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The Role Of Progesterone 


The placenta continues to produce both progesterone and estrogen for the duration of the pregnancy and the levels get higher and higher right up to just before the birth.


The following chart shows the average growth in the level of progesterone within the body during a pregnancy.  The dotted line shows what would happen if no fertilization happened during normal menstrual cycles.




During the pregnancy, the progesterone is needed in the following ways, (mostly in conjunction with estrogen):

  • Makes the endometrium develop and secrete fluids after being primed by estrogen

  • Maintains the functions of the placenta and fights off unwanted cells near the womb that could cause damage to the placenta or foetus.

  • Keeps the endometrium in a thickened condition

  • Stops the uterus making spontaneous movements

  • Stimulates the growth of breast tissue

  • Prevents lactation until after the birth (with estrogen)

  • Strengthens  the mucus plug covering the cervix to prevent infection.

  • Strengthens the pelvic walls in preparation for labour.

  • Stops the uterus from contracting (thus keeping the baby where it is)


At the end of the pregnancy, the levels of progesterone secreted by the placenta drop off.  It is this action that stimulates the beginning of the contractions that will lead to birth.



The effects on a woman due to raised levels of progesterone can include any or all of the following:

  • Constipation

  • Heartburn

  • Runny and irritable nose

  • Eyesight problems (blurring or headaches)

  • Increased kidney infection risk.


A minimum level of about 10ng/ml is required to sustain a pregnancy through the very early stages.  It is because of this that doctors can measure the levels of progesterone in the blood to assist in the diagnosis of abnormal pregnancies such as ectopic pregnancy.



Supplements of natural progesterone are sometimes prescribed as a fertility treatment or to assist in reducing the risk of miscarriage. This is found in the form of a cream that is rubbed into the abdomen and thighs or else it arrives as a vaginal suppository or an injection (tablets don't work very well).



1. Progesterone production

2. The role of progesterone

3. The role of estrogen (oestrogen)

4. Summary of some other hormones






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