When Is the Best Day to Take a Hormone Test? A Complete Guide to Cycle Day 3 Testing

If you’ve ever been told to get bloods done on day 3 and wondered why that specific day matters, or felt frustrated trying to time a test around an unpredictable cycle, this guide is for you. We’re going to explain not just when to test, but why the timing matters at a biological level, what each hormone is actually measuring, and what to do when life doesn’t cooperate with your cycle.

First: What does “day 3 of your cycle” mean?

Day 1 of your cycle is the first day of your period, this means full menstrual flow, not spotting. If you see light spotting on Monday and Tuesday but don’t experience a proper flow until Wednesday, Wednesday is your day 1. Count forward from there: day 3 is the third day of your period (in this scenario that would be Friday). If your period is less than 3 days, day 3 is the third day after your period starts. 

This matters because starting the count from spotting rather than full flow is one of the most common reasons people test at the wrong time.

How does the menstrual cycle affect hormone levels?

To understand why day 3 matters, it helps to have a basic picture of what’s happening in your body across the menstrual cycle.

Your menstrual cycle is divided into two main phases separated by ovulation. The follicular phase always begins on day 1 of the menstrual cycle and ends with ovulation. In a 28-day cycle, the follicular phase extends from day 1 to approximately day 14. The luteal phase then follows ovulation and typically lasts 14 days, ending when your next period begins.

The follicular phase is a period of rapid hormonal change, making it significant for hormone testing. When the previous menstrual cycle completes, levels of oestrogen and progesterone decrease. This triggers the release of follicle-stimulating hormone (FSH) into circulation. 

Therefore, the days right around day 3 are when your body’s hormone system essentially resets and returns to its baseline. This is precisely why it’s the ideal time to take a snapshot of your reproductive hormones.

Which hormones can be tested on day 3,
and what does each one tell us?

At Hertility, we test a broader panel than many providers. Here’s a detailed breakdown of each hormone in our Advanced At-Home Hormone and Fertility Test and why its timing matters.

The Cycling Hormones

These are tested on day 3 as mentioned earlier because they are at their ‘baseline’ around the first few days of your cycle.

FSH (follicle-stimulating hormone)

FSH is made by the pituitary gland in the brain and is the primary driver of egg development. FSH stimulates the production of oestradiol and eggs (oocytes) during the first half of the menstrual cycle. 

Your FSH on day 3 might tell us whether the body is working as we would expect, or a little bit harder to induce follicular growth which may indicate reduced ovarian reserve, suggesting the egg supply could be beginning to decline. 

Oestradiol (E2)

Oestradiol is the primary form of oestrogen produced by the ovaries, and it plays a complex, interconnected role with FSH. Oestradiol serves as the brakes for the brain’s production of FSH. It travels from the ovaries to the brain and signals it to dial down FSH levels. 

This is why FSH and oestradiol are always measured together. Not only their results, but their interpretation relative to each other is important for our clinicians to determine whether there is anything going on. 

LH (luteinising hormone)

LH is best known as the hormone that surges dramatically at mid-cycle to trigger ovulation. But measuring it at baseline on day 3 also tells us something important. If LH is too high on day 3, it may signal a condition like polycystic ovary syndrome (PCOS). An elevated LH:FSH ratio in the early follicular phase is one of the hormonal patterns clinicians look for when investigating PCOS and irregular ovulation.

AMH (anti-Müllerian hormone)

AMH is one of the most valuable markers for assessing ovarian reserve, and it works quite differently from the cycling hormones. 

Historically, it has been thought that AMH doesn’t fluctuate dramatically across the cycle in the same way, so could be measured at any point during the menstrual cycle. However, research does suggest there may be some variation, which is why at Hertility, we standardise AMH testing to the days 2-5 window. This allows us to negate any potential fluctuation and ensure our results are consistent and comparable over time.

AMH tells us about egg quantity (how many follicles are available) but it’s important to note it doesn’t directly measure egg quality. It should always be interpreted alongside your other results and your clinical history.

Thyroid hormones (TSH and free T4)

Thyroid hormones don’t fluctuate with the menstrual cycle, so strictly speaking they don’t need to be tested on day 3. We include them in the same panel because thyroid dysfunction, both overactive and underactive thyroid can significantly disrupt ovulation, cycle regularity, and fertility outcomes. Testing them alongside your reproductive hormones gives a more complete picture of your overall hormonal health in a single sample.

Androgens (including testosterone)

Androgens like testosterone are relatively stable across the menstrual cycle, making cycle timing less critical for these markers. That said, testing during the early follicular phase, when oestrogen is at its lowest means androgens aren’t being masked or influenced by rising oestrogen levels. For women investigating conditions such as PCOS, elevated androgens are an important part of the diagnostic picture.

Prolactin

Prolactin can technically be tested on any day. What does affect prolactin is the time of day and lifestyle factors. Prolactin naturally rises during sleep and can remain elevated for some hours after waking. Stress, physical activity, and even eating can temporarily raise levels. This is why Hertility asks you to take your sample first thing in the morning, before eating or exercise, to capture the most stable reading.

The science behind day 3 testing:
what does the research actually say?

Day 3 testing has been the clinical standard for decades, and the evidence supporting the days 2-5 window is robust.

A landmark study  examined whether it mattered exactly which day within the early follicular window you tested, measuring FSH and oestradiol variability across days 2-5. The researchers found no clinically significant differences between days and concluded that flexibility within this window was scientifically justified, while confirming the early follicular phase as the correct time to test.

The World Health Organisation’s 2025 guidelines on the prevention, diagnosis and treatment of infertility recommended antral follicle count (AFC), AMH, or day 2/3 FSH testing as the gold standard approaches for ovarian reserve assessment. This reflects the global clinical consensus built over several decades of research.

Most fertility treatment protocols also begin ovarian stimulation around days 3-5, which is why clinicians have accumulated decades of reference data from this window. Testing at the same point in the cycle ensures your results can be meaningfully compared to those established reference ranges.

Why can’t you test your hormones
on any day of your cycle?

The short answer is that the cycling hormones – FSH, LH, and oestradiol – shift dramatically depending on where you are in your cycle. At ovulation, LH surges to many times its baseline level. Oestradiol peaks in the late follicular phase before dropping again. FSH is suppressed once a dominant follicle starts producing oestrogen.

If you measure FSH, LH, or oestradiol on a 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. 

Equally important is the matter of reference ranges. Laboratories establish ‘normal’ values for these hormones based on the cycle phase. If you test at the wrong time, your result may fall outside the expected range simply because of where you are in your cycle rather than because anything is actually wrong. Or conversely, a real issue could be masked by the hormonal environment of a different cycle phase.

Testing outside the days 2-5 window doesn’t make results worthless, but it does make them harder to interpret accurately and impossible to compare meaningfully over time.

What do abnormal day 3 results mean?

It’s important to remember that a single result outside the typical range doesn’t automatically indicate a problem, this is why it is always standard clinical practice to get a retest done. Hormone levels can vary from cycle to cycle, and a range of factors including illness, stress, recent travel, and sleep disruption can affect results. Our doctors will always interpret your results in the context of your full panel, your symptoms, your age, and your medical history.

Can you test your hormones while
on hormonal contraception?

Hormonal contraception, including the combined pill, the mini-pill, the patch, the injection, and the hormonal coil (IUS) works in part by suppressing the natural cycling of FSH, LH, and oestradiol. Because these hormones are suppressed, baseline testing for them isn’t meaningful while you’re using hormonal contraception.

However, several important hormones are less affected by contraception and can still be tested on any day including thyroid hormones, AMH, androgens and prolactin can similarly be assessed.

If you’re on hormonal contraception and want a picture of your reproductive health, Hertility can still provide meaningful insights, your test can be taken on any day of your cycle, and our clinical team will interpret your results in the context of your contraception use.

Hertility’s approach to hormone testing

At Hertility, we don’t just look at one or two hormones in isolation. We assess a comprehensive panel in the context of your medical and lifestyle history that gives a fuller picture of your reproductive health than a standard GP blood test may be able to provide.

Our recommendation is straightforward:

If you’re not on hormonal contraception, aim for cycle day 3. Days 2 and 4 are also valid. If you miss the window, simply test the next cycle, there’s no rush, and the results will be just as reliable. If you’re on hormonal contraception, you can test on any day.

Everything we do is grounded in clinical guidelines, peer-reviewed research, and the understanding that reproductive health is personal. A result is never just a number, it’s a data point that our clinical team interprets in the context of your whole picture.

Frequently asked questions

How do I count cycle days accurately? 

Day 1 = the first day of full menstrual flow, not spotting. If spotting precedes your full flow, don’t count those days. Start your count when you have a proper bleed, enough to need a pad, tampon, or menstrual cup.  If your period is less than 3 days, day 3 is the third day after your period starts. 

What if my periods are very irregular?

Irregular cycles can make timing tricky, but it doesn’t mean you can’t get meaningful results. Our clinical team has extensive experience interpreting results from women with irregular periods, PCOS, and other conditions that affect cycle regularity. We’ll guide you on the best approach.

Can I get a hormone test if I’m on the pill? 

Yes, though the interpretation differs from someone not on hormonal contraception. We won’t assess your cycling hormones (FSH, LH, oestradiol), but we can still look at thyroid function, androgens, AMH, and prolactin. Your sample can be taken on any day.

Does the time of day matter for hormone testing? 

For most hormones, the time of day has minimal impact. The key exception is prolactin, which is affected by sleep, stress, and physical activity. For this reason, Hertility recommends taking your sample first thing in the morning, before eating or exercise.

How often should I retest my hormones? 

This depends on your situation. Some women benefit from annual monitoring; others may retest sooner if a result came back outside the expected range, if their circumstances have changed, or if they’re actively planning a pregnancy. Our clinical team can advise based on your individual results.

Will stress affect my results? 

Acute stress can transiently affect prolactin levels, which is one reason we recommend morning testing. Chronic stress can also influence cycle regularity, which may indirectly affect the timing of your test. Let us know about anything significant that might be relevant when you submit your sample.

References

• Leslie M. Hansen, Frances R. Batzer, Jacqueline N. Gutmann, Stephen L. Corson, Maureen P. Kelly, Benjamin Gocial, Evaluating ovarian reserve: follicle stimulating hormone and oestradiol variability during cycle days 2-5, Human Reproduction, Volume 11, Issue 3, 1 March 1996, Pages 486–489, https://doi.org/10.1093/HUMREP/11.3.486
• Khodavirdilou, R., Pournaghi, M., Rastgar Rezaei, Y. et al. Does Anti-Müllerian hormone vary during a menstrual cycle? A systematic review and meta-analysis. J Ovarian Res 15, 78 (2022). https://doi.org/10.1186/s13048-022-01006-z 
• WHO, 2025. Guideline for the prevention, diagnosis and treatment of infertility. [Available online]. Accessed via: https://www.who.int/publications/i/item/9789240115774