The fertile window is shorter than most people think

The fertile window is defined by two lifespans that are wildly unequal.

An egg, once released, is viable for around 12 to 24 hours. Sperm, in the presence of fertile cervical mucus, can survive up to about five days in the reproductive tract.

Put those together and the window is roughly six days: the five days before ovulation, plus the day of ovulation itself. And the probability is not evenly distributed across them — the highest chances are in the two or three days before ovulation, not on the day.

Why the "before" part matters

This is the single most useful and most counterintuitive fact in natural conception timing.

If you wait until you detect ovulation and have sex then, you are aiming at the end of the window rather than the middle. Sperm need time to travel and to undergo capacitation, the process that makes them capable of fertilising. Having sperm already present and waiting when the egg is released is a better position than starting the journey afterwards.

The practical implication: aim for the days when fertile cervical mucus appears, not the day a temperature rise confirms ovulation happened.

Which signals arrive in time

Cervical mucus is the earliest usable signal. As oestrogen rises, it becomes clear, slippery and stretchy — the raw-egg-white texture. That change typically precedes ovulation by a few days, which makes it prospective and therefore useful. It is also the physiological reason sperm survive that long; without it, they do not.

LH tests detect the surge 24 to 36 hours before ovulation. Useful and precise, though they tell you the window is closing rather than opening.

Basal body temperature confirms ovulation after it happened. Valuable across months for confirming you ovulate and roughly when, useless for timing this cycle.

How to tell when you ovulate goes through each in more detail, including where LH tests mislead in PCOS.

Why app predictions miss

An app that only knows your period dates is not detecting ovulation. It is calculating, from your recent cycle lengths, when ovulation usually happens for someone with those cycles.

That prediction fails exactly when it matters most. Because the follicular phase is the elastic half of the cycle, a stressful month, an illness or a training block pushes ovulation later — while the app is still pointing at day 14. Why your cycle length changes covers the mechanism.

Apps that take in observed data — an overnight temperature from a wearable, logged mucus, LH results — are doing something categorically better: recording rather than predicting.

How often, and the rest of the practical advice

Guidelines converge on every one to two days across the fertile window. Daily is fine and does not meaningfully deplete sperm quality in most men. Longer abstinence to "save up" is counterproductive — sperm DNA fragmentation increases with prolonged abstinence.

Some other things worth knowing, because they come up constantly:

• Position and lying down afterwards have no demonstrated effect. Sperm reach the cervix within minutes.
• Most commercial lubricants impair sperm motility. Fertility-friendly formulations exist, or use mineral oil.
• Stress does not prevent conception directly, but it can delay ovulation, which shifts the window. That is a real effect with an indirect mechanism — how stress affects your cycle.
• Neither partner should be optimising alone. Male factor contributes to roughly half of infertility cases, and a semen analysis is a simple, early, under-used test.

Timing is not usually the problem

Worth saying plainly, because the fertility content ecosystem implies otherwise: for most couples, imperfect timing is not what is standing in the way. Conception is a probabilistic process even when everything is right, and the per-cycle chance is modest for everyone.

Roughly 80 percent of couples conceive within a year of trying, and a substantial share of the remainder conceive in the second year. How long conception actually takes has the numbers and the point at which it is reasonable to ask for help — six months if you are 35 or over, twelve if you are under.

Optimising timing raises your per-cycle probability somewhat. It does not overcome a structural problem, and spending two years perfecting technique instead of getting a basic workup is a common and costly detour.

A few cycles of observation beats a year of assumption

The most valuable thing you can do early is establish whether and when you actually ovulate — not according to an average, but according to your own signals across a few cycles. That answer changes what you do next, and it is the first thing a fertility clinic will want.

Naked is built to keep that record: cervical mucus, temperature, LH results and symptoms logged alongside your cycle, sleep, stress and everything else, so you can see your real pattern rather than a predicted one — including the months when it moved, and what moved it.