Cycle and fertility
Period dates, ovulation, the fertile window and early-pregnancy hormones — every figure computed in your browser and sent nowhere. Predictions are rendered as windows, because that is what the underlying data supports.
These numbers never leave your browser
Cycle dates are among the most sensitive things a person can type into a website. Every calculator in this wing runs entirely in your browser: the dates you enter are not transmitted, not logged, and not stored on any server, and every page in this wing says so on the page rather than burying it in a privacy policy.
The consequences are deliberate and visible. There is no account to create, no sign-in, and nothing that follows you between visits unless you choose to save it locally. Two of these tools - the implantation calculator and the hCG doubling time calculator - also carry no advertising at all, because of who is likely to be reading them and what they are likely to be reading them for. A test in this repository fails the build if an advert ever appears there.
Why every answer here is a window
Ovulation is estimated, not observed. This site counts backwards from the next expected period rather than forwards to a fixed day 14, because the follicular phase is the part that varies most: Bull and colleagues put its 95 percent interval at 10 to 30 days across 612,613 cycles.
The same dataset found a mean cycle length of 29.3 days and only 13 percent of cycles exactly 28 days long, and a mean luteal phase of 12.4 days with a 95 percent interval of 7 to 17 days. Mapping that interval onto an entered cycle length gives a band about ten days wide around any estimated ovulation date. A single confident date would be hiding ten days of published uncertainty behind one number.
The forecasting tools compound this honestly. A six-cycle forecast applies the same assumed cycle length six times, so its error compounds six times too, and the copy on the page says as much rather than presenting cycle six with the same confidence as cycle one.
What the fertile window is, and what the evidence for it is
ACOG's patient guidance puts the fertile window from five days before ovulation through one day after it. Wilcox, Weinberg and Baird found in 1995 that conceptions arose only from a six-day window ending on the day of ovulation - no pregnancy in that study came from intercourse after ovulation had occurred.
This site shows both. The displayed window follows ACOG, and the evidence window from Wilcox is drawn alongside it, so the extra day is visible as what it is: an allowance for uncertainty in ovulation timing and sperm survival, not a day on which a conception was ever recorded.
Early pregnancy: arithmetic, and nothing beyond it
Implantation was dated by Wilcox, Baird and Weinberg in 1999 through the first appearance of chorionic gonadotropin in daily urine samples: 6 to 12 days after ovulation, with 84 percent of pregnancies on day 8, 9 or 10. That is a hormone measurement, and this site reports it as one. There is no symptom list here, because the study that dated implantation did not find symptoms, and no earliest test date, because home test sensitivities differ too much for a printed date to be meaningful.
The hCG tool is the clearest statement of this wing's posture. A doubling time is division and a logarithm applied to two values and the hours between them - it is not a prognosis. The published reference point is Barnhart 2004, which observed a slowest rise of 24 percent over one day and 53 percent over two among symptomatic women who went on to have a viable intrauterine pregnancy. Those figures replaced an older 66-percent-in-48-hours teaching that had labelled viable pregnancies abnormal, and the authors' own conclusion was that intervention should be more conservative, not less.
So the tool returns arithmetic and a comparison against a published minimum, and stops. No verdict, no probability, no normal-or-abnormal flag. Somebody using it may be having the worst week of their life.
What a prediction here can and cannot be checked against
A cycle prediction is testable in a way most health estimates are not: the next period either arrives when the arithmetic said or it does not. That makes recorded cycle lengths the most valuable input you can give any of these tools, and it is why the period calculator accepts an average of your own recent cycles rather than assuming 28 days.
What cannot be checked from a calendar is ovulation itself. Nothing in this wing observes it; the date is inferred from a cycle length by subtracting an assumed luteal phase. Ovulation predictor kits and basal temperature tracking measure or infer something closer to the event, and a clinic can do better still. Where a measured date exists, enter it - every tool here accepts an ovulation date directly in preference to deriving one.
Two things follow from that division. A tool that prints a single ovulation date without a band is overstating what the arithmetic supports, and one that prints an ovulation date derived from an unrecorded cycle length is stacking two assumptions. This site shows the band and names the assumption in both cases.
In-depth guides
Calculators for this topic
A period calculator projects your next period dates from the first day of your last one and your average cycle length. It lists the next six cycles with the fertile window in each, and shows how wide the prediction really is: in 612,613 tracked cycles, only 13 percent were exactly 28 days long.
This ovulation calculator counts back from your next expected period rather than forward from your last one, because the first half of the cycle is what varies. It dates estimated ovulation, ACOG's fertile window of five days before to one day after, and the six-day conception window Wilcox measured in 1995.
A conception date calculator subtracts 266 days from an estimated due date, or adds 14 days to the first day of a last period, following the convention ACOG publishes. It returns a conception window rather than a single day, because sperm survive for several days and ovulation timing varies between cycles.
An implantation calculator dates the window in which a pregnancy implants, from your estimated ovulation date. Wilcox and colleagues measured this directly in 1999: implantation happened 6 to 12 days after ovulation, and 84 percent of it on days 8, 9 or 10. This page carries no ads.
This works out the doubling time between two hCG results and the percentage change between them, using the interval you enter. It shows the arithmetic and the slowest rise Barnhart and colleagues recorded in 2004 for a viable intrauterine pregnancy. It does not interpret your result. This page carries no ads.
Common questions
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