Pregnancy
Ovulation & fertile window
Ovulation is counted backwards from your next period, not forwards from your last. The luteal phase holds steady at about 14 days whatever your cycle length, while the first half is what stretches.
How this is calculated
ovulation = next period − luteal phase. Counting forward to "day 14" is the common shortcut and is wrong for anyone whose cycle is not 28 days: a 35-day cycle ovulates around day 21.
Common questions
Isn't ovulation always on day 14?
Only in a textbook 28-day cycle. The luteal phase — ovulation to next period — holds fairly steady at about 14 days whatever your cycle length, while the first half is what stretches. So a 35-day cycle ovulates around day 21, not day 14. That is why this counts backwards from your next expected period.
Why does the fertile window open five days before ovulation?
Sperm can survive up to five days in the reproductive tract, so intercourse before ovulation can still result in conception. The egg itself is only viable for about 12-24 hours, which is why the window closes on the day of ovulation.
Can I use this to avoid pregnancy?
No. Ovulation timing shifts between cycles even for people with regular ones, and this is a prediction from a calendar rather than a measurement. Calendar-based prediction is not a reliable method of contraception.
How soon can I take a pregnancy test?
Around 12 days after ovulation is the earliest most home tests become reliable. Testing before then produces false negatives, because implantation may not have happened yet and hCG has not risen enough to detect.