A new government report shows the rate of twin births is at a historic high.
More women gave birth to twins last year than ever before; it's not entirely clear what's behind the shift.
Researchers say it could be the fact that more women are giving birth later in life, a time when twins may be a more common occurrence.
Women who use in vitro fertilization can also have a higher chance of conceiving twins.
Rate of twin births at historic high, report says
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