Fertility by the numbers
Numbers about fertility are easy to find. What almost nobody publishes is what each number does not say. Below, a reproductive medicine specialist lists the figures with their sources — and the limit of each one.
The egg is viable for about 24 hours after ovulation.
✕ What this number does NOT say：Timing sex *on* ovulation day can already be late — it's the sperm that waits.
Source: 生理学 / Physiology
Sperm can survive around a week in the tubes.
✕ What this number does NOT say：Surviving is not the same as staying fully fertile — the days just before ovulation matter most.
Source: 生理学 / Physiology
The fertile window runs from 5 days before ovulation to 1 day after; the peak is the 2 days before ovulation through the day itself.
✕ What this number does NOT say：Ovulation shifts between cycles — there is no reliably "safe" day.
Source: 生理学 / Physiology
Your period arrives about two weeks after ovulation — so ovulation is ~2 weeks before your next expected period.
✕ What this number does NOT say：This backward calculation fails if your cycles are irregular.
Source: 生理学 / Physiology
The rise confirms that ovulation happened.
✕ What this number does NOT say：By the time you see it, the fertile window has closed — BBT cannot predict.
Source: 生理学 / Physiology
It climbs 30 points between months 3 and 6, but only 10 points between months 6 and 12.
✕ What this number does NOT say：Waiting longer buys much less than people assume — month 6 is the decision point.
Source: 書籍本文 / Book, Fact 17（タイミングを特に合わせない前提の保守的な提示）
A German prospective study of 346 couples using natural family planning to time intercourse.
✕ What this number does NOT say：These figures do not apply to untimed intercourse — the precondition changes the result substantially.
Source: Gnoth C, et al. Hum Reprod. 2003;18(9):1959-66.
This is the evidence behind treating six cycles as the sensible limit for watchful waiting.
✕ What this number does NOT say：The other half have no identifiable problem — six cycles without success is not itself abnormal.
Source: Gnoth C, et al. Hum Reprod. 2003;18(9):1959-66.
782 couples across seven European centres, adjusted for timing and frequency of intercourse.
✕ What this number does NOT say：Absolute sterility stays around 1% at every age. What rises with age is difficulty, not impossibility.
Source: Dunson DB, et al. Obstet Gynecol. 2004;103(1):51-6.
A model accounting for miscarriage and age-related infertility. Within four years: 91% / 84% / 64%.
✕ What this number does NOT say：ART recovers only about half of the births lost by delaying from 30 to 35, and under 30% of those lost from 35 to 40. "We can always do IVF" does not hold.
Source: Leridon H. Hum Reprod. 2004;19(7):1548-53.
The twenties are the most fertile years — and therefore the highest-risk years for unintended pregnancy.
✕ What this number does NOT say：The same number reads as good news or as a warning depending on what you want.
Source: 書籍本文 / Book, Fact 11
About 20% even after six more months of trying.
✕ What this number does NOT say：Continuing to "let nature take its course" does not restore the odds — it only spends time.
Source: 書籍本文 / Book, Fact 1
A marker of how many eggs remain; it approaches zero around menopause (~52).
✕ What this number does NOT say：It says nothing about egg quality, and varies enormously between women of the same age.
Source: 書籍本文 / Book, Fact 4
Below 1.0 is treated as low; 1.0–2.0 as mildly low. The ESHRE Bologna criteria define diminished reserve as under 1.1 ng/mL.
✕ What this number does NOT say：Low reserve does not mean you cannot conceive — egg quality still tracks your age, not your AMH.
Source: ESHRE Bologna criteria (2011) ほか
From the JSOG PCOS diagnostic criteria (2024). No high-side判定 is applied from age 40, as the criteria do not specify one.
✕ What this number does NOT say：A high AMH alone does not diagnose PCOS — cycle irregularity, ultrasound findings and hormone levels are all required.
Source: 日本産科婦人科学会 PCOS診断基準 2024 / JSOG PCOS criteria 2024
Depending on how many children you hope for, it can justify moving faster.
✕ What this number does NOT say：Low does not mean you cannot conceive, and it does not predict this cycle.
Source: 書籍本文 / Book, 付録3
Eggs run out early — potentially right around the average age of marriage.
✕ What this number does NOT say：There are no symptoms until reserve is nearly gone; a shifting cycle length is the first clue.
Source: Luborsky JL, et al. Hum Reprod. 2003;18(1):199-206.
Women who conceive smoothly often sit in the 28–32 day range.
✕ What this number does NOT say：A short 25–26 day cycle can signal falling reserve; 35–40 days means ovulation timing varies widely.
Source: 書籍本文 / Book, Fact 6
Around 20% cumulatively over 3–5 cycles.
✕ What this number does NOT say：Each single cycle fails more than 90% of the time — plan in blocks, not in months.
Source: 書籍本文 / Book, Fact 19
Around 30% cumulatively over 3–5 cycles.
✕ What this number does NOT say：It only delivers sperm closer — everything after fertilisation is the same as natural conception.
Source: 書籍本文 / Book, Fact 19
The most effective treatment, and efficient because one retrieval yields several eggs.
✕ What this number does NOT say：It cannot increase how many eggs you have or improve their quality. Over 40, it falls below 20%.
Source: 日本産科婦人科学会ARTデータ / JSOG ART registry
It grows about 2mm a day near ovulation, so ultrasound can pinpoint the day.
✕ What this number does NOT say：This works even when cycles are irregular or stress delays ovulation.
Source: 書籍本文 / Book, Fact 19
A single exposure to an infected partner transmits it 30–50% of the time.
✕ What this number does NOT say：It is usually silent — untreated, it can block the tubes and cause infertility.
Source: 書籍本文 / Book, Fact 9
90% clear it naturally within two years.
✕ What this number does NOT say：A minority persists and can progress to cervical cancer over 5–10 years — with no symptoms along the way.
Source: 書籍本文 / Book, Fact 10
The second leading cause of cancer death in women aged 25–40 there.
✕ What this number does NOT say：Screening every 1–2 years plus HPV vaccination prevents most of it — these are largely preventable deaths.
Source: 書籍本文 / Book, Fact 10
A method a woman controls herself; about ¥4,000/month in Japan.
✕ What this number does NOT say：It does not prevent STIs — condoms are still needed alongside it.
Source: 書籍本文 / Book, Fact 11
Around five years of protection from one insertion, with fertility returning immediately after removal.
✕ What this number does NOT say：It requires a clinic procedure.
Source: 書籍本文 / Book, Fact 11
Must be taken within 72 hours; ¥20,000–30,000 per use in Japan.
✕ What this number does NOT say：Not a routine method — starting the pill in advance is both more reliable and cheaper.
Source: 書籍本文 / Book, Fact 11
Keeping it in range lowers miscarriage and preterm birth risk.
✕ What this number does NOT say：Thyroid function alone does not explain irregular cycles.
Source: 書籍本文 / Book, 付録3
If deficient, 25–50 µg (1,000–2,000 IU) daily; reported to help cycle regularity and the implantation environment.
✕ What this number does NOT say：Fat-soluble and accumulates — do not overdose, and retest after 1–2 months.
Source: 書籍本文 / Book, 付録3
Below 1:32, vaccination is recommended before trying.
✕ What this number does NOT say：It is a live vaccine — avoid pregnancy for 2 months after, and check your partner too.
Source: 書籍本文 / Book, 付録3
These are not guidelines. They are the thresholds the author — a practising reproductive medicine specialist — uses in clinic. Where they deliberately differ from the standard advice, the reasoning is given.
If you are under 30, start testing after 6 months; if you are 30 or older, after 3–6 months
Why：Deliberately earlier than the standard 12-month definition of infertility: months 6–12 add only ~10 points of cumulative pregnancy rate, while egg quality and quantity keep falling throughout.
When cycles shorten to 25–26 days, AMH is often around 0.4–0.5; once cycles stretch to 40–60 days or periods stop for 3+ months, AMH is often below 0.1
Why：As reserve declines, cycles first shorten and then lengthen. The change in cycle length is the first sign you can notice yourself.
Plan backwards: about 2 years per child — 2 years for one, 4 for two, 6 for three
Why：Conception, pregnancy and recovery before the next attempt take roughly two years each. Ideally the family is complete by around 35.
◯ What tests can tell you
✕ What tests cannot tell you
Timed intercourse and IUI are, by definition, treatments carried out without knowing which step is failing.