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Why "cycle day 3" is so important in the world of fertility hormone testing

Why "cycle day 3" is so important in the world of fertility hormone testing

10 min read

Depending on how long you’ve spent getting to know the ins and outs of the fertility world, you might have heard about the concept of “day 3 testing.” In a nutshell, day 3 testing (also called “cycle day 3” or “CD3” testing) refers to the clinical standard of measuring fertility hormones via a blood test on the third day of your period.

With the Modern Fertility Hormone Test, we follow the same standards of fertility hormone testing that they do in a fertility clinic. (In fact, we’ve conducted a peer-reviewed clinical study showing that the results from our home test kit are as accurate as what you’d get from a traditional lab draw.)

Two key points before we dive in:

  1. If you’re not on hormonal birth control and you get a regular period, your Modern Fertility Hormone Test will measure what are called day 3-dependent hormones, and your testing instructions will specify that you take your sample the third day of your period.
  2. If you are on birth control, the hormones you’ll test for (and the day you’ll need to take your test) depend on the type of birth control you’re on and whether or not you get a regular period.

What’s so special about cycle day 3, anyway? In this post, we’ll break down why some hormones are day 3-dependent, how you can find your day 3, and what your options are for testing if you don’t have a period.

The biggest takeaways

  • Cycle day 3 is when many reproductive hormones are at the level doctors consider "baseline."
  • Days 2-4 are okay for testing as long as you're still experiencing flow. If you have a super-short period (lasting fewer than 3 days), testing on day 2 is the way to go.
  • Whether you get a period or not and what birth control you're on will dictate if testing day 3 hormones is an option for you right now.

First, a refresher on the menstrual cycle

So what makes day 3 the magic day for fertility hormone testing? To answer this question, let’s zoom out for a quick refresher on the menstrual cycle. Basically, you can think of your menstrual cycle as a play with three acts:

  • Act 1: The follicular phase, which includes the days you’re on your period, the maturation of ~15-20 ovarian follicles (tiny sacs that house and release eggs) and the thickening of the uterine lining.
  • Act 2: Ovulation, or when one ovary releases the most developed egg and it travels through the fallopian tube to meet up with sperm for fertilization.
  • Act 3: The luteal phase, or when your body is ready to support implantation and pregnancy if an egg is fertilized. If an egg isn’t fertilized, then you get your period (and begin the follicular phase all over again).

In each of these “acts,” certain hormones play a leading role in moving the events of the menstrual cycle forward. And, of course, as soon as the curtains close on your menstrual cycle, another performance starts up again and repeats throughout your reproductive years (until menopause). You can read the Modern guide to the menstrual cycle for more details.

Like in any good drama, individual hormones have their own character arcs and moments to shine. In the menstrual cycle, some hormones are pretty steady throughout, but others experience high highs and low lows.

Anti-Mullerian hormone (AMH), for example — which is the best hormone for measuring ovarian reserve, or your egg count — has a constant presence throughout the entire cycle. This means you can measure it on any day of the month and expect a similar result. The same can’t be said, though, for all fertility hormones. The ones that fluctuate depending on where you are in your menstrual cycle are the ones we care about measuring on day 3 specifically.

Meet your cycle day 3-dependent hormones

Estradiol, follicle-stimulating hormone (FSH), and luteinizing hormone (LH) all need to be tested on day 3 of the menstrual cycle to get the most clinically meaningful results.

As a reminder, the hormones included in your Modern Fertility Hormone Test depend on your birth control (if you take any) and the regularity of your period. You’ll test E2, FSH, and LH if you fall into any of the following categories:

  • You don’t use birth control, and you get a period.
  • You use the minipill or hormonal IUD and get a period (not just spotting).
  • You use a copper IUD and get a period.

Here’s a quick recap on the role each of these day 3-dependent hormones plays:

  • FSH: Follicle-stimulating hormone, or FSH, is produced by the pituitary gland. As the name implies, FSH is responsible for growing the follicles that hold your eggs each menstrual cycle and getting one egg ready for ovulation. FSH levels are typically at their lowest at the beginning of your cycle (the time around day 3.) As FSH levels rise, the follicles grow and ~15-20 eggs begin to mature.
  • E2: Estradiol, or E2, is a hormone produced by the ovaries and by the placenta during pregnancy. E2 plays many roles, including kickstarting the process that leads to ovulation in the first half of the menstrual cycle. FSH and E2 are typically measured and interpreted together on day 3. This is because E2 serves as the brakes for the brain’s production of FSH. E2 travels from the ovaries to the brain and signals to dial down FSH levels. If someone has an atypically low FSH, it could indicate that there’s something going on with the brain areas involved in hormone production, or it could be a result of E2 pressing the brakes too hard. In these instances, if E2 was lower, FSH would likely be within the normal range. So, E2 is important in its own right, but also for providing crucial context for understanding FSH levels.
  • LH: Luteinizing hormone, or LH, is produced by the pituitary gland. Its primary role is regulating the length of your menstrual cycle and triggering ovulation. LH is lower at the beginning of the menstrual cycle and surges right before ovulation. That’s why many ovulation tests (like the Modern Fertility Ovulation Test) measure LH to help pinpoint when ovulation is coming up. While urinary LH is a great predictor of when you’re about to ovulate, measuring this hormone via blood test on day 3 of your cycle provides more insight into your baseline LH level. Atypically low or high LH in blood test results may indicate irregular ovulation patterns.

Does it really matter if you test E2, FSH, and LH on day 3?

Since E2, FSH, and LH fluctuate quite a bit throughout the menstrual cycle, you’d likely get a totally different value for the same hormone if you tested on any other day. Doctors call day 3 the “baseline” because that’s when E2, FSH, and LH are most stable, before they begin to rise. Aiming to test on day 3 (for most people) puts you right at the beginning of the follicular phase and is the best time to get a clear read of these baseline levels.

We hear from customers that it can be a bummer to have to wait for your period to test, and we totally get that feeling. It can be hard to wait for this info! While it’s true that you can technically measure these hormones on any day of your cycle, it won’t be clinically meaningful.

Imagine you’re checking in with your cardiovascular health and measuring your resting heart rate. If you were just moving around a lot or feeling anxious for any reason, you might expect a higher-than-average heart rate. The heart rate measured at that time wouldn’t necessarily be representative of your actual resting heart rate, unaffected by movement or nerves — and it might not be the same the next time you measured it.

Testing your fertility hormones works the same way. If you measure E2, FSH, or LH on any random day, your levels could be higher or lower because of where you are in your cycle. If you used those results to track changes over time against levels measured during another day in your cycle, you wouldn’t be able to compare them in a meaningful way — they’d be like apples and oranges.

Another reason why testing your fertility hormones on day 3 is important comes down to reference ranges, which we’ll explain in the next section.

How are day 3 results interpreted?

For any kind of hormone test — including Modern Fertility’s and other fertility hormone tests — your results are interpreted using a “reference range”. Testing and interpreting hormone levels is not as straightforward as measuring something like weight or height, where you can expect to get the same reading using any measurement tool. This is because hormones are tested using extremely precise tools that need to be calibrated by each individual lab.

This is where the concept of reference ranges comes in: Reference ranges make results interpretable outside the walls of an individual lab by marking the bounds of “low/normal/high” results for people your age who were also tested at that lab. While you can’t necessarily compare the exact numerical value you get from testing your hormones at multiple labs, you can compare the range (“low/normal/high”) your results are in.

Adding to the complexity, day 3 hormones have cycle-phase specific reference ranges. So when a doctor goes to interpret your day 3 results, they’ll compare your result to what counts as “low/normal/high” for the follicular phase in premenopausal women within their lab’s reference range. Knowing where your results fall relative to the “normal” range can help you figure out if there are any indicators of irregular ovulation patterns, or conditions associated with decreased ovarian function, like primary ovarian insufficiency (POI) or menopause.

(Fun fact: There are reference ranges for mid-cycle, mid-luteal, and postmenopausal when it comes to FSH, but baseline follicular phase reference ranges are what are used in this case).

FAQs on testing day 3 hormones

How can you pinpoint day 3 of your cycle?

There are a couple of things to keep in mind when pinpointing your day 3. First, you’ll count “day 1” as the first day you experience “full flow,” not just spotting, before 5pm. You can think of “full flow” as enough flow to need a tampon, pad, menstrual cup, or other menstrual product. If you spot on Monday and Tuesday, then start getting enough flow to need a menstrual product on Wednesday, you’ll count Wednesday as day 1. This would put your day 3 as Friday.

Another handy thing to know is that unless you have a super-short period, there are actually a few days that are okay for testing. Through having (literally) hundreds of conversations with customers, we’ve found it’s easiest to have a single day to aim for — this is why we lead with talking about “day 3 testing.” But you can take your test on days 2, 3, or 4 and get accurate results, as long as you’re still bleeding. If your period is really short (you only experience full flow for one or two days), you should test on day 2.

Can you test day 3 hormones if you’re on birth control and get a period?

Some people who use a continuous form of contraception, like the minipill (progestin-only) or hormonal IUD, get regular periods. In these causes, day 3 hormones will be included in your test.

Not all periods are created equal — in fact, for some birth control methods, the “periods” you experience aren’t actually periods. If you are using the regular pill (which contains both estrogen and progestin), the vaginal ring, or the birth control patch, the bleeding you experience during the placebo days/week is actually “withdrawal” bleeding — your body’s response to the sudden drop in hormones. And because this withdrawal bleeding is caused by the changes in hormones of your birth control, it’s not a reflection of your body’s hormones during a natural period. If you’re looking to test your day 3 hormones, we recommend waiting three months after going off these birth control methods for your levels to return to non-birth control levels.

Can you test day 3 hormones if you’re on a medication-induced bleed?

Some medications, like Provera, are used to induce a withdrawal bleed for those with irregular or absent cycles. If you’ve been prescribed a medication that induces a bleed, you should talk to your doctor before taking the Modern Fertility Hormone Test.

Is fertility hormone testing valuable even if you don’t get a period?

If you’re looking for insight into your fertility but you’re on hormonal birth control, not getting a period, or your periods are so irregular you don’t want to wait for the next one to test, you can absolutely still get really valuable info from testing with Modern Fertility. For these situations, we test AMH and thyroid-stimulating hormone (TSH). AMH and TSH levels are more stable throughout the menstrual cycle and can be tested on any day of the month — even if you don’t get a period.

Here's a little more about what you can learn from these two super important hormones:

  • AMH is the best biomarker for understanding ovarian reserve, or egg quantity. AMH tells you how many eggs you have "waiting in the wings" and can give you insight into menopause timing, as well as to potential responses to procedures like in-vitro fertilization (IVF) and egg freezing. You can read more about AMH testing here.
  • TSH is a hormone that regulates the thyroid gland, which plays a role in your metabolism, heart function, nervous system, bone maintenance, mood, fertility, and even sexual function. Your TSH can give you insight into thyroid conditions like hyper- or hypothyroidism (which are marked by low and high TSH, respectively) — both of which can impact conception and pregnancy. Find out more about how your thyroid and fertility are connected here.

It’s also worth noting that your results are only one piece of what you get when you take the Modern Fertility Hormone Test. You’ll also get access to ongoing fertility education, a weekly webinar (we call them "egginars"), a 1:1 consultation with our fertility nurse, and an online community where you can connect with other people with ovaries and get your questions answered by fertility experts.

Customers sometimes ask us if they can guess when their period might have been to test their day 3 hormones. For all the reasons we’ve discussed above, it’s important to test day 3-specific hormones while you’re on your period to get clinically meaningful information.

One final take-home message for the no-period folks who aren’t able to test on day 3: If you’re not on hormonal birth control and you experience sudden cycle changes, spotting between periods, bleeding for more than a week, extremely painful/unusually heavy periods, or if you’ve gone 90 days without getting a period, it’s important to touch base with your healthcare provider. Cycle abnormalities could be a sign of an underlying health condition. If you’re concerned about your cycle, click here for more info.

Here’s the bottom line: Testing your day 3 hormones is a great tool for getting more insight into your reproductive health, and collecting your sample at the right time is essential for getting clinically meaningful results. That’s why it’s especially handy to have your test at home so you can test when the timing works for you — instead of waiting for your next period and rushing to schedule a doctor’s appointment. With the Modern Fertility Hormone Test, you’ll have everything you need to test your fertility hormones at home!

This article was medically reviewed by Dr. Sharon Briggs, Modern Fertility's head of clinical product development.

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Jen Lehr

Jen is a member of Modern Fertility's content team. She's a distance runner, outdoor adventure enthusiast, and humbly claims to have mastered the art and science of snacking.

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