💡 Bottom Line Up Front
Even with perfect timing, healthy partners, and no fertility issues, the per-cycle conception probability (fecundability) is only 20–30% for couples under 30. This isn't a design flaw — it's biological quality control. Over half of fertilized eggs have chromosomal errors and self-destruct before or during implantation. Add timing misses, egg quality variation, and the sheer difficulty of the sperm journey, and the 20–30% rate starts to seem reasonable.
Where the Losses Happen
| Stage | Approximate Loss Rate | Cumulative Survival |
|---|---|---|
| Sperm survive to cervix | ~80% lost | ~20% of sperm survive |
| Sperm reach fallopian tube | 99.99% lost | ~200 of 300 million arrive |
| Egg is released (ovulation occurs) | ~5% of cycles anovulatory | ~95% chance egg is available |
| Sperm and egg meet (timing) | ~30–40% miss the window | ~60–70% chance of encounter |
| Fertilization succeeds | ~20–30% of encounters fail | ~50% chance at this point |
| Embryo is chromosomally normal | ~30–50% are abnormal (age-dependent) | ~25–35% viable embryo |
| Implantation succeeds | ~30–40% fail to implant | ~15–25% established pregnancy |
| Pregnancy survives to detection | ~15% early loss | ~20–25% detectable pregnancy |
The single biggest factor is chromosomal abnormality in the embryo. The egg's final meiotic division is error-prone, and the error rate increases dramatically with age. At age 25, perhaps 10–25% of eggs are chromosomally abnormal. At 40, it's 60–80%. These abnormal embryos may fertilize normally and even begin dividing, but they almost always fail to implant or miscarry early.
Why This Is Quality Control, Not a Flaw
Human reproduction is remarkably selective compared to other species. Many animals have litters of 6–12 offspring with minimal quality filtering. Humans typically carry one pregnancy at a time and invest enormous biological resources in each child. The “wasteful” 70–80% monthly failure rate is actually a stringent screening process that eliminates embryos unlikely to develop into healthy pregnancies.
⚠ Chemical pregnancies: more common than you think
A chemical pregnancy is when an embryo implants and produces enough hCG to be detected by a test, but fails before it can be seen on ultrasound (typically before 5–6 weeks). These account for 50–70% of all pregnancy losses and were invisible to previous generations who didn't have sensitive early tests. If you're tracking with early-detection tests, you may experience chemical pregnancies that feel devastating but are actually a normal part of the biological process — the embryo likely had a chromosomal abnormality.
The Cumulative Math
| Months Trying | Cumulative Probability (Under 30) | Cumulative Probability (35–37) | Cumulative Probability (40+) |
|---|---|---|---|
| 1 month | 25–30% | 15–20% | 5–10% |
| 3 months | 55–65% | 40–50% | 15–25% |
| 6 months | 75–85% | 60–70% | 25–40% |
| 12 months | 90%+ | 75–85% | 40–55% |
| 24 months | 95%+ | 85–92% | 50–65% |
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