We know that sex hormones fluctuate substantially throughout the menstrual cycle, but are other blood biomarkers influenced by your period, too? Yes—research shows that many blood biomarkers (including those in your iron, lipid, and inflammation groups) can vary substantially according to menstrual cycle phase. And these fluctuations can have important implications when interpreting blood test results. In general, it's best practice to get blood tests done at the same point in your menstrual cycle each time for consistent comparisons.
A woman’s period can be a very personal topic.* But here at InsideTracker, we strive to provide evidence-based science to empower individuals to take control of their health—so let's talk about it. In this article, we’ll take a look at why a regular cycle is important and how blood testing with InsideTracker can be best executed according to your menstrual cycle.
A summary of the natural hormonal changes at each menstrual cycle phaseA woman's menstrual cycle has four different phases—the menstrual, follicular, ovulation, and luteal phases—but they are often broken down more broadly into just the follicular and luteal phases.
First comes the menstrual phase, when a woman gets her period and her levels of the hormones estrogen and progesterone drop. This typically lasts 3-7 days but can vary between individuals.
This process also kicks off the follicular phase, which begins on day one of a woman’s period and continues until ovulation, for a total of about 16 days. During the follicular phase, the pituitary gland releases follicle-stimulating hormone (FSH)—this is essentially the time in which the ovaries are preparing to release an ovum (AKA “egg”).
Finally, we enter the luteal phase, which begins with ovulation. During the ovulation phase, peak estrogen levels trigger a spike in luteinizing hormone (LH), which in turn stimulates the release of a mature ovum from the ovary. Wondering if you’re ovulating? One sign is a slight rise in body temperature, which happens around day 14 of your cycle.
After ovulation, the luteal phase is characterized by a slight rise in estrogen levels and a more pronounced one in progesterone, followed by a drop in both hormones and the restart of the cycle (barring pregnancy). The luteal phase is when women experience PMS symptoms, like bloating, headache, weight changes, food cravings, and trouble sleeping. This phase lasts 11-17 days.
Why is a regular menstrual cycle important?Each of these phases contribute to what's characterized as a regular cycle, and is a good indicator that a woman’s body is working normally and has the physiologic ability to become pregnant. An irregular menstrual cycle can make it more difficult for a woman to become pregnant. And even if pregnancy is not in your short or long-term goals, a regular monthly cycle is critical for overall health as a woman—beyond impacting reproductive health, an irregular cycle is associated with other health problems including anemia, depression, osteoporosis, heart disease, and stroke.
How does the menstrual cycle affect blood biomarkers?Some of the biomarkers included in InsideTracker blood tests can be impacted differently based on the menstrual phase a woman is in at the time of the blood draw. Here’s what it means for your results.
IronConventional wisdom (and multiple research studies) has suggested that blood loss during menstruation is the major cause of lower iron levels in women. Typically, women lose about 16mg of iron for the average menstrual period, though it can be as high as 36mg or above for women with menorrhagia (clinical excessive menstruation). Given that a woman’s average body iron levels tend to be around 3,000-4,000 mg, the average woman’s menstrual blood loss equates to about 0.4–0.5% of total body iron. But this seemingly negligible amount does indeed have the power to significant impact markers of iron status in healthy, menstruating women.
Evidence from one study shows that meaningful clinical markers, “including hemoglobin, ferritin, and percent transferrin saturation, tend to be lower in blood collected from menstruating women in comparison to blood collected from women in their luteal phase.” In other words, critical markers of iron status can be markedly lower during your period. Other studies, however, have come to conflicting conclusions—that the menstrual cycle has no effect on the above clinical markers in iron-depleted women.
Regardless, it is advised that premenopausal women reach a dietary iron intake of 18mg per day to compensate for menstrual blood losses. (NIH) If you’re a premenopausal woman, you exercise regularly and/or adhere to a vegan or vegetarian diet, you should pay particular attention to iron levels—this blog goes deeper into the reasoning.
The takeaway: Your iron group blood biomarkers may be slightly lower during menstruation—and can result in lower levels if you get a blood test during this time.
Research has shown that lipid levels can fluctuate throughout a woman’s menstrual cycle as a normal response to the fluctuations in estrogen levels. Draper et al. showed significant reductions in total cholesterol, HDL (good) cholesterol, and triglyceride concentrations in the luteal phase relative to the follicular phase, which is in agreement with previous research on the topic. In addition, women who were undergoing estradiol treatment (but not progesterone) saw a 30% reduction in triglycerides in the luteal phase.
Figure 1: Reprinted from “Mumford, Sunni L et al. “Variations in lipid levels according to menstrual cycle phase: clinical implications.” Clinical lipidology vol. 6,2 (2011): 225-234. doi:10.2217/clp.11.9
The takeaway: Due to the known fluctuations in lipid levels, it is best to test at the same phase of your cycle each month for consistent results. Caution is warranted when interpreting cholesterol and triglyceride blood test results that traverse menstrual cycle phases.
Vitamin D levels can also be affected by cycle phase—in fact, vitamin D has consistently shown to be higher during a woman's period than other phases of the menstrual cycle. Interestingly, researchers have found that vitamin D supplementation can improve PMS symptoms in some women.
The takeaway: Like other biomarkers, vitamin D levels will fluctuate throughout your cycle. If you suffer from PMS and your blood test results show lower levels, a vitamin D supplement may help relieve symptoms.
A prospective cohort study among 259 healthy menstruating women ages 18-44 (note: none were taking oral contraceptives) showed that levels of hsCRP varied significantly across the menstrual cycle. HsCRP was highest during menstruation, decreased steadily during the follicular phase, reached its lowest on the expected day of ovulation, and increased back to baseline in the luteal phase. Symptoms of PMS (observed in the luteal phase) were also associated with elevated CRP. [2,4,5]
The takeaway: HsCRP levels will fluctuate throughout each phase of the cycle, and the timing of blood tests should be taken in consideration when looking at results. If PMS is an issue, focus on anti-inflammatory diet and lifestyle interventions, as they may help to relieve symptoms.
How can blood tests reflect sex hormone levels if they fluctuate significantly across the cycle?We are often asked why InsideTracker blood tests don’t evaluate estrogen, follicle-stimulating hormone (FSH), luteinizing hormone (LH), or T4. At this time, medication is presently the only intervention that can meaningfully impact these hormones, and InsideTracker does not provide medical recommendations. In addition, the levels of each of these hormones fluctuate substantially each month due to a woman's menstrual cycle.
For a clearer, more stable picture of a woman's reproductive health, we test sex hormone-binding globulin (SHBG) and an estrogen precursor called DHEAS. You can read more about each of these biomarkers and how they relate to reproductive health in this blog.
InsideTracker promises to help individuals monitor, improve, and ultimately optimize their well-being through personalized diet, exercise, supplements, and lifestyle recommendations. As such, we only test biomarkers scientifically proven to be affected by such interventions. For more on how our science team chooses the biomarkers found in our blood tests, check out this blog.
How will hormonal contraceptives affect my results?Hormonal contraceptives can impact a number of biomarkers, even after you stop taking them. We recommend checking out this detailed blog on how birth control can affect your biomarkers to get a more in-depth picture of their impact.
- Regardless of phase, we always recommend that blood tests be done at the same point in a woman's menstrual cycle for consistent comparisons.
- Cholesterol levels are more likely to be stable during a woman’s menstrual period. This phase of the cycle is also more easily identified than the others, making clear the ideal window for a blood test. This is particularly true if your lipid profile is of concern.
- Vitamin D levels will fluctuate throughout the menstrual cycle. Testing in the same phase of your cycle will help show consistent results.
- If you suffer from PMS, you may benefit from anti-inflammatory diet and lifestyle interventions, as elevated hsCRP levels are associated with PMS. Vitamin D supplementation may also help relieve symptoms if blood levels are low.
- Overall, the evidence indicates that certain markers of oxidative stress, inflammation, and cholesterol vary across the menstrual cycle in healthy, premenopausal women with a regular cycle.
*We acknowledge that not everybody who menstruates identifies as a woman.
Stevie Lyn Smith, MS, RDN, CSSD, CDN
- Stevie Lyn is a Content Strategist and Team Nutritionist at InsideTracker. As a Registered Dietitian and Ironman triathlete, she enjoys combining her passions to help educate others on how to fuel for overall health and performance. When she’s not swimming, biking, or running with her dog, you’ll find her in the kitchen working on a new recipe to improve her biomarkers.
 Belza A, Henriksen M, Ersbøll AK, Thilsted SH, Tetens I. 2005. Day‐to‐day variation in iron‐status measures in young iron‐deplete women. Br J Nutr 94:551–556.