Conception Calculator
Estimate the days you are most likely to conceive, the ovulation and intercourse windows around them, the earliest sensible pregnancy-test day, and the due date if you conceive — from the first day of your last period, projected over six cycles.
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The fertile window
Conception — the start of a pregnancy, when an egg is fertilized — can only happen inside a short fertile window each cycle. Sperm remain viable inside the body for up to five days (in some cases seven), while the egg survives only 12–24 hours after ovulation. Regular intercourse across the five days before ovulation and on the day itself therefore covers the window — and within it, the last three days are the most fertile of all: intercourse in that stretch results in pregnancy in approximately 30% of cases. The calculator's "most probable conception days" run from two days before the estimated ovulation day to three days after the window opens — the extra day at the end reflecting the egg's short life after release.
Making the window work for you
The single best preparation is simply regular sex — two to three times a week — rather than saving everything for one predicted day; it keeps favorable, fertile-quality cervical mucus present, which protects sperm and extends their lifespan. To aim more precisely, layer your own signals onto the calendar. Basal body temperature: your lowest temperature in 24 hours, taken with a sensitive basal thermometer the moment you wake, before even sitting up, at about the same time each morning. Before ovulation it sits around 97.2–97.7 °F; two or three days after ovulation it rises by about half a degree to a degree and stays up until your period. Cervical mucus: dry for several days after your period, then cloudy and sticky like paste, then — a few days before ovulation — clear, slippery, and stretchy like raw egg white, in greater quantity, lasting until ovulation day.
If your ovulation day wanders month to month (day 14, then 17, then 12), combine the signals — the mucus change or the temperature shift usually keeps a consistent lead time even when the calendar day doesn't. And if charting itself becomes a stressor, drop it: sex every two days covers the fertile window almost as well, since sperm live up to seven days. Feeling good is not a distraction from the process; it is part of it.