Naegele's rule and the 280-day calculation
The estimated due date used by this calculator follows Naegele's rule, the standard convention in obstetrics for more than two centuries. It adds 280 days, which is 40 weeks, to the first day of the last menstrual period. The commonly taught shorthand version, add one year, subtract three months, and add seven days, produces the same date in most months.
The calculator implements the arithmetic directly: due date equals the last menstrual period date plus 280 days. Working in days rather than months avoids the ambiguity that arises when adding calendar months across February or across a month with 31 days, so the result is consistent regardless of where in the year the date falls.
Naegele's rule assumes a 28-day cycle with ovulation on day 14. That assumption is built into the arithmetic and is not adjustable here. Cycles longer or shorter than 28 days shift the actual conception point, and the rule does not correct for this, which is one reason the estimate is described as an estimate.
Last menstrual period mode versus conception date mode
The tool accepts either a last menstrual period date or a conception date, and converts between them using a fixed 14-day offset. In LMP mode, the conception date shown is your entered date plus 14 days. In conception mode, the LMP is derived as your entered date minus 14 days, and the 280-day calculation then runs from that derived LMP.
The practical consequence is that a conception date produces a due date 266 days later, which is 38 weeks. This is why the two modes give different answers from the same calendar date: entering 5 January as an LMP and entering 5 January as a conception date describe pregnancies two weeks apart.
Gestational age is conventionally counted from the LMP, not from conception, which means a pregnancy is described as two weeks along at the moment of conception. This convention exists because the LMP is an observable date while the moment of conception usually is not. The week figure this tool reports follows that convention.
A worked example
Suppose the first day of the last menstrual period was 5 January 2026. Adding 280 days lands on 12 October 2026, which is the estimated due date. The derived conception date is 5 January plus 14 days, or 19 January 2026.
The current week is calculated as the number of whole days elapsed since the LMP, divided by seven and rounded down. On 15 September 2026, 253 days have passed since 5 January, and 253 divided by 7 is 36.1, so the tool reports week 36. Days remaining is the difference between today and the due date, rounded up, which is 27 days in this example.
Note that the tool reports completed weeks only. Clinical notation normally writes gestational age as weeks plus days, so the same point would be recorded as 36 weeks and 1 day. If you need the days component, take the elapsed days and find the remainder after dividing by seven.
How trimesters are defined here
This calculator uses gestational week boundaries: weeks 0 through 12 are labelled first trimester, weeks 13 through 26 are second trimester, and week 27 onward is third trimester. A date before the LMP produces a week below zero and is labelled as not yet pregnant.
Trimester boundaries are a convention rather than a biological fact, and different sources place them differently. Some define the second trimester as starting at week 14, and some define the third as starting at week 28. A result near a boundary may therefore be labelled differently by another calculator or by a clinician without either being wrong.
The week count itself is unambiguous once the LMP is fixed, so if you need to compare against a different trimester convention, use the week number rather than the label.
Why the estimate and the actual date usually differ
An estimated due date is a single point drawn from a distribution. Births cluster around 40 weeks but spread across a range of weeks on either side, and a birth on the estimated date itself is the exception rather than the rule. The estimate exists to anchor the timeline of a pregnancy, not to predict a birthday.
The 28-day cycle assumption is the largest source of variation in the LMP method. Someone with a consistently 35-day cycle ovulates later than day 14, so a due date calculated by Naegele's rule from the LMP will tend to run early relative to the actual point of conception. Irregular cycles, recent hormonal contraception, and uncertainty about the LMP date all add further variability.
Dating by ultrasound, particularly a first-trimester scan, measures the pregnancy directly rather than inferring it from a cycle assumption, and clinicians will revise the estimated date when scan measurements differ from the LMP-based figure by more than a defined margin. Where a clinician has given you a dating, that is the one to work from. This calculator is a convenience for tracking dates and is not a substitute for care from a healthcare provider.