Updated · Built and checked by the PlinzoTools team, formulas tested against published sources
Calculate current gestational age and trimester from the last menstrual period.
44w 6d
This tool calculates how far along a pregnancy is -- gestational age in weeks and days, plus trimester -- from the first day of the last menstrual period (LMP), the standard clinical dating convention (pregnancy is dated from LMP, not conception).
Enter the first day of your last period and the date you want the calculation as of (defaults to today). Gestational age, trimester, and estimated due date all appear instantly.
Days Pregnant = (As-Of Date - LMP Date)
Weeks = floor(Days / 7), Days = Days % 7
Trimester 1: weeks 0-12, Trimester 2: weeks 13-26, Trimester 3: weeks 27+. Due date estimated as LMP + 280 days (Naegele's rule).
This is a standard dating estimate based on average cycle length; actual gestational age can differ, especially with irregular cycles. A healthcare provider's ultrasound-based dating is more precise. This tool is for general information, not medical advice.
Assume, purely as a hypothetical, that the first day of the last period was 1 January 2026 and you want the result as of 12 March 2026. The number of days between those dates is 70 (31 in January, 28 in February, plus 11 in March). Dividing by 7 gives exactly 10 weeks and 0 days, and because 10 is below 14 the tool reports trimester 1. The estimated due date is the LMP plus 280 days, which lands on 8 October 2026. The tiles also show the total days counted, 70 in this case.
Change the as-of date to 20 March 2026 and the total becomes 78 days, or 11 weeks and 1 day. The weeks and days format matches how clinicians talk about pregnancy, such as "11 plus 1," so you can compare it directly with what a provider tells you.
Days = As-of date - LMP. Weeks = whole weeks in Days; extra days = Days mod 7. Due date = LMP + 280 days
The tool uses completed weeks to assign trimesters: weeks 0 to 13 are trimester 1, weeks 14 to 27 are trimester 2, and week 28 onward is trimester 3, following the ACOG convention (first trimester runs through 13 weeks 6 days). A few sources draw the boundaries one week differently, so a result at the very edge of a boundary may be labeled differently by your provider. The label is a convenience; the week count is the meaningful figure.
The 280 day rule assumes a 28 day cycle with ovulation around day 14. If your cycles are longer, shorter, or irregular, real gestational age can differ by days or even a couple of weeks. Early ultrasound measurements are often used to confirm or adjust the due date. When your provider gives you a date, follow that one. This calculator is not a diagnosis and does not replace prenatal care, and any concern about symptoms, bleeding, or pain should go to a healthcare professional promptly.
People use the same calculation to plan appointment windows, leave from work, or when to share news. Partners and family often ask "how many weeks is she now," and the as-of date field answers that for any day you choose.
It's the standard clinical convention (Naegele's rule) since the exact conception date is often unknown, while the last period's start date is typically easier to identify.
A due-date calculator gives one target date; this tool tells you how far along you are right now (as of any date you choose), which changes week to week.
No -- it's an estimate (only a small percentage of babies are born exactly on the calculated due date); actual delivery timing varies by several weeks in either direction for a normal pregnancy.
It means 70 full days have passed since the first day of the last period. Clinicians write this as 10w0d, and the following day would be 10w1d.
Not directly. The tool is built on the LMP. Conception typically occurs about two weeks after the LMP in a regular cycle, so entering a conception date would understate gestational age by roughly that amount.
The tool applies a single fixed rule, LMP plus 280 days, so it returns one estimate. Actual birth commonly occurs some time before or after it.
No. If an ultrasound has set a due date, your provider's dating is more precise than a calendar-based calculation.
Reference: Centers for Disease Control and Prevention