Estimate your due date from LMP, conception, ultrasound, or IVF transfer and see how far along the pregnancy is today.
How to use our Due Date Calculator
- Choose a method in Calculate from based on the date source you trust most: Last menstrual period (LMP), Conception date, Ultrasound date, or IVF transfer date.
- Enter only the fields shown for that method, such as First day of last menstrual period (date) and Average cycle length (days), or the matching ultrasound or IVF details.
- Click Calculate to see Estimated due date, Gestational age today, Estimated conception date, and trimester milestone dates.
- Sanity-check the result by reading Dating method note and confirming the timeline matches what you know; if an early ultrasound or IVF record disagrees with LMP, that method is often the better dating basis.
Definitions
Calculate from: The dating basis you choose for the estimate.
First day of last menstrual period (date): The first day your last period started.
Average cycle length (days): Your usual number of days from one period start to the next.
Gestational age at ultrasound (weeks) and Gestational age at ultrasound (days): How far along the scan said the pregnancy was on that exam date.
Embryo age at transfer (days): The embryo age used in IVF, usually 3 or 5 days.
Estimated due date: The expected date the pregnancy reaches 40 weeks by the chosen method.
Gestational age today: How far along the pregnancy is today, shown as completed weeks plus extra days.
Estimated conception date: The likely fertilization date calculated from the due date.
Dating method note: A short explanation of how the selected method works and when it is stronger or rougher.
Common mistakes and quick fixes
Mistake: Entering the last day of bleeding in First day of last menstrual period (date) .
Fix: Use the first day bleeding started, because the LMP method counts from that date.
Mistake: Typing a future date into Conception date (date) , Ultrasound date (date) , or IVF transfer date (date) .
Fix: Enter a real past calendar date. Future dates are blocked because they cannot be used for dating.
Mistake: Entering 7 or more in Gestational age at ultrasound (days) .
Fix: Keep days from 0 to 6 only, and put full weeks in Gestational age at ultrasound (weeks) .
Mistake: Using an irregular guess in Average cycle length (days) for LMP dating.
Fix: Enter your usual whole-number average cycle length, from 20 to 45 days, or leave the default 28 if that is closest.
Mistake: Choosing Last menstrual period (LMP) when you have a more exact IVF record.
Fix: Switch Calculate from to IVF transfer date and choose the correct Embryo age at transfer (days) .
Mistake: Treating Estimated due date as the exact birth date.
Fix: Read it as an estimate and use Dating method note to understand why the date may shift with better records.
Limitations & Key Assumptions / Boundary Conditions
- This calculator gives an estimate, and real birth dates often happen before or after the estimated due date.
- LMP dating assumes a standard 280-day pregnancy from the first day of the last period, with a 28-day cycle unless you adjust Average cycle length (days) [1].
- If cycles are very irregular, the LMP-based result may be less reliable than a well-documented early ultrasound or IVF record.
- Ultrasound mode depends on the scan date and the gestational age entered for that exact date. Entering the wrong weeks or days will shift every result.
- IVF mode only supports embryo ages of 3 or 5 days because those are the transfer options in this calculator.
- Gestational age today can be negative if the implied pregnancy start date is still in the future relative to today; that usually means an entered date needs review.
- Clinicians may revise dating when medical records, early ultrasound findings, or IVF documentation provide a better pregnancy timeline [1].
Methodology
How the calculator works
The calculator first finds one estimated due date from your selected dating basis. It then uses that same due date to calculate the other timeline outputs so the dates stay consistent.
Formulas used
EDD = LMP + 280 days + (cycle_length - 28 days)
EDD = conception_date + 266 days
EDD = ultrasound_date + (280 days - (ultrasound_weeks x 7 + ultrasound_days))
If embryo_age = 5, EDD = transfer_date + 261 days
If embryo_age = 3, EDD = transfer_date + 263 days
GA_today_days = today - (EDD - 280 days)
estimated_conception_date = EDD - 266 days
What each result means
Estimated due date and 40-week mark are the same date in this calculator. End of first trimester is set at 13 weeks 6 days, and End of second trimester is set at 27 weeks 6 days. Gestational age today is converted from total days into completed weeks plus leftover days.
Mini example
If First day of last menstrual period (date) is January 1, 2026 and Average cycle length (days) is 28, the due date is October 8, 2026.
EDD = Jan 1, 2026 + 280 days = Oct 8, 2026
estimated_conception_date = Oct 8, 2026 - 266 days = Jan 15, 2026
That means the pregnancy timeline is anchored to an estimated start date 280 days before the due date, and all milestone dates are counted forward from that same timeline.
Method notes
LMP is the common starting method, but it assumes ovulation around day 14 of a 28-day cycle unless cycle length is adjusted. Conception mode uses 266 days from fertilization. Ultrasound mode treats the scan's reported gestational age on the exam date as the anchor. IVF mode uses transfer date plus an embryo-age-specific offset. ACOG notes that pregnancy dating may be revised when better clinical dating information is available, especially with ultrasound or assisted reproduction records [2].
Assumptions and limits
This tool uses calendar-day arithmetic, including leap years and month boundaries. It does not diagnose pregnancy health, detect complications, or replace clinical dating. Results can differ from medical records if entered dates are uncertain or if a clinician uses a different official dating basis.