3 Labs to Ask For After a Miscarriage (That Your Doctor Might Not Mention)
Miscarriage Support · Evidence-Based
3 Labs to Ask For After a Miscarriage (That Your Doctor Might Not Mention)
The standard advice is often “this happens, keep trying.” Here’s what the research actually says about three overlooked labs, and why they’re worth advocating for.
Summary
After a pregnancy loss, the standard advice is often “this happens, keep trying.” But three specific labs — vitamin D, thyroid antibodies, and progesterone — are backed by research as meaningful, actionable factors in miscarriage risk, and they’re frequently left out of a standard post-loss workup. Here’s what the evidence actually shows about each one, and why a normal-looking result on one test (like TSH) doesn’t rule out a problem on another (like thyroid antibodies).
If you’ve experienced a miscarriage, you’ve probably heard some version of: this is common, it happens, just try again. And while it’s true that many losses are due to factors outside anyone’s control, there’s a meaningful category of miscarriage risk that comes down to things that can actually be tested and, in some cases, treated. Three labs in particular are worth asking for directly, especially if your provider hasn’t brought them up.
1 Vitamin D
Vitamin D deficiency is common, and the research connecting it to pregnancy outcomes has grown substantially in recent years. A review by Tommy’s National Centre for Miscarriage Research analyzing existing studies found that women with low vitamin D levels are at significantly increased risk of miscarriage. A broader umbrella review of pregnancy outcomes similarly found that vitamin D deficiency in pregnancy is associated with increased risk of several complications, including recurrent miscarriage.
It’s worth being balanced here: the research isn’t unanimous. A 2024 Mendelian randomization study specifically looking for a causal link between vitamin D and miscarriage found minimal association, and the same Tommy’s review noted that while the deficiency-risk link is well established, there isn’t yet enough trial data to confirm that supplementing vitamin D before conception directly reduces miscarriage risk.
2 Thyroid Antibodies — Even With a “Normal” TSH
This is a big one, and it’s often missed because a standard thyroid workup stops at TSH (thyroid-stimulating hormone). But TSH and thyroid antibodies measure different things. A meta-analysis of 22 studies found a significant association between thyroid peroxidase antibodies (TPO-Ab) and recurrent miscarriage, with antibody-positive women showing more than double the risk. Multiple studies have also found that thyroid antibody-positive women had a higher average TSH than antibody-negative women, even when both were technically within the “normal” reference range, meaning antibody status can carry additional risk information that TSH alone doesn’t capture.
A related study of nearly 900 women found that half of the thyroid antibody-positive group had a history of prior miscarriage, compared to only 14% in the antibody-negative group. So a normal TSH result on paper doesn’t rule out immune-mediated thyroid involvement in pregnancy loss — thyroid antibodies are a distinct, separately worthwhile test.
3 Progesterone — With an Important Timing Caveat
The evidence on progesterone is more nuanced than “just take progesterone,” and it’s worth understanding the actual research landscape. The PROMISE trial found no overall benefit of progesterone for women with a history of unexplained recurrent miscarriage. But the follow-up PRISM trial — which studied over 4,000 women with early pregnancy bleeding specifically — found a different pattern: women with a history of one or more prior miscarriages who were currently bleeding saw a meaningfully higher live birth rate with progesterone treatment, and that benefit grew larger for women with three or more prior losses, with live birth rates of 72% on progesterone versus 57% on placebo in that subgroup.
This lines up with a clinically useful takeaway: progesterone testing and support appears most relevant for early losses (generally before a heartbeat is confirmed, roughly the 8-week mark) and especially for anyone with a pattern of short luteal phases, early spotting, or chemical pregnancies — not as a blanket recommendation for every pregnancy loss regardless of timing or symptoms. Testing is ideally done 7 days post-ovulation in a subsequent cycle, not on a fixed calendar day.
A Note on Why This Matters
None of this is about assuming every loss has a fixable cause — some don’t, and that’s genuinely not anyone’s fault. But for the losses that do involve an identifiable, modifiable factor, not testing for it means missing a real opportunity to change the outcome next time.
Know Your Numbers
Want to Know Exactly Which Labs to Run — and What Your Numbers Should Actually Look Like?
Most lab ranges you get back are just population averages, not numbers optimized for fertility or a healthy pregnancy. Fertility Labs 101 walks you through the full lab workup used with fertility clients — for both partners — so you can understand what’s being tested, why, and what the optimal ranges actually are, not just whether you fall in a wide “normal” bracket.
Get Fertility Labs 101 →Sources & Further Reading
- Tamblyn, J., et al. Vitamin D deficiency and miscarriage risk: a review. Fertility and Sterility / Tommy’s National Centre for Miscarriage Research, 2022.
- Effects of vitamin D in pregnancy on maternal and offspring health-related outcomes: an umbrella review of systematic reviews and meta-analyses. Nutrition & Diabetes, 2024.
- Vitamin D and miscarriage risk: a Mendelian randomization study. Human Reproduction Online, 2024.
- Xie, J., et al. Effect of antithyroid antibodies on women with recurrent miscarriage: a meta-analysis. American Journal of Reproductive Immunology, 2020.
- Thyroid Antibodies and Miscarriage: Where Are We at a Generation Later? PMC, review article.
- Bagis, T., et al. Thyroid antibody positivity and prior miscarriage in pregnant women (cohort of 876 Turkish women).
- Coomarasamy, A., et al. A Randomized Trial of Progesterone in Women with Bleeding in Early Pregnancy (PRISM trial). New England Journal of Medicine, 2019.
- Micronized vaginal progesterone to prevent miscarriage: a critical evaluation of randomized evidence. American Journal of Obstetrics and Gynecology, 2020.
There’s a 50% Chance Male Fertility Is Keeping You From Getting Pregnant
Half of all infertility cases involve a male factor. Yet sperm testing is still the last thing most couples are told to do — and the numbers being called “normal” are far from it.
- Male factor is involved in 50–70% of infertility cases — yet sperm testing is still routinely the last step, not the first.
- The WHO’s lower limit for “normal” sperm concentration is 15 million/mL — the 5th percentile. At that level, estimated per-cycle pregnancy odds are around 5%.
- Optimal sperm concentration sits at the 50th percentile of men who successfully conceived: approximately 60–73 million/mL.
- Below 30 million/mL, pregnancy probability drops off sharply. Above 50 million/mL it plateaus near the natural per-cycle ceiling of 20–25%.
- Sperm concentration is not fixed. A targeted, data-driven approach can produce significant improvement — often within 3–6 months.
When a couple struggles to conceive, the instinct — in clinical settings and at home — is to start with her. Her cycle, her hormones, her ovulation. This is understandable. It is also costing couples months, and sometimes years, they do not need to lose.
The data is unambiguous: male factor is involved in somewhere between 50 and 70% of infertility cases. Not as a rare secondary concern — as a primary driver, half the time. Yet semen analysis is still treated as the test you run after everything else has been checked, if it gets run at all.
What the statistics actually show
Infertility is clinically defined as 12 consecutive months of trying to conceive without a pregnancy. When researchers break down the causes, the picture looks like this:
This does not change based on prior pregnancies. Sperm quality and concentration shift over time, affected by age, health, stress, illness, and lifestyle. A previous successful conception — in this relationship or a past one — is not a current result. Testing still matters.
Why “normal” doesn’t mean what it sounds like
The reference range most clinics use comes from WHO guidelines. Under those guidelines, a sperm concentration of 15 million per millilitre is considered normal. A man who hits that number is typically told everything looks fine.
What that framing leaves out: 15 million/mL is the 5th percentile. It was derived from a dataset of men who did achieve pregnancy with their partners within 12 months — but the threshold was drawn at the very bottom of that group. Ninety-five percent of those men had better numbers. The threshold was not placed where success is likely. It was placed at the floor of the range where it has occasionally been shown to work.
At 15 million/mL, the estimated per-cycle probability of conception is approximately 5%. That is the number being rubber-stamped as acceptable in clinical semen analysis reporting.
The 50th percentile of that same dataset — the midpoint of men who successfully conceived — sits around 73 million per millilitre. A practical target for optimisation is above 60 million/mL. That gap, between what clears the bar and what actually reflects fertility potential, is not a minor technical distinction. It is where most couples who are told “nothing is wrong” are quietly losing time.
Where the real drop-off happens
Pregnancy probability by sperm concentration is not a linear curve. The steepest decline happens below 30 million/mL — which is still well above the WHO minimum threshold that triggers no clinical concern.
Above 50 million/mL, per-cycle pregnancy probability plateaus near 20–25% — a ceiling that reflects biological limits, not sperm count. Getting from 60 million to 100 million makes little practical difference. Getting from 15 million to 60 million makes a significant one.
Subfertility vs. infertility — and why the gap matters
One reason male factor gets missed is that clinical concern is triggered by infertility thresholds, not subfertility. If a result clears 15 million/mL, it does not prompt a referral or a follow-up conversation. But a man at 25 million/mL is not in the same position as one at 70 million/mL — and treating those results as equivalent is how couples end up in a prolonged stretch of “keep trying” that didn’t need to last as long as it did.
Subfertility — results that are technically within range but far from optimal — is common and, in many cases, improvable. The research base for male fertility nutrition, lifestyle, and supplementation is more robust than most people realise, partly because sperm concentration is a measurable, repeatable endpoint that is straightforward to study. Randomised controlled trials exist. The evidence base is there. But it cannot be applied well without knowing what the actual numbers are.
Getting tested — and reading the results correctly
A clinical semen analysis — where the sample is produced and analysed on-site at a lab — remains the most reliable method. At-home testing has improved; one option used in fertility-focused clinical programmes is Fellow, which runs morphology and returns actual numerical data rather than a simplified pass/fail result. If at-home results come back low or irregular, confirming with a clinical sample before building any treatment plan is the sensible next step.
When results come back, the question is not just whether concentration clears 15 million/mL. It is where concentration sits relative to the 50th-percentile target. Below 60 million/mL, there is room for improvement. Below 30 million/mL, understanding why — structural, hormonal, or lifestyle-related — should come before any intervention is started.
Testing is the beginning of a plan, not the end of a conversation.
The Sperm Count Blueprint
A focused resource for anyone whose sperm concentration is below optimal — or who doesn’t yet know where it stands. Covers how to interpret your numbers, the most common reasons concentration is low, and how to build a targeted plan based on what the research actually supports.
Get the Blueprint → Includes the Full Male Lab GuideReferences
- World Health Organization. (2021). WHO laboratory manual for the examination and processing of human semen (6th ed.). WHO Press. https://www.who.int/publications/i/item/9789240030787 — Source for sperm concentration reference ranges, including the 5th percentile lower threshold of 15 million/mL used in clinical semen analysis reporting.