Be careful how you share this one with your siblings: In a recent Australian study involving 13,000 sisters, the firstborn were found to be 29% more likely to be over-weight than the second born; they also had a higher risk for high blood pressure.

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The Auckland U. researchers think it may have to do with earlier babies getting less blood and nutrients in utero, because of the narrower vessels that typically accompany first pregnancies.

The syndrome may be advantageous for men (and future linebackers), though: Previous studies showed that firstborn males are taller and heavier.

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