Gestational Age Calculator (LMP)

The appointment tells you 12 weeks and 3 days, and a week later you have lost track. The field labelled DUM takes the first day of your last menstrual period; the one labelled Data referência takes the date you want to check. The answer comes back as 12s 4d, the same weeks-plus-days shorthand a prenatal card uses: completed weeks first, leftover days after.

The maths is the plain difference in days between the two dates, divided by seven. The convention is what trips people up: obstetric dating starts on the first day of the last period, not at conception. Since ovulation usually falls around day 14 of the cycle, by the time an embryo exists the pregnancy already counts as two weeks along. Gestational age therefore runs about two weeks ahead of the embryo's actual age.

Check the reference date before reading the number, because it loads with a fixed default rather than today. This is pure calendar arithmetic: no adjustment for irregular cycles, no ultrasound input, no warning when the result stops being plausible. If your cycle is not close to 28 days, or the last period is a guess, the first-trimester scan is what sets the dating. No date leaves your browser.