Why Should You Get Hormone Tests on Day 3 of Your Cycle?

I have been super-irregular since having a D&C procedure after my miscarriage. After menstruating while ovulating for the past few months, my doctor wants me to run some tests on the third day of my cycle. What is it about the third day, and what will my doctor be looking for? I’d love to get pregnant again. I’m 35 and have PCOS.

—Anonymous

One reason it is difficult to check reproductive hormones is because they are supposed to fluctuate throughout your menstrual cycle. While we have ranges for hormones including estrogen and progesterone at different points in the menstrual cycle, at some points in the cycle those ranges are quite large. And if we don’t know on what day of your cycle the lab tests were drawn, it can be very difficult to interpret the results. For example, progesterone levels should be low before ovulation and rise after ovulation. The levels we are interested in are most reliable early in the follicular phase, on day 3.

Your doctor will likely be looking at a series of different tests, such as estrogen, progesterone, follicle-stimulating hormone, luteinizing hormone, prolactin, testosterone, sex hormone-binding globulin, and anti-Mullerian hormone. Each of these lab tests can tell us about different hormones and glands that play a role in the menstrual cycle and whether they are functioning normally. Taken together, they give us clues as to why a woman’s cycles may be irregular. The entire panel together will help your doctor to determine how you might best proceed.

That series of tests can tell us whether your hypothalamus and pituitary are functioning properly to drive egg maturation and ovulation. It can provide information about the quality and number of eggs being recruited for possible ovulation. Your doctor will also get a sense of whether you are producing enough estrogen to build a good uterine lining. On day 3, there are some hormones that we expect to be quite low, in particular progesterone, which should be elevated only after ovulation.

My patients are often concerned about which day is day 3. When exactly does the clock start? Day 1 is defined as the first day of full menstrual flow. While certainly the goal is to be as exact as possible about the timing of the lab tests, different people might identify day 3 slightly differently.

My patients also often worry that day 3 will fall on a Sunday or holiday, when a lab near them might not be open. When I am doing initial testing, I am happy to have results from blood tests drawn on day 4 rather than wait another several weeks to get day 3 labs. But there are times when precision is critical. If that is the case, your doctor should be able to direct you to a lab that has weekend or holiday hours. You should ask your doctor how they would want you to handle this particular problem.

Ultimately, we draw hormone lab tests on day 3 of your menstrual period because levels at those times can tell doctors the most about why your menstrual cycle may be irregular and the quality and quantity of eggs being prepared for ovulation during that cycle.