Insight | Know your fertility numbers before you need them

The Short Version

Reproductive endocrinologist Natalie Crawford, Andrew Huberman and Stanford urologist Michael Eisenberg all argue that fertility is something you should measure long before you need it, not something you discover is a problem after months of trying [1][2][3]. For women, that means an AMH blood test and an antral follicle count to estimate ovarian reserve, ideally repeated to set a baseline; for men, it means a semen analysis, repeated every few years, and banking sperm while young [1][2][3]. The reason is simple arithmetic: a woman's egg supply shrinks continuously from before birth and egg quality drops sharply after about 37, while a man's sperm quality, testosterone and genetic integrity decline with age, weight, heat, drugs and testosterone therapy [1][2][3]. A number in hand turns an unpleasant surprise at 38 or 42 into a set of choices at 30: freeze eggs or sperm, start trying sooner, change habits, or look for a hidden medical cause [1][2][3]. The one real dissent is institutional: the American College of Obstetricians and Gynecologists advises against routine AMH screening, and Crawford and Huberman explain plainly why they disagree [1].

What They Said

Natalie Crawford: the egg "vault" and why you should look inside it early

Crawford, who is double board-certified in obstetrics and gynecology and in reproductive endocrinology and infertility, builds her case on one image: a vault of eggs in each ovary [1]. A female fetus has the most eggs she will ever have, about six to seven million, at around 20 weeks of gestation, and loses more than half of them before she is even born [1]. From then on, every month, a group of eggs leaves the vault regardless of what else is happening, and the more eggs inside, the more leave [1]. That loss is continuous and independent of periods, pregnancy or the pill [1].

Two practical consequences follow. First, egg freezing and IVF do not use up future eggs: the medications simply rescue the group already leaving the vault that month, which would otherwise die. As she puts it, "You're making the withdrawal no matter what" [1]. Second, earlier puberty does not end fertility earlier, because loss from the vault runs on its own clock [1]. The birth control pill does not slow the withdrawals either; it stops FSH so each month's group dies without ovulating, which is why she uses it routinely in IVF to synchronize egg growth and why it does not cause infertility [1].

Because you cannot see the vault directly, Crawford relies on two proxies. The first is an AMH (anti-Müllerian hormone) blood test, for which she cites a cash price of around $79 [1]. The second is an antral follicle count, a transvaginal ultrasound in which the small follicles look like chocolate chips in a cookie [1]. Her typical totals across both ovaries are about 16 to 20 at age 30, 14 to 16 at 35, 8 to 10 at 40 and 2 to 4 at 44, with a steep decline around 37 [1].

She is openly at odds with official guidance here. ACOG advises against routine AMH screening because AMH does not predict a woman's monthly chance of natural conception: a woman with five eggs leaving the vault still ovulates one, just like a woman with 20 [1]. Crawford strongly disagrees, and Huberman is outspoken in agreeing with her [1]. Her argument is that low reserve is actionable even if it does not change this month's odds: you could freeze eggs, start trying sooner, talk differently with a partner, or investigate causes such as genetic mutations or autoimmune disease [1]. She asks why parenthood is the one life goal where people say, "I'll just wait and see if it's a problem later" [1]. She notes that many OB-GYNs will order AMH if you ask, and suggests that when doctors ask women about contraception they should also ask whether they want to be pregnant someday [1].

She adds a free, everyday signal: the menstrual cycle itself. The luteal phase is fixed at 12 to 14 days, so changes in cycle length come from the follicular phase [1]. Cycles from about 21 to 35 days can be normal, but consistency matters most; her rule of thumb is that periods should be less than 35 days apart and predictable within a couple of days on a calendar [1]. She warns about "irregularly regular periods", cycles that jump between, say, 24 and 34 days, which suggest faulty hormonal communication [1]. And a cycle that shortens, for example from 28 to 30 days down to 24, can be an early sign of falling ovarian reserve, because fewer eggs respond faster to FSH [1]. Thyroid and prolactin problems can also change cycles [1].

One testing caveat she flags: the pill can lower AMH by up to 30%, reversibly, so a woman on the pill who tests low may stop for a few months and retest [1].

Crawford on quality, not just quantity

The count is only half the picture. Egg quality falls with age because chromosomes held in position by spindles since before birth accumulate wear and tear, which raises aneuploidy, the wrong number of chromosomes [1]. Miscarriage reaches about 40% at age 40, and at 40 she expects only about 20 to 25% of embryos to be chromosomally normal [1]. Natural pregnancy odds at 43 are under 3% per month, she says, while acknowledging that people will always cite exceptions [1].

That is why she names about 32 to 33 as the optimal age to freeze eggs if you are not ready to start a family, the point where quality and quantity still intersect well [1]. She walks through the attrition that makes early action matter: of 20 eggs frozen at age 30, about 18 survive thawing, about 14 fertilize, about seven reach the blastocyst stage, and roughly 60 to 70% of those are chromosomally normal, leaving perhaps four embryos [1]. Each genetically tested normal embryo carries up to about a 65% chance of live birth, rising cumulatively to about 88% after two transfers and 95% after three, which is why more than one round is often needed for more than one child [1]. Egg freezing technology has improved: roughly 40% of eggs survived thawing a decade ago, versus about 90% now, while embryos survive about 99% of the time [1]. She generally advises freezing eggs rather than committing to a sperm source you don't intend to have a child with [1]. Her clinic allows egg freezing or IVF up to 45 with informed consent [1].

She also describes what the process involves, so the option is not an abstraction: typically 12 to 14 days of small under-the-skin injections of FSH and LH, often after a few weeks on the pill, a trigger shot, then a roughly 20-minute retrieval usually under IV sedation [1]. She cautions against clinics pushing repeated low-stimulation cycles for profit when full stimulation gives better odds, and says patients should know what sedation their clinic uses [1].

Crawford on the male side

Crawford is not only an egg specialist. She explains that sperm are constantly made, about 72 days to form plus 18 to travel out, so suppressing FSH and LH halts production, and that after prolonged testosterone use about 25% of men may not recover it [1]. Heat harms sperm because the testes need to be cooler, while saunas and hot tubs don't affect ovaries [1]. She says advanced paternal age effects become clear around 50, that the strongest known association with autism is paternal age over 50, that freezing sperm costs around $400, and: "If you're going to get a vasectomy, go freeze sperm first" [1].

Andrew Huberman: baselines, repeated tests and the 90-day window

In his solo episode on fertility, Huberman frames the subject as mattering to everyone, since "this is really the aspect of our biology that allowed us to be here," and because the practices that protect fertility also support vitality and longevity [2]. He describes the same fixed egg "vault" that is formed while a female is still an embryo, with no new eggs made afterward [2].

His testing advice is explicit. The number of antral follicles (2 to 9 millimeters) recruited each month correlates with the size of the ovarian reserve and can be measured by ultrasound, alongside AMH in a blood test [2]. He says many women and OB-GYNs wish these tests had been done earlier, ideally several times over months, to get a baseline and make informed decisions such as egg freezing [2]. For men, a sperm analysis after 48 to 72 hours of abstinence is relatively inexpensive, and he relays urologists' advice to repeat it about every five years and to consider freezing sperm when younger [2]. He stresses that normal-looking ejaculate can still contain few or no sperm, and that problems like round-headed sperm may only be caught by analysis [2]. Baseline hormone panels are valuable too [2].

He grounds the urgency in age-based numbers for women. Women 30 or younger have about a 20% chance of conceiving per cycle, so OB-GYNs typically suggest trying for five or six months before seeking help [2]. At 31 to 33 it is about 18% (six to seven months), at 34 to 37 about 11% (nine months to a year), at 38 to 39 about 5%, and at 40 or older about 1 to 3%, so women 38 and up may want to see an OB-GYN at the outset [2]. Miscarriage risk climbs too: about 25% of fertilizations for women 35 or older and about 50% by 40, often from chromosomal errors when the polar body forms [2].

For men, he emphasizes timing. Sperm production takes about 60 days plus roughly two more weeks of transit, so anyone hoping to conceive should think in a 90-day window [2]. He notes that in about one in five couples with fertility problems the issue lies with the sperm, and that autism risk rises when fathers are 40 or older, though the overall probability remains lower than people imagine [2]. He also points out that population sperm counts appear to be declining, mentioning Shanna Swan's work on endocrine disruptors like phthalates and BPA as one possible factor [2]. Clinicians like to see at least about 15 million sperm per milliliter and more than two milliliters of ejaculate, with ideally more than half the sperm motile [2].

Two of his less obvious points make the case for testing before trying. A serious viral illness in the previous 70 to 90 days can reduce sperm count and quality and skew a sperm analysis [2]. And he was surprised to learn that about 1 in 25 men carry one copy of a cystic fibrosis mutation, which can malform the vas deferens so the ejaculate lacks sperm, something fixable by surgical repair or sperm extraction but invisible without testing [2]. He also urges testing for sexually transmitted infections, especially chlamydia, which often has no symptoms and raises the risk of ectopic pregnancy and miscarriage [2].

His summary of what he learned from OB-GYNs and urologists: "What is good for the woman is good for the egg and for fertility and for pregnancy," and the same holds for men and sperm [2].

Michael Eisenberg: semen quality as a vital sign

Eisenberg, a Stanford urologist with more than 300 published papers, explains what a semen analysis measures: "We look at the amount of a jacket, semen that comes out. We look at the sperm, how many there are. We look at their motility or movement" and their shape, with advanced tests for DNA fragmentation and epigenetic profiles in rare cases [3]. He insists that "fertility itself is just team sport," so no man can be labeled fertile or infertile without considering his partner [3].

His strongest argument for early male testing is that men are routinely ignored. "When couples do seek careful fertility, the man is bypassed probably a third of the time," he says, "even though when you look at the reasons for infertility, the man contributes probably half of the time to infertility" [3]. IVF with intracytoplasmic sperm injection means a single sperm can suffice, which he thinks has dampened research on male fertility; as Huberman puts it in the conversation, "It only takes one sperm which is the point I was true for IVF but more is better if you're trying to conceive naturally" [3]. Couples trying naturally probably need 20 to 40 million moving sperm [3].

He also explains why guessing doesn't work. Asked whether obesity reliably predicts low testosterone, he answers, "I would say that you cannot predict," adding that "more information is always better" [3]. Some fit men who exercise five to seven days a week have very low testosterone, which is why objective testing matters [3]. About 15% of men have low semen quality and about 1% have no sperm at all, often without knowing it, since normal ejaculation does not mean normal sperm [3].

Eisenberg goes further than fertility: he believes semen quality should be treated as something like a sixth vital sign [3]. Men with higher semen quality tend to live longer, see doctors less and have lower cancer rates, and fertility evaluations sometimes uncover cancer, diabetes or genetic conditions [3]. Lower testosterone and lower semen quality are both linked to higher risk of heart disease, diabetes and mortality [3]. He would like the CDC to add semen analysis to its national health surveys [3].

Huberman, who began getting semen analyses and banked sperm about eight years before the conversation, encourages men in their 20s and 30s to get a baseline, and Eisenberg agrees, welcoming easier at-home options and noting that insurance increasingly covers fertility testing [3].

On age, Eisenberg says men tend to assume their fertile window is infinite, but it is not [3]. The average age of US fathers has risen from about 27.5 to about 31 over 40 years [3]. Older fathers are linked to higher risk of achondroplasia, autism, bipolar disorder, schizophrenia, lower school performance, slightly smaller newborn brain size on MRI, and some childhood and adult cancers [3]. Sperm accumulate roughly two new mutations per year, and some mutations give sperm a proliferative edge, called selfish spermatogonial selection [3]. There is no current way to pick out sperm with undamaged DNA, since testing a sperm's DNA destroys it [3]. His mitigation advice is simple: try to conceive earlier or freeze sperm [3].

Finally, he flags the most common self-inflicted cause of male infertility he sees: testosterone therapy. "Estimates say maybe about one in 20 infertile men are that way because of testosterone," he says, and it has even been tested as a contraceptive [3]. Exogenous testosterone shuts down the brain's release of LH and FSH, and testosterone inside the testicle, normally at least tenfold higher than in the blood, falls sharply, halting sperm production [3]. Many men are never told this [3]. His view: "before starting it everybody should be aware of all the risks," and for testosterone, "reproduction is certainly one of them" [3]. Options include freezing sperm before starting, or low-dose hCG, typically 500 to 1,000 units every other day, to maintain production [3].

Where They Agree, and Where They Don't

They agree that testing should come early and be repeated. Crawford wants AMH and follicle counts to be routine [1]; Huberman says women and OB-GYNs wish tests had been done earlier and several times over months, and relays a five-year cadence for men's sperm analyses [2]; Eisenberg and Huberman both encourage men in their 20s and 30s to get a baseline [3].

They agree that age is the dominant variable for both sexes. Crawford's follicle counts fall steeply around 37 and miscarriage reaches about 40% at 40 [1]; Huberman's per-cycle odds fall from about 20% at 30 or younger to 1 to 3% at 40 or older [2]. For men, all three flag paternal age, though at different thresholds: Crawford says effects become clear around 50 [1], Huberman says autism risk rises when fathers are 40 or older [2], and Eisenberg lists a broad set of conditions linked to older fathers and a steady accumulation of about two mutations per year [3]. The difference is less a contradiction than a matter of where each draws the line on a gradual slope.

They agree freezing is the main hedge. Crawford gives 32 to 33 as the sweet spot for eggs and says to freeze sperm before a vasectomy [1]; Huberman says to consider freezing sperm when younger [2]; Eisenberg says to conceive earlier or freeze sperm [3].

They agree men are overlooked. Huberman puts the sperm share of couples' fertility problems at about one in five [2], while Eisenberg says the man contributes probably half the time and is bypassed about a third of the time [3]. Those figures differ, likely because Huberman counts couples where the issue lies with the sperm and Eisenberg counts cases where the man contributes at all, but both point to testing the man, not assuming the problem is the woman's.

They agree testosterone therapy threatens sperm. Crawford says about 25% of men may not recover production after prolonged use [1]; Huberman says it dramatically reduces sperm production unless offset with HCG, FSH or clomiphene under a doctor's care [2]; Eisenberg says about one in 20 infertile men in his clinic are infertile because of it [3].

Where they disagree: official guidelines versus the clinicians. ACOG advises against routine AMH screening because AMH doesn't predict monthly natural conception [1]. That reasoning is correct on its own terms, and Crawford accepts the biology behind it: a woman with low reserve still ovulates one egg a month [1]. Her disagreement is about what the test is for. It is not a monthly fertility forecast; it is information for planning, and low reserve can change when you start, whether you freeze and whether you look for an underlying cause [1]. A similar tension exists for men: Eisenberg notes US screening guidelines recommend against routine testicular self-exams because of anxiety over false alarms, but he does not discourage them, saying "no one knows your scrotum better than you" [3].

Minor disagreements on exposures. Huberman cites a 2021 meta-analysis of 18 studies with 4,280 samples finding mobile phone use reduces sperm quality, and advises keeping phones out of the front pocket [2]; Eisenberg says the phone evidence is not convincing, though some lab studies show slightly higher DNA fragmentation with close proximity [3]. On alcohol, Huberman says zero is best and two drinks a week is the upper limit for healthy adults not trying to conceive [2], Crawford says at most one or two drinks a week when trying to conceive [1], while Eisenberg says effects on semen quality appear above about 20 drinks a week and generally advises moderation [3]. On cannabis, Crawford and Huberman describe clear harms [1][2], while Eisenberg notes a study of about 1,200 men found daily users had worse sperm but other studies show no effect [3]. On abstinence before trying, Huberman says 48 to 72 hours maximizes sperm concentration [2], while Crawford says saving up doesn't help and daily intercourse gives the highest odds, noting the two- to three-day abstinence before a sperm test exists only because normal values were set that way [1].

Why It Works

Egg loss is a one-way clock. Because the vault is fixed before birth and empties continuously, there is no way to "save" eggs, only to measure how many remain and act while enough of them are good [1][2]. AMH and antral follicle counts track the size of the group recruited each month, which correlates with the size of the reserve [1][2]. A shortening cycle is a behavioral read-out of the same thing: fewer eggs respond faster to FSH [1].

Quality declines on its own schedule. Egg chromosomes are held by spindles from before birth and accumulate wear, raising aneuploidy and miscarriage [1]; Huberman ties miscarriage to chromosomal errors when the polar body forms, and notes mitochondria matter for egg spindles [2]. Freezing stops that clock for the eggs you bank, which is why freezing at 32 to 33 beats freezing at 40, when only about 20 to 25% of embryos are normal [1].

Sperm are renewable, which cuts both ways. Because sperm are made continuously over about 72 plus 18 days, or roughly 90 days by Huberman's count, lifestyle changes show up within a few months, but so does damage from heat, illness, binge drinking or testosterone [1][2]. A single binge of five or six drinks can affect sperm across the whole 60- to 90-day production window [2], and cannabis even once can impair motility of sperm produced over the following 60 days [2]. That is also why one test can mislead and a baseline plus repeats is more useful [2].

Testosterone therapy shuts the factory. Outside testosterone triggers negative feedback that suppresses LH and FSH; intratesticular testosterone collapses; sperm production stops [2][3].

Male age adds mutations. About two new mutations per year, plus selfish spermatogonial selection and epigenetic changes, plausibly explain the conditions linked to older fathers [3].

Semen quality may reflect whole-body health. Eisenberg offers possible reasons: roughly 10% of the male genome is devoted to reproduction and overlaps with other systems, adult disease may have developmental origins, and partnership and children are themselves linked to longer life [3].

Inflammation is a shared enemy. Crawford says "inflammation is just toxic to eggs and sperm," which is why she names sleep as perhaps the most overlooked factor and emphasizes that eggs and sperm are affected over roughly a 90-day window [1].

Where It Applies, and Where It Doesn't

Testing is most valuable for people who want children someday but not yet, particularly women approaching their mid-30s, since the steep decline in follicle counts comes around 37 and the best freezing window is 32 to 33 [1]. It is also strongly indicated for men considering testosterone therapy, a vasectomy or delayed fatherhood [1][3], for anyone with shortening or erratic cycles [1], and for couples who have been trying longer than their age bracket suggests [2].

The limits matter. AMH does not tell a woman her chance of getting pregnant this month, which is ACOG's objection [1]; it measures quantity, not quality, and quality is driven mostly by age [1]. The pill can lower AMH by up to 30%, so a low result on the pill may be misleading [1]. A sperm analysis can be skewed by a serious viral illness in the prior 70 to 90 days [2], and reference ranges were set with a particular abstinence period [1]. Normal ejaculation does not mean normal sperm, and you cannot judge a man's testosterone or sperm by looking at him [3]. Eisenberg also cautions that fertility is a team sport, so one partner's numbers never tell the whole story [3]. And freezing is not a guarantee: 20 eggs at 30 may yield about four normal embryos, and one embryo carries up to about a 65% chance of live birth [1].

Many of the numbers here are population averages; Eisenberg notes huge individual variation in testosterone, including 80-year-olds with very high levels [3], and Crawford notes people will always cite exceptions to the age statistics [1]. For all of this, the results should be interpreted with an OB-GYN, reproductive endocrinologist or urologist rather than on your own.

What To Do With It

1. Women: ask for an AMH test and an antral follicle count now, not when you start trying. Many OB-GYNs will order AMH if asked, and Crawford cites a cash price of around $79 [1]. If possible, repeat it over several months to set a baseline [2]. If you are on the pill and the result is low, consider stopping for a few months and retesting [1].

2. Track your cycle as a free early-warning system. Periods should come less than 35 days apart and be predictable within a couple of days; watch for jumps between, say, 24 and 34 days, or a cycle that shortens from 28 to 30 days down to 24, and raise either with your doctor [1].

3. Men: get a semen analysis in your 20s or 30s after 48 to 72 hours of abstinence, and repeat it about every five years [2][3]. Avoid testing within 70 to 90 days of a serious viral illness, which can skew the result [2]. Ask about baseline hormones too [2].

4. If the numbers are low or your timeline is long, freeze. For eggs, Crawford's target is about 32 to 33, and she prefers freezing eggs over committing to a sperm source you don't plan to have a child with [1]. For sperm, bank while young, before a vasectomy, and before starting testosterone therapy [1][2][3]; Crawford puts the cost at around $400 [1].

5. Know the age thresholds for seeking help. Under about 30, try five or six months before seeing a specialist; at 34 to 37 allow nine months to a year; at 38 and up, consider seeing an OB-GYN at the outset [2].

6. Men considering testosterone therapy: get the full picture first. Address sleep, exercise, alcohol, smoking and body weight before starting, and if you do start, discuss sperm banking or low-dose hCG to preserve production [3].

7. Protect what you have, on a 90-day horizon. Sleep well (seven to nine hours is linked to the best semen quality) [3], avoid smoking, vaping and cannabis [1][2], keep alcohol to one or two drinks a week or less when trying [1], keep heat away from the testes (hot tubs, saunas, laptops, heated car seats) [2][3], and remember "it's the choices that you make every single day" that count [1].

8. Screen for hidden causes. Get tested for sexually transmitted infections, especially chlamydia [2]; if your reserve is low, ask about genetic or autoimmune causes [1]; men with low counts may have a varicocele, a hormonal problem or a genetic condition worth evaluating [2][3].

9. Test both partners. Because the man contributes to roughly half of infertility and is bypassed about a third of the time, insist that he is evaluated alongside the woman [3].

Sources

1. Dr Natalie Crawford Female Hormone Health Fertility Vitality · Huberman Lab
2. How to Optimize Fertility in Males & Females · Huberman Lab
3. Dr. Michael Eisenberg: Improving Male Sexual Health, Function & Fertility · Huberman Lab