The Forgotten Hormone

Proov founder Amy Beckley has spent a decade on the least studied stretch of the menstrual cycle. The field's own guidance now concedes much of her argument, and the research that would settle the rest depends on a federal budget Congress has not yet set.

Every month, the roughly two weeks between ovulation and a period determine whether a pregnancy can take hold. Progesterone made during those days prepares the lining of the uterus and holds it in place.

When progesterone drops, the lining sheds and a period starts.

Medicine calls this stretch the luteal phase, and more than 70 years after doctors first described problems with it, there is still no agreed test for whether it went right.

The field's own guidance says so. In July, the American Society for Reproductive Medicine and the Society for Reproductive Endocrinology and Infertility reissued their guidance on luteal phase deficiency, a second half of the cycle that is too short or too weak, in the journal Fertility and Sterility.

It finds that no test reliably separates women who can conceive from women who can't, and that no randomized trial has ever tested progesterone for women with the condition. It also says the condition "has not been proven to be an independent entity" causing infertility or miscarriage.

The guidance treats progesterone as essential to early pregnancy. What it lacks is the research to say when a woman has too little.

The core conclusions carry over from 2021. The one new line goes further against progesterone, stating it has not been shown to help women with unexplained recurrent miscarriage, based on a 2025 Cochrane review of nine randomized trials.

Amy Beckley has built a company on that gap. A PhD scientist, she is the founder and CEO of Proov, an at-home hormone testing company she started in her basement after years of her own infertility. Its urine progesterone test was the first of its kind to receive FDA clearance in 2020. Beckley calls the luteal phase "the hidden secret of a woman's cycle that nobody really knows about," and she has spent the past decade arguing that it deserves the attention the fertile window gets.

"The luteal phase literally is an indicator of health," she told The Luteal.

Her own blood work kept coming back normal. Her progesterone looked strong on the standard test, drawn about a week after ovulation. "But then like literally the next day or the day after, I would start spotting and my period would start," she said. She would conceive and lose the pregnancy within days. By her account, she went through two rounds of IVF, seven miscarriages, three years of infertility, and surgery for endometriosis before she found the answer.

The test had captured her progesterone on a single day. In her view, nobody had checked whether it stayed high long enough for an embryo to implant.

Beckley's account of the testing problem matches the guidance closely.

"You're getting a snapshot in time," she said. "And you're not really assessing, is it high enough long enough for that period of time?" The guidance explains why. Progesterone is released in bursts and can rise or fall eightfold within 90 minutes, and no minimum blood level has ever been established as normal. The proposed fix, three blood draws over five days, has never been validated. Beckley's verdict on it is short.

"Who goes in 3 times to get their blood drawn to look at the luteal phase? Nobody."

IVF gave her the idea that changed her outcome. Clinics routinely give women progesterone after embryo transfer, and she wanted to know why nobody tried it in an unassisted cycle. "All the big studies are done by the big pharma companies in the context of IVF because that's where the money's at," she said.

For her next pregnancy, she asked her doctors to prescribe progesterone starting right after ovulation. "I will be the guinea pig. Let's just try it," she told them. "Short answer is it worked." Her daughter turned 13 this month.

Her result is one woman's experience, and the published trials are narrower than her story. ASRM finds no evidence that progesterone improves fertility in unassisted cycles, though it does help after fertility treatment. The largest trial of progesterone for pregnancy loss, PRISM, published in the New England Journal of Medicine in 2019, enrolled 4,153 women with bleeding in early pregnancy. Live births were 75% with progesterone (1,513 of 2,025) and 72% with placebo (1,459 of 2,013), a difference that was not statistically significant. Among 285 women with three or more prior miscarriages, the gap widened to 72% (98 of 137) against 57% (85 of 148). PRISM started progesterone after a positive pregnancy test, not after ovulation.

The trial that would test Beckley's approach, in women trying to conceive without IVF, has not been run.

Her line about pharma explains why.

Progesterone has been sold as a low-cost generic for years, so no manufacturer has a patent to protect by paying for that trial. Research with no commercial sponsor falls to public funders, which in the United States mostly means the National Institutes of Health.

The first obstacle she names is awareness. "Nobody knows what progesterone is. Nobody cares," she said. "We know estrogen." Doctors she approached told her the condition wasn't part of their training. "They were like, well, we don't even learn the word luteal phase defect in school. That doesn't exist." Her name for progesterone is "the forgotten hormone."

Proov's test reads progesterone's breakdown product in urine, so a woman can test at home each morning across the second half of her cycle instead of going in for blood work. The FDA's clearance covers one thing, confirming that ovulation happened, with a yes-or-no result at a single cutoff. "We had to call it confirming ovulation," she said.

The company built a progesterone score on top, which she compares to the sleep and readiness scores on an Oura Ring, a wellness measure rather than a diagnosis.

Getting there took outside capital. "We are a venture-backed company because there's no way you can go through FDA clearance and bootstrap a company," Beckley said.

The FDA's own filing fee is the small part, $26,067 for a standard clearance review in fiscal 2026. The larger cost is the testing before a submission, which for Proov included precision studies and a trial of 101 women using the strips at home. The company raised a $9.7M Series A in December 2021, led by Hambrecht Ducera Growth Ventures.

After Proov's clearance, the FDA listed the category as exempt, so competing urine progesterone tests can sell under the same classification without a clearance of their own. "It created a category that all these entrants came into and didn't have the same hurdles," Beckley said.

Clearance also opened no doors with doctors or insurers. "We can't go to a doctor, be like, oh, it's FDA cleared. They don't care." The first company through the gate carried the regulatory cost, and the market it opened pays no premium for that work.

She places the resistance from clinicians in the payment system rather than in the doctors themselves. "It's not necessarily that they don't like it," she said. "They're not able to make a good living anymore doing what they do by taking insurance." Proov is building its own telehealth service to connect women with low scores directly to a doctor. "We can't change the healthcare system. We just can't."

Asked what a healthy luteal phase looks like, she didn't overstate it. "It's a complicated question that the proper answer doesn't exist yet."

Her working definition is at least 10 days of elevated progesterone, including a seven-to-ten-day stretch above the test's cutoff. "We call that high enough, long enough." The cutoff is the one the FDA reviewed for confirming ovulation. The time requirement is Proov's own, and testing it against birth outcomes is the research the company is trying to fund.

How many women fall short depends on where the line sits. "30 to 40% of women do not have healthy cycles," Beckley said of Proov's data.

A National Institutes of Health study published in 2014 followed 259 women with regular cycles. Of 463 cycles, 41 (8.9%) had a luteal phase shorter than 10 days, and 20 (4.3%) also had low progesterone. None of those lines has been tied to whether women go on to conceive. Proov's figure comes from women already looking for fertility help, which likely explains why it runs three to four times higher.

Her answer is to measure each woman against herself, because "women don't respond to hormones the same way." She compares it to heart rate. "My heart rate's 60, your heart rate might be 75. They're both normal. But if mine goes to 75, that's not normal for me." Proov is building a membership it calls Proov Continuous, with 30 straight days of home urine testing repeated every six months to a year. The company is working toward separate scores for estrogen, progesterone and overall cycle health. "Patterns are very real," she said.

She believes the same blind spot runs into midlife. In her view, perimenopause begins with falling progesterone, years before estrogen drops. "There's a 10-year period in hormone decline that she feels terrible," she said. "Everything upstream is progesterone-related." Proov applied for an NIH grant to track daily hormones for a year in women aged 35 to 45, and reviewers passed on the first submission because doctors can already treat on age and symptoms.

"We pushed back and we said it's not menopause, it's perimenopause," she said. The reviewers' position matches practice. UK guidance from NICE advises diagnosing perimenopause in women over 45 from symptoms such as hot flushes and irregular periods, without lab tests, because hormone levels fluctuate too much during perimenopause for a test to help.

That grant now sits inside a larger federal decision. "The US government is cutting down all their funding. They're shutting down research programs," she said. The White House's fiscal 2027 budget asks for $41.4B for the NIH, down from the $47.2B Congress enacted for 2026. A stopgap law signed in early September holds funding at 2026 levels through December 11, which leaves the real number to be set after the November midterms.

As The Luteal reported this month, the National Academies found in 2024 that several of the most common gynecologic conditions have no NIH institute responsible for them, and the agency has stopped publishing its category-level spending estimates for 2026 and 2027.

Her skepticism extends to the new wave of continuous hormone wearables. "I'm not 100% convinced we need continuous hormone monitoring," she said. "Can you imagine someone who has a sensor that goes up and down like this, they're going to freak out."

She sees a clear use in IVF, where clinics already test blood repeatedly to time medication. "You want that very fine-tuned tracking." Everywhere else, she says, the daily pattern has to be understood first.

Every part of the system stops at the same line. The FDA cleared a test that answers whether ovulation happened. The professional society says no test defines whether the weeks that followed were adequate. No drug company has a reason to fund the trial, and the public funder that would is waiting on Congress.

What the market is missing is a definition of normal tied to outcomes, built from years of daily hormone data, and that is what Beckley is building toward. "If we don't get the research, we can't build new products," she said. "You're just going to get the same old crap in a different wrapper."

Congress has until December 11 to set the NIH's budget. Until someone pays for that trial, the work of defining a normal luteal phase falls to companies like Proov, one morning urine test at a time.

This article is for informational purposes only and is not medical advice. Proov's figures and plans are company-stated.

--- Sources

Primary

ASRM and SREI Practice Committees, "Diagnosis and treatment of luteal phase deficiency: a Committee Opinion," Fertility and Sterility, published online July 7, 2026, doi:10.1016/j.fertnstert.2026.06.014

Haas DM et al., "Progestogen for preventing miscarriage in women with recurrent miscarriage of unclear etiology," Cochrane Database of Systematic Reviews, 2025, CD003511 (as cited in the ASRM 2026 committee opinion)

Coomarasamy A et al., "A Randomized Trial of Progesterone in Women with Bleeding in Early Pregnancy" (PRISM), New England Journal of Medicine, 2019

Schliep KC et al., "Luteal Phase Deficiency in Regularly Menstruating Women," Journal of Clinical Endocrinology and Metabolism, 2014 (NIH BioCycle Study)

US Food and Drug Administration, 510(k) Decision Summary K191462, Proov Test, cleared February 27, 2020

US Food and Drug Administration, Product Classification Database, product code QKE (Class I, 510(k) exempt)

US Food and Drug Administration, Medical Device User Fee Amendments fee schedule, fiscal year 2026

National Institutes of Health, Office of Budget, FY 2027 President's Budget Request

Congressional Research Service, Report R43341, National Institutes of Health Funding, FY2026 appropriations

National Institute for Health and Care Excellence, NICE guideline NG23 and quality standard QS143

Interview with Amy Beckley, PhD, founder and CEO of Proov, September 16, 2026

Secondary

Nextgov/FCW and the Committee for a Responsible Federal Budget, reporting on the continuing resolution funding the government through December 11, 2026

The Luteal, women's health research funding investigation, September 2026

Company-stated

Proov, the 30 to 40% cycle figure, healthy-cycle definition, Proov Continuous and planned scores, telehealth service, NIH grant application and review, and the FDA's reasoning for requiring clearance

Proov, $9.7M Series A announcement, December 2021, led by Hambrecht Ducera Growth Ventures, as reported by TechCrunch and MobiHealthNews