pregnancy trimesters by week
The due date calculator classifies the trimester from a single number: the total complete days since the LMP. The cutoffs are not approximate, they are exact days, and that same day total also feeds the pregnancy progress percentage. This page shows both trimester cutoffs and what happens to the percentage once the pregnancy passes the EDD.
One day before the 2nd-trimester cutoff. Dividing 97 by 280 gives 34.6%, the highest number the calculator still classifies as 1st trimester.
A single day after the previous example flips the displayed trimester from 1st to 2nd, and the percentage rises to a round number, 35.0%, because 98 is a multiple of 14.
Same pattern as the previous cutoff: 196 is also a multiple of 14, so the percentage lands exactly on 70.0% on the first day of the 3rd and last trimester the calculator recognizes.
Naegele's Rule adds 280 days (40 weeks) to the last menstrual period date to estimate the due date. It assumes a 28-day cycle and ovulation on day 14. Longer or shorter cycles shift the actual time of ovulation, which is why ultrasound is more accurate for confirming or revising the individual EDD.
The calculator has no defined "end" for the 3rd trimester, it starts on day 196 and keeps showing as 3rd trimester even past the EDD, on day 280. What changes after the EDD is only the counter's text, which switches from "X days until the estimated date" to "X days past the estimated date".
Because the formula caps the result at 100%: complete days divided by 280, but never above 100. At 281 days, one day past the EDD, the percentage still reads 100.0%, and it stays that way on any later day, it is the days-past-EDD counter that keeps rising, not the progress bar.
They are calendar cutoffs applied to the count from the LMP, the same criterion as ACOG's FAQ156, not a measurement of that specific baby's development. Two pregnancies with the same total days since the LMP always land in the same trimester in this calculator, even if their actual development varies.
Medical notice: this calculation is educational, based on population-level averages, and does not replace prenatal care, ultrasound, diagnosis or guidance from a doctor or obstetrician.
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