Due Date Calculator
Calculate an estimated due date and a clear pregnancy-week timeline without sending your date anywhere.
- Runs in your browser — files never leave your device
- Fewer clicks, keyboard friendly
- Features others lock behind paywalls
Your date is calculated in this browser and is never sent to a server.
Enter a date to see an estimated due date and week-by-week milestones.
How to use
- Choose whether you know the first day of your last period, an estimated conception date, or an IVF transfer date.
- Enter that date.
- For a last-period estimate, enter your average cycle length or leave the 28-day default.
- For IVF, choose the embryo age recorded at transfer.
- Read the estimated due date, gestational age today, and calendar dates for each pregnancy milestone.
How the due-date estimate is calculated
The usual last-menstrual-period (LMP) convention adds 280 days, or 40 weeks, to the first day of the LMP. This is often called Naegele’s rule and assumes a 28-day cycle with ovulation around day 14. This calculator shifts that estimate by the difference from 28 days: a 32-day average cycle moves the estimate 4 days later. If you know conception instead, the estimate is conception plus 266 days. These are calendar-day calculations, so daylight-saving changes do not shift a displayed date.
IVF transfer dates
For pregnancies from assisted reproductive technology, ACOG advises dating from the embryo age and transfer date rather than LMP. The standard offsets are 263 days after a day-3 embryo transfer and 261 days after a day-5 embryo transfer. Use the embryo age documented by the fertility clinic. The displayed estimated conception date is a calendar anchor for the timeline, not a statement about a biological moment of conception.
Weeks, trimesters, and term milestones
Pregnancy weeks are counted from the LMP-based dating start, which is about 2 weeks before conception in a typical cycle. The first trimester ends at 13 weeks 6 days; the second begins at 14 weeks 0 days; and the third begins at 28 weeks 0 days. ACOG defines early term as 37 weeks 0 days through 38 weeks 6 days, full term as 39 weeks 0 days through 40 weeks 6 days, late term as 41 weeks 0 days through 41 weeks 6 days, and postterm as 42 weeks 0 days or later. The calendar cards expose the start dates that are most useful for planning.
Why a due date can change
An LMP estimate is less reliable when the period date is uncertain, cycles are irregular, or ovulation occurred earlier or later than usual. ACOG notes that first-trimester ultrasound is the most accurate method for establishing or confirming gestational age; when a clinician establishes a due date, that documented date should be used. A due date is an estimate, not a prediction of the exact birth date. Contact a qualified clinician for care, symptoms, or individual medical decisions.
Frequently asked questions
How is a pregnancy due date calculated from the last period?
A standard LMP-based estimated due date is the first day of the last menstrual period plus 280 days, or 40 weeks. This calculator also shifts that estimate by the number of days your average cycle differs from 28 days.
Why is pregnancy 40 weeks when conception happens later?
Clinical gestational age starts on the first day of the last menstrual period, which is about 14 days before conception in a typical 28-day cycle. That convention makes a full-term due date 40 weeks from LMP but about 38 weeks, or 266 days, from conception.
How is an IVF due date calculated?
For IVF dating, ACOG gives an estimated due date of 263 days after a day-3 embryo transfer or 261 days after a day-5 embryo transfer. The fertility clinic’s documented embryo age and clinician-established date should be followed.
When do the pregnancy trimesters start?
The first trimester covers 0 weeks 0 days through 13 weeks 6 days, the second begins at 14 weeks 0 days, and the third begins at 28 weeks 0 days. This calculator converts each boundary to a calendar date from your entered date.
Is an early ultrasound more accurate than a due-date calculator?
A first-trimester ultrasound is the most accurate method for establishing or confirming gestational age, particularly when the LMP is uncertain or cycles are irregular. A clinician-established due date takes priority over this calculator’s estimate.
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Updated 2026-10-13