The CD 3 tests-how important are they and what do they predict?

If you are familiar with fertility treatments or if you just had an assessment of your ovarian reserve because you are trying to conceive, the term "3 day tests" rings a bell to you.

Day 3 testing (can also be done and day 2 or 4) consists of bloodwork that measures your hormone levels (FSH, Estrogen, Progesterone, LH) and a scan, meant to count and measure your Antral Follicles. Your AMH level can be measured anytime and if you need to have it measured more than once, it is preferred to do so using the same laboratory, because measurement scales vary for every lab.

E2 (Estrogen) is the main female reproductive hormone. It is being secreted by the ovary and helps to stimulate follicle growth and prepare the lining for implantation, in case conception occurs. The majority of the fertility clinics would want you to have an E2 level under 50 (some under 80) on your day 3. Too high an estrogen level on this day might suggest you have an estrogen producing cyst, in which case stimulation is usually not recommended. Feeding it stims, the cyst might not only "eat up" the meds destined for your other normal sized antrals, but also grow and grow until it bursts. A too low estrogen level is not ideal either, suggesting diminished ovarian reserve and possibly  peri/premenopause. Also, very important, when your Estrogen levels are high, the value of the FSH is artificially lowered. Hence the importance of always measuring the two of them.

FSH (Follicle-stimulating hormone) as the name tells us, is the hormone that stimulates the ovary to make the eggs grow. Released by the brain, the FSH tends to get higher and higher as we age, and our ovaries struggle more and more to produce an egg. The ideal FSH level is under 10, but the lower the better. Some clinics would accept you for fertility treatments with an FSH under 15, but there are many studies out there showing success rates diminish seriously as 3 day FSH levels increase. That might explain why older women with high FSH have better chances getting pregnant naturally than with IVF treatments. This being said, if you need help to procreate, there are lots of clinics who offer natural IVF for women with high FSH-meaning no meds, egg retrieval for possibly just one egg, and fertilisation as needed (normal or ICSI, IMSI)

LH (Luteinising hormone) helps mature the follicle and eventually, when an LH surge occurs in the end of the follicular phase, helps the release of the mature follicle. The ideal level is under 7 mIU/ml with a ratio LH:FSH of 1:1. An LH much higher than the FSH might be an indication of PCOS (Polycystic Ovary Syndrome).

P4 (Progesterone) should remain low during the follicular phase (under 1ng/ml) and rise after ovulation, as proof the ovary released the egg. The low limit used as indicator for ovulation at 7dpo is 5, but the higher the better. Some women with low progesterone might need progesterone supplementation in order to maintain pregnancy.

AMH (Anti Mullerian Hormone) is a free circulating hormone released by the small antral follicles present in your ovaries, and it is used to assess your ovarian reserve, as in "how many eggs do you still have". Taken alone, it doesn't amount for much, and it is far more reliable when discussed in conjunction with the other day 3 levels, and most important, with the day 3 scan. Also, it is important to know that the AMH level has been proven to be artificially lowered by low Vitamin D levels.

Ovarian ultrasound/scan: it is meant to count and measure the antral follicles. The antral follicles are small follicles (between 2-10 mm) found in your ovaries at the beginning of the follicular phase. They are an extremely important and very useful assessment of the way your body might respond to fertility treatments. Each antral contains an immature egg that might develop and ovulate. During natural cycles, the body recruits what is thought to be the best follicle, and makes it grow and eventually ovulate once the Estrogen level is high enough (200-600 E2 level/mature follicle) and the LH surge occurs. In stimulated cycles, all antrals have potential to grow, and even sometimes, some more follicles pop up during stims.

Those are the main tests performed during the day 3 assessment. The list is not exhaustive, though. Depending on your clinic and your health issues, you might have your prolactin and thyroid levels checked, or any other test your doctor might consider appropriate.

Unfortunately, there are some clinics who perform this testing once a year, and consider it available in subsequent cycles. Whether for logistics or financial reasons, this is a bad idea. Hormones fluctuate every God given month, and once you have your period, they are reset and you start the new month with a clean slate.

It is totally possible to have an FSH of 6 in January, and an FSH of 14 the next month, and you surely won't have the same response to meds during those two months if you are to cycle. Hence the importance of demanding those 3 day tests at the beginning of each and every cycle using stimulation meds, to spare you the heartbreak, the false expectations and the waste of money.

Sources:

https://www.fertstert.org/article/S0015-0282(14)02518-7/fulltext

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3719363/