Markers · Hormones
A progesterone of 4 on day 21. Progesterone, read against the day it was drawn.
What progesterone measures, what it does in the body, why it's tested and the two ranges it's read against, all from Merlin's own biomarker catalogue, then how the day it was drawn changes what it means.
What progesterone is
Progesterone is a steroid hormone produced primarily by the corpus luteum after ovulation, and by the placenta during pregnancy. It plays a critical role in regulating the menstrual cycle, preparing the uterine lining for embryo implantation, and maintaining early pregnancy. Low levels can indicate anovulation, luteal phase deficiency, or risk of miscarriage, while evaluation in context of cycle phase is essential for accurate interpretation.
What it does in the body
Progesterone prepares and maintains the endometrial lining for embryo implantation, suppresses uterine contractility during pregnancy, promotes breast tissue development, modulates immune tolerance during pregnancy, regulates the hypothalamic-pituitary-gonadal axis via negative feedback, supports myelination in the nervous system, and acts as a precursor to cortisol, aldosterone, and testosterone synthesis.
Why it's tested
Progesterone testing is used to confirm ovulation, evaluate luteal phase adequacy, assess risk of miscarriage or ectopic pregnancy, monitor progesterone supplementation therapy, investigate infertility, evaluate irregular menstrual cycles, and assess adrenal or ovarian function disorders.
The two ranges
The reference range reflects population statistics. The target reflects physiology.
- Lab reference range
- Follicular phase: 0.1-1.5 ng/mL; Luteal phase: 2.5-28.1 ng/mL; Pregnancy (1st trimester): 9-47 ng/mL; Pregnancy (2nd trimester): 17-147 ng/mL; Pregnancy (3rd trimester): 55-200 ng/mL; Postmenopausal: <0.1-0.8 ng/mL; Males: 0.2-1.4 ng/mL
- Functional target
- Follicular phase: 0.1-0.9 ng/mL; Luteal phase (mid-luteal/optimal): 7-25 ng/mL (levels >10 ng/mL suggest adequate ovulation); Pregnancy (1st trimester): 11-44 ng/mL; Postmenopausal: <0.5 ng/mL
Those are different numbers, and you deserve to know which one you're looking at. Merlin reads your result against both.
The day it was drawn
Progesterone comes from the corpus luteum, which only forms after ovulation, so a mid-luteal progesterone is the one blood test that confirms an egg was released. An LH strip can't stand in for it: the strip catches the surge that comes before ovulation.
Drawn about 7 days after ovulation, and off any progesterone, 10 ng/mL or more confirms ovulation occurred and the corpus luteum is producing well. Between about 3 and 10 points to ovulation from a smaller follicle or a softer luteal phase. Under about 3 suggests ovulation likely didn't occur in the cycle it was drawn in, or the timing was off.
Count the draw from ovulation, not from day 1. Day 21 lands a week after ovulation only in a cycle that ovulates around day 14. In a longer cycle, a day-21 draw comes early and reads low for that reason alone. When you log your ovulation signs in Merlin, it checks whether the draw landed in the readable window, about 5 to 9 days after ovulation, before it reads the number.
If you were taking progesterone when it was drawn, the number is your dose plus whatever your ovaries made, and it can't confirm or rule out ovulation in either direction. A draw taken off any progesterone answers it. On a contraceptive method that stops ovulation, a low progesterone is the method working.
If you're trying to conceive: a low luteal progesterone matters most with a history of miscarriage, PCOS or endometriosis. Where a deficit is documented, progesterone support is a clinical decision, made on labs, under supervision, not a self-directed add-on.
Where this lives
A Field Guide to Female Fertility
The field guide that covers progesterone in depth, whole and public.
In a real read
The real read on this site is a Cardiovascular one, so there's no hormone panel in it. What it shows is the shape: one system, thought all the way through, with the score, the narrative and the layers underneath. A hormone panel gets one of these too, with progesterone read against the day it was drawn and the ovulation signs you've logged.
A real Cardiovascular surface from a real anonymized panel: the score, the narrative, the layers underneath. Yours will be built the same way, on your own numbers.
Educational context for your conversations with your licensed healthcare provider, not a diagnosis or a treatment plan. Merlin reads results; it doesn't diagnose.
Read next to
No progesterone result yet? You can order it inside Merlin: it's on Ovulation Check - Luteal ($122), Female Hormone ($179) and Preconception (Female) ($257), which takes two draws, the second about 7 days after ovulation.
Bring the panel this number came from. Merlin reads it against everything else you have, every system and every year, then shows you what it means and what's worth finding out next. Start with your first month.