Pregnancy can be dated three ways in routine and fertility care: from the last menstrual period, from a known conception or ovulation date, or from an IVF embryo transfer date. Each method uses a different day offset to reach the same kind of answer — gestational age and an estimated due date — because each anchors to a different point in the reproductive timeline. This article explains how the methods relate, how to read your result, and when the calculator's estimate should defer to an ultrasound.

How the three dating methods differ

Last menstrual period (LMP) dating is the traditional default: Naegele's rule adds 280 days to the first day of the LMP, assuming a 28-day cycle with ovulation on day 14. If a person's average cycle runs longer or shorter than 28 days, the calculator adjusts the estimated due date accordingly — a longer cycle pushes ovulation (and the due date) later, a shorter cycle pulls it earlier.

Conception-based and IVF-based dating skip the LMP assumption entirely and count from an actual, known event. A known conception or ovulation date adds 266 days (280 minus the ~14 days between LMP and ovulation). IVF transfer dating goes one step further: because the embryo's age at transfer is precisely known in a lab setting, the offset accounts for it directly — 263 days for a Day-3 transfer, 261 days for a Day-5 (blastocyst) transfer. All three methods are built on the same 280-day (40-week) standard pregnancy length, so a correctly entered date from any method should produce a consistent estimated due date.

Reading gestational age, EDD, and trimester

Gestational age is always expressed relative to the LMP-equivalent starting point — even on IVF dating, where the transfer date itself already corresponds to a gestational age of 2 weeks 3 days (Day-3) or 2 weeks 5 days (Day-5), not zero. This is a common point of confusion for IVF patients comparing their calendar to friends dating from LMP.

The trimester shown follows standard obstetric boundaries in completed weeks and days (first trimester through 13w6d, second trimester 14w0d-27w6d, third trimester from 28w0d), not calendar months. The Trimester Breakdown tab also shows ACOG/SMFM term-timing classification once gestational age reaches 37w0d, since "due date" is a single point estimate but a wide range of delivery timing (37w0d-41w6d) is considered term.

Limits — when ultrasound dating should take precedence

This calculator implements the standard formulas but does not perform ultrasound-based dating or redating. Per ACOG Committee Opinion No. 700, first-trimester ultrasound (particularly crown-rump length) is the most accurate method for establishing gestational age, and when it disagrees with LMP-based dating by more than about 5 days in the first trimester (roughly 7-10 days later in pregnancy), the ultrasound estimate generally takes precedence.

This tool is for education and clinical reference only. LMP dates can be uncertain or irregular cycles can shift the true ovulation day, and conception/IVF dates depend on accurate record-keeping. Any discrepancy between dating methods, or any decision about pregnancy management, should be resolved with the treating obstetric provider based on the complete clinical picture — including, when available, ultrasound measurement.

Looking for a fuller pregnancy-planning experience with milestone tracking and a multi-method comparison? See the Due Date Calculator.