Estrogen is responsible for building the uterine lining (creating a healthy and happy home for growing baby) and progesterone is responsible for maintaining that lining. Progesterone is produced by the corpus luteum AFTER ovulation has happened at the ovary. There is no standardized “ideal” level of serum progesterone in the luteal phase – other than the fact that if you’re above 18 nmol/L we can say with confidence, yes ovulation happened that cycle, but beyond that, serum progesterone levels are finicky to read heavily into.
Progesterone has cyclical variation (ie – elevated in the luteal phase, low in follicular phase), but it also has pulsatile secretion throughout the day.
“Serum progesterone levels can fluctuate 8-fold in a 90-minute period during the mid-luteal phase and range from 7.3 to 127.5 nmol/L during a 24-hour period in the same healthy subject” Mesen et.al (2015)
Due to this large variation, it is difficult to say what the standard of progesterone should be in the luteal phase to maintain a pregnancy. And of course – it’s going to depend on the individual and other aspects of health!
The image on this post is a graphic from a paper looking at healthy pregnancies and pregnancies with threatened miscarriage, and their respective progesterone levels. Keep in mind – there are MANY factors that go into a healthy pregnancy, and MANY factors that contribute to a miscarriage. Progesterone (what this study is looking at) it just ONE of those factors, so I want to be clear, it is not the be all end all cure for miscarriage and recurrent pregnancy loss, however it is a modifiable factor (a simple prescription) to increase progesterone levels and lengthen luteal phase length – which may be indicated in certain cases.
In general, a luteal phase of 12-14 days is a healthy length to allow for adequate embryo implantation (again, not always the case for certain individuals, but in general!) and more robust progesterone secretion is always good to see. In early pregnancy, you also want to be seeing gradual increase in progesterone. In those early days – it should be increasing by roughly 1-3 mg/mL every couple of days until it reaches the peak for the trimester.