What it measures.
Progesterone is a steroid hormone produced by the corpus luteum after ovulation. A serum level above 3 ng/mL confirms ovulation occurred; levels above 10 ng/mL during the mid-luteal phase indicate adequate luteal function for implantation. In IVF, optimal mid-luteal progesterone of 41-60 ng/mL is associated with the highest live birth rates in fresh embryo transfer cycles.
The concentration of progesterone in blood, reflecting corpus luteum function after ovulation. Progesterone prepares the endometrium for implantation, maintains early pregnancy, suppresses further follicular development, and causes the basal body temperature rise used in fertility charting.
Why it matters.
Progesterone is the definitive ovulation confirmation test. Without ovulation, there is no corpus luteum and no progesterone rise. Sub-optimal progesterone (luteal phase deficiency) is associated with impaired implantation and early pregnancy loss. In IVF, progesterone levels guide supplementation dosing.
Physiology.
After ovulation, the ruptured follicle transforms into the corpus luteum under LH stimulation and secretes progesterone. Peak levels occur 7 days post-ovulation (Day 21 in a 28-day cycle). If implantation occurs, hCG from the trophoblast rescues the corpus luteum. If not, the corpus luteum involutes after 12-14 days and progesterone falls, triggering menstruation.
Testing & preparation.
How to prepare
- No fasting required
- MUST be timed to 7 days post-ovulation (Day 21 for 28-day cycles)
- For non-28-day cycles: count 7 days after positive OPK
- 35-day cycle = test on Day 28; 21-day cycle = test on Day 14
- Note: testing on 'Day 21' of a 35-day cycle catches the wrong phase
When to test
Mid-luteal phase to confirm ovulation. Each cycle if monitoring ovulatory status. During IVF luteal phase support.
How often
Each cycle when monitoring for ovulation. Not needed once ovulation is confirmed and regular.
Interpretation.
High progesterone
Common causes:
- Normal luteal phase (expected rise after ovulation)
- Pregnancy (first trimester: 11-90 ng/mL)
- Progesterone supplementation (IVF support)
- Molar pregnancy or ectopic (contextual)
- IVF: optimal is 41-60 ng/mL at fresh ET
Implications:
- Confirms ovulation occurred
- Adequate luteal support for implantation
- Very high progesterone in IVF (>60 ng/mL) paradoxically reduces live birth rates
Low progesterone
Common causes:
- Anovulatory cycle (<3 ng/mL — no corpus luteum formed)
- Luteal phase deficiency (3-10 ng/mL — weak corpus luteum)
- Incorrect timing (tested too early or too late in luteal phase)
- Hypothalamic amenorrhoea
- PCOS (anovulatory cycles)
Implications:
- <3 ng/mL = anovulatory — no ovulation this cycle
- 3-10 ng/mL = borderline — may indicate inadequate luteal support
- Recurrent low progesterone suggests chronic anovulation — investigate cause
- May benefit from progesterone supplementation in fertility treatment
Optimization.
Diet
- No foods directly raise progesterone meaningfully
- Adequate cholesterol intake — progesterone is synthesised from cholesterol
- Vitamin B6 studied for luteal phase support (evidence modest)
Lifestyle
- Stress reduction supports corpus luteum function via LH pulsatility
- Adequate sleep — progesterone has sedating effects and supports sleep architecture
- Maintain healthy weight — both underweight and overweight impair ovulation
Supplements
- Vitamin C (750mg/day) studied for luteal phase support (small RCTs)
- Vitex (chasteberry) — traditional use for luteal support; modest evidence
- No supplement replaces medical progesterone supplementation when indicated
FAQs.
Why is it called 'Day 21' progesterone if my cycle isn't 28 days?
Day 21 assumes ovulation on Day 14 of a 28-day cycle (7 DPO). If your cycle is 35 days, ovulation is around Day 21, so test on Day 28. The key is testing 7 days after ovulation, not literally Day 21. Use OPKs to identify your ovulation day if cycles are irregular.
What if my progesterone is low but I have regular periods?
Regular bleeding can occur without ovulation (anovulatory cycles). The lining builds from estrogen alone and sheds irregularly. Only mid-luteal progesterone above 3 ng/mL confirms true ovulation. This is a common and important distinction.
Should I take progesterone supplements to help fertility?
Only if prescribed. Progesterone supplementation is standard in IVF and sometimes used after IUI. For natural conception, supplementation is not routinely recommended unless luteal phase deficiency is documented. Self-supplementation with over-the-counter progesterone creams has not been shown to improve fertility outcomes.