Estimate due date from last menstrual period or conception date with trimester timeline and milestones.
Give it the first day of your last menstrual period, or the conception date if you know it, and this tool returns the estimated due date 280 days later, the standard 40-week count, plus a trimester-by-trimester timeline and milestone dates. The surprise for most first-time users is that gestational age is counted from the LMP, so you are already about two weeks pregnant at the moment of conception. The other surprise: only about 4-5% of babies arrive on their exact due date. Treat the result as an estimate, not a promise.
A concrete input and expected output from the current implementation.
Input
Last menstrual period: January 1, 2026
Expected output
Estimated due date: October 8, 2026 (40 weeks from the LMP). First trimester: January 1 - April 2. Second trimester: April 3 - July 9. Third trimester: July 10 - October 8. Anatomy scan window (18-22 weeks): May 7 - June 4.
The due date is the LMP plus 280 days, which lands on October 8, 2026; Naegele's rule of nine months plus seven days gives the same date for a January start. Trimester boundaries sit at day 91 (April 2) and day 189 (July 9) of the 280-day count, and the scan window at days 126-154 (May 7 - June 4).
The tool adds 280 days (40 weeks) to the first day of the last menstrual period to get the estimated due date. This is Naegele's rule, which assumes a 28-day cycle with ovulation on day 14. If you enter a conception date instead, it adds 266 days (280 minus 14), the embryo's true age at birth. Because it counts calendar days, month lengths and leap years affect the result.
From the due date it walks the 40 weeks forward and back, assigning weeks 1-13 to the first trimester, 14-27 to the second, and 28-40 to the third, then flagging milestones along the way: the end of the first trimester, the anatomy scan window around 18-22 weeks, and full term at 39 weeks. Every date is simply the LMP plus a whole number of weeks.
Gestational age is counted from the first day of your last menstrual period, roughly two weeks before ovulation and conception. At the moment of conception you are already about 2 weeks along by this count, which is why a pregnancy is called 40 weeks even though the embryo itself is only about 38 weeks old. This is the standard medical convention, not a quirk of this tool.
The LMP calculation assumes ovulation on day 14 of a 28-day cycle. With a 35-day cycle, ovulation typically falls around day 21, so the true due date is roughly a week later than the formula gives. The most accurate method is a first-trimester ultrasound, which is why most providers use one to confirm or adjust the date.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
Calculate percentages, percentage change, and what percent one number is of another with step-by-step breakdowns.
Calculate tips and split bills between multiple people with customizable tip percentages and rounding options.
Calculate monthly payments, total interest, and generate amortization schedules for loans and mortgages.
Calculate Body Mass Index from height and weight with category classification and healthy range guidance.
Calculate sale prices, savings amounts, and final costs after single or stacked percentage discounts.
Compute mean, median, mode, standard deviation, variance, and other descriptive statistics from a data set.