Twin Pregnancy

A twin pregnancy is when there are two fetuses in the uterus at the same time. Twins can be identical, meaning they developed when a single fertilized egg split early in development, or fraternal, ...