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The Pill & Mirena - How they work?

birth control mirena the pill womens bodies Mar 14, 2023

The Mirena

The mirena is a hormonal IUD (intrauterine device). It is a T-shaped device that is inserted into the uterus and it releases the hormone PROGESTIN.

What is progestin?

It is artificial progesterone.

How does the mirena prevent us from falling pregnant?

It does 2 things:

  1. It thickens cervical mucous so that sperm cant swim or get to the egg to fertilize it
  2. It thins the uterine lining and partially suppresses ovulation (mainly in first year of use)

Normally after about 6 months of having the mirena your period will cease, or become almost non-existent. Why? Because your uterine lining is so thin and the messages from your pituitary gland have been blocked by the progestin.

Your brain and your ovaries no longer communicate when using synthetic hormonal birth control methods.

You also block any signs from your body of hormonal imbalances. Which tend to return once the mirena is taken out - eg. heavy bleeding, hormonal headaches, absent periods etc.


The Pill

There are 2 types of pills - the combined pill and the progestin-only pill.

Combined pill eg. Yasmin, Melodene, Yaz, Triphasil, Diane

These contain a synthetic oestrogen and progestin combination. They work by suppressing ovulation, thinning the lining of the uterus and thickening cervical mucous - which makes is difficult for sperm to swim towards the fertilized egg.

These pills tricks your pituitary gland into thinking you are pregnant all month, so it doesn't ovulate (ie. it doesn't produce FSH and LH to stimulate ovulation). 

Progestin-only pill eg. Mini-pill, Camila

This is often given to breastfeeding moms, as it doesn't interfere with milk production or to women with a high risk of blood clots or heart disease. 

These pills only contain progestin, no oestrogen. They work by thinning the lining of the uterus and thickening cervical mucous to make the sperm unable to swim through it. The may suppress ovulation too - but not every month, they do this by preventing your pituitary gland from producing LH. 

Birth control pills are taken in 21-24 day cycles with a ‘sugar-pill’ for 5-7 days after that. The body has what is called a ‘withdrawal bleed’ over that time, not a real menstrual cycle.



Both the pill and the mirena trick your body into thinking it's pregnant all month. The pituitary glad then stops producing FSH (follicle stimulating hormone) and LH (Luteinizing hormone). The HPO (hypothalamic pituitary ovarian) axis is affected.  In simple language- your brain and your ovaries STOP communicating. We then expect our bodies to just start communicating when we want a baby? Food for thought.

Do you want to know more about Kelly's Inner Work Fertility Course. It was created in line with her own journey to motherhood, after battling unexplained infertility for 3 years!

Inner Work Fertility Course Information

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