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Trying to conceive

Which of your fertile days actually count?

Not every fertile day is worth the same. This ranks all six by the chance of conception measured for that day, so you can see where the window actually peaks.

Your dates stay on this device. Nothing you enter here is saved or sent to Flowy.

Also called fertile days calculator, conception chance calculator. Same calculation, one page.

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Which days actually count?

First day of your last period

The calculation

How this estimate works

The tool adds your cycle length to your last period to find the next one, then counts back by your luteal phase length to place ovulation. The fertile window is the five days before that date plus ovulation day, which is the span over which sperm can survive and still meet an egg.

Each day then carries the conception probability measured for that position in the window. The two days before ovulation score highest because sperm are already waiting when the egg arrives.

Good to know

Leave room for real life

A calendar cannot see an LH surge. It can only tell you where ovulation usually lands for a cycle shaped like yours. Cervical mucus changes and ovulation tests both respond to what your body is doing this month, which a date never will.

Timing is the input people can control, so it gets the attention. It is rarely the binding constraint. Age, sperm quality, tubal health, and ovulatory disorders all move the odds more than hitting the exact right day.

Common questions

A little more context

Which days in the fertile window matter most?

The two days before ovulation and ovulation day itself carry the highest chance. Sperm can wait several days for an egg, so intercourse before ovulation beats intercourse after it. The egg only survives about 12 to 24 hours once released.

Where do the percentages come from?

They come from Wilcox, Weinberg and Baird (New England Journal of Medicine, 1995), which tracked intercourse timing against confirmed pregnancies. Each figure is the chance of pregnancy from intercourse on that single day, not the chance of conceiving in the cycle overall.

What are my odds across a whole cycle?

For couples under 35 with no known fertility problem, roughly 20 to 25 percent per cycle. Most who are going to conceive do so within a year of trying. Age moves that figure more than timing does.

Why does the calculator ask for my luteal phase length?

The luteal phase, from ovulation to your next period, is the steadier half of the cycle. Counting back from your next period places ovulation more accurately than assuming it falls at the midpoint, which matters most if your cycles run long or short.

When should we talk to someone about fertility?

General guidance is after a year of regular unprotected sex, or after six months if you are 35 or older. Sooner if periods are absent or very irregular, or if there is a known condition such as endometriosis or PCOS.

Keep exploring

Ovulation calculator

Estimate a six-day fertile window and likely ovulation date.

Safe days calculator

Map the calendar method across your cycle, with its real failure rate.

BBT chart

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