Pregnancy Milestone Calculator
Trimesters, due date and milestone planner.
These calculators are for informational purposes only and do not constitute legal, financial, or professional advice.
How the Pregnancy Milestone Calculator works
The Pregnancy Milestone Calculator projects a due date and the two trimester boundaries from a single input: the first day of your last menstrual period (LMP). It applies Naegele's rule — adding 280 days (40 weeks) to the LMP — the same standard used across most clinical intake forms and pregnancy-dating tools.
The 12-week and 27-week markers it also returns are counted from the same LMP date, not from the due date backward, so all three figures move together if the LMP date changes. Naegele's rule assumes a 28-day cycle with ovulation on day 14; for irregular cycles, or once a first-trimester ultrasound has produced a more precise dating, treat this projection as a starting estimate rather than a final one.
This tool is built for planning purposes — timing leave, travel, or milestone-related decisions — not as medical guidance. Only a small share of pregnancies deliver on the exact calculated due date; a range of a couple of weeks on either side is the clinical norm. Confirm and adjust your due date with your prenatal care provider, who has access to ultrasound dating this calculator does not.
Worked example
A last menstrual period starting April 7 projects a due date of January 11 the following year (280 days later), an end-of-first-trimester marker around June 30 (12 weeks), and an end-of-second-trimester marker around October 13 (27 weeks) — all three computed from the same April 7 anchor date.
Frequently asked questions
Why does the calculator ask for the last period date instead of the conception date?
Because the first day of the last menstrual period is the one date most people can state with confidence, while the actual date of ovulation or fertilization usually is not known precisely. Naegele's rule is built around the LMP for exactly this reason.
What if my cycle is not 28 days?
The formula assumes a 28-day cycle with ovulation on day 14, so longer or irregular cycles can shift the real due date away from the LMP-based estimate by several days to over a week. A first-trimester ultrasound is generally considered more accurate than LMP-based dating alone when the two disagree by more than about a week.
Is this the same due date my doctor will give me?
It should closely match an initial LMP-based estimate, since both use the same underlying rule. Your provider may later revise the date based on an ultrasound measurement — that revision reflects better information, not an error in either calculation.