Progesterone is the reproductive hormone that prepares the uterus to receive a fertilized egg. If the egg is not fertilized, the menstrual cycle will start and progesterone levels will fall. Progesterone levels rise after ovulation for about five days. Then they start falling. 

During pregnancy, it’s the placenta that produces high levels of progesterone. Thus, levels are higher if you are carrying twins or triplets.

Progesterone is a hormone that thus balances estrogen. An imbalance between the two hormones can cause health issues, some that are quite serious. For example, in women, an imbalance in estrogen and progesterone can cause infertility, fibroids, ovarian cysts, brittle bones, or even adrenal gland cancer. 

Healthy progesterone levels help balance blood sugar levels. They stimulate growth of the bones and good bone density. They also allow you to sleep well at night and reduce anxiety during the day.

Normal Ranges for Progesterone in ng/mL or nmol/L:

Normal Progesterone levels vary depending on the time of the menstrual cycle, the trimester of the pregnancy, and whether or not one is menstruating or is male. 

Progesterone Levels for Young Girls (Pre-Puberty): 0.1-0.3 ng/mLProgesterone Levels for Menstruating Females:

  Day 1-14 during the menstrual cycle: <1 ng/mL or 0.5-2.3 nmol/L

  Day 15-29 during the cycle: 2-25 ng/mL or 6.4-79.5 nmol/L

During Pregnancy:

   1st trimester:  10-44 ng/mL or 32.6-140 nmol/L

   2nd trimester:  19.5-82.5 ng/mL or 62-262 nmol/L

   3rd trimester:  65-290 ng/mL or 206.7-728 nmol/L

 Menopause: <1ng/mL or <2 nmol/L

 Men:  <1 ng/mL or <3.2 nmol/L

Critical Range: Anything outside of the normal range

Sources: 

- http://www.lifeextension.com/Vitamins-Supplements/itemLC004317/Progesterone-Blood-Test

- https://www.webmd.com/women/progesterone-15286#1

- https://www.urmc.rochester.edu/encyclopedia/content.aspx?ContentTypeID=167&ContentID=progesterone