Progesterone Blood Test (Day 21 Ovulation Check)

Progesterone (P4) is a key hormone produced mainly by the corpus luteum after ovulation and, in pregnancy, by the placenta. This simple blood test measures the level of progesterone in your bloodstream to help assess whether ovulation has occurred, evaluate luteal phase function, and support the investigation of fertility concerns, menstrual irregularities, and early pregnancy viability. It is commonly requested by fertility specialists, gynaecologists, and GPs as part of a broader fertility workup, and is often timed to a specific day of your cycle to give the most meaningful result.

What this test measures

• Progesterone (P4): The primary progestogenic hormone. After ovulation, rising progesterone prepares and maintains the uterine lining for possible implantation. A single measurement can indicate whether ovulation has taken place and how well the corpus luteum is functioning.

Why it matters

Progesterone is central to ovulation, implantation, and the maintenance of early pregnancy. Low or absent progesterone can point to anovulation, a short luteal phase, or other hormonal imbalances that may affect fertility. Measuring P4 at the right point in your cycle gives you and your clinician clear, actionable information to guide diagnosis, treatment, and next steps in your fertility journey.