Why "Day 21" testing is sometimes inaccurate

During a fertility work-up, or any hormone work-up, day 3 and day 21 hormone testing is usually a great place to start. Day 3 means the third day of your bleed (day 1 meaning the first day of your bleed, not including spotting). The hormones tested are typically estradiol, progesterone, FSH, LH, prolactin, and testosterone. These tests can start to paint a picture of a women’s hormonal state.

The one test that doesn’t provide much information on your day 3 is progesterone. That is because it is the main hormone secreted in the luteal phase of your cycle (post ovulation). You don’t expect to see much of it in your system until you ovulate.

That is why progesterone testing is usually done on Day 21, 1 week after ovulation. This assumes that you ovulated on Day 14 and that you have a regular cycle every 28 days. This is rarely the case in most women. What if your cycles are 35 days long and you tend to ovulate on Day 19. Testing Day 21 may give an inaccurate reading of your peak progesterone level.

So when should progesterone testing be done? The most accurate time to test is 7 days post ovulation, regardless of how long your cycle is. For example, if you ovulate on Day 17, your progesterone should be tested on Day 24, this will provide a much more accurate level of your peak progesterone.

Why is progesterone testing important? It can determine if ovulation is occurring or if progesterone levels are not in optimal ranges to conceive. Progesterone is one of the main hormones for sustaining a pregnancy. It maintains the uterine lining and ensures a healthy environment for implantation.

It can also be a great tool when timing conception or any assisted reproductive techniques (IUI, IVF, etc). It is also used as a natural contraception tool, indicating your fertility window (when to avoid intercourse).

Progesterone is an important hormone in any women’s case, not just in fertility. Low levels can also contribute to premenstrual symptoms, hormonal acne, mood swings, and estrogen dominant conditions (endometriosis).

You always want to ensure hormone testing is done at the appropriate time in YOUR cycle, not what the standard testing says. If not done properly, certain conditions can be missed or an inaccurate diagnosis could be made. Proper testing is an important factor in the diagnosis of your infertility.

IN good health,

Dr. Kirti Deol