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Contraception health centre

How contraceptive implants and injections work

NHS ChoicesMedical Reference

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Contraceptive implants and injections steadily release the hormone progestogen into your bloodstream.

Progestogen is very similar to the natural hormone progesterone, which is released by a woman's ovaries during part of the menstrual cycle.

The continuous release of progestogen:

  • stops you from releasing an egg every month (ovulation),
  • thickens the mucus from the cervix, making it difficult for sperm to pass through to the womb and reach an unfertilised egg, and
  • makes the lining of the womb thinner, so it is unable to support a fertilised egg.


  • Uterus : The uterus (or womb) is a hollow, pear-shaped organ in a woman where a baby grows during pregnancy.  
  • Ovaries: Ovaries are the pair of reproductive organs that produce eggs and sex hormones in women.
Medical Review: October 12, 2009

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