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Ella aims to prevent ovulation for as long as five days after unprotected sex.
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All work by preventing ovulation, not by stopping the implantation of a fertilized egg.
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The drugmaker has said its pill, also known by its chemical name ulipristal, works primarily by preventing ovulation of a woman's egg.
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It must be taken within 3 days of unprotected sex and works by preventing ovulation or by stopping sperm from reaching an egg.
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It's a hat trick of defence, preventing ovulation, thickening cervical mucus to impede sperm and thinning the lining of the womb to guard against implantation.