Eleven calculators, one dating method each, named on the page.
Someone building this site could have shipped a due date box that spits out a date with no explanation, the way most of them do. Instead, every tool here names the arithmetic behind it, so a person who wants to check the math by hand, or ask a provider a sharper question about it, actually can.
Type a last period into a typical pregnancy app and you get a date with no visible reasoning. Ask what happens when an ultrasound later disagrees with that date, and most apps have no answer at all. PregnancyTools starts from the opposite direction: Naegele's rule for the last-period math, ACOG's own thresholds for exactly when a scan should overrule it, and a dated source for every other figure, listed in full on editorial standards.
Chris Terry built this site, along with the rest of the calculator sites under Encore Promotional Products' Encore Editorial umbrella, and he owns and edits it. The dating guides are written by Naomi Foster. Neither one has a clinical background, and the site does not pretend otherwise; the authors page spells out who writes what, what gets fact-checked before a guide goes up, and against which source. Content review here does not involve a clinician; it is a check against the published ACOG and CDC documents named on the editorial standards page, and it is not a substitute for medical advice.
A cycle length or an hCG figure is not the kind of thing that belongs on a stranger's server. Every calculator here runs client-side, in the browser tab you already have open; nothing you type gets sent anywhere, logged, or saved between visits. Close the tab and the fields are empty again next time. The privacy page spells out the rest.
Naegele's rule assumes a 28-day cycle with ovulation on day 14, an assumption plenty of real cycles do not match, and once a first-trimester ultrasound exists, ACOG treats it as the more reliable dating method. A number from this site is a planning estimate built on that logic, not a diagnosis and not a stand-in for a scan. If a result here disagrees with something your provider already told you, go with the provider.
Display ads and a handful of affiliate links pay for hosting and the time spent checking sources; clicking one and buying something may earn a small commission at no cost to you. None of that changes which formula a calculator runs. The full disclosure is at the bottom of every page.