Due Date Calculator
Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer
Last updated: August 19, 2026Three Starting Points, One Due Date
Not every pregnancy is dated the same way. Most are dated from the last menstrual period, but IVF pregnancies have a known fertilization date that makes LMP dating unnecessary and less accurate, and some people simply know their conception date more precisely than their last period. This calculator handles all three starting points and converts each into a due date using the arithmetic appropriate to it.
The Formulas
From conception date: Due Date = Conception Date + 266 days
From IVF transfer: Due Date = Transfer Date + (266 − Embryo Age at Transfer)
280 days (40 weeks) is counted from the last period; 266 days (38 weeks) is counted from conception itself, which happens roughly two weeks into a standard cycle. The IVF version starts from a known transfer date and simply subtracts however many days old the embryo already was at transfer.
The Three Methods Agree, Given Consistent Dates
For a hypothetical last period start of January 5, 2026 on a 28-day cycle (implied conception January 19, implied blastocyst transfer January 24):
| Mode | Input date used | Computed due date |
|---|---|---|
| From last period | Jan 05, 2026 (cycle 28) | Oct 12, 2026 |
| From conception date | Jan 19, 2026 | Oct 12, 2026 |
| From IVF transfer (Day 5) | Jan 24, 2026 | Oct 12, 2026 |
| From IVF transfer (Day 3) | Jan 22, 2026 | Oct 12, 2026 |
All four rows land on the same date because the underlying inputs are equivalent — this is the arithmetic check, computed with the calculator's own formulas, not a claim that real cycles are always this tidy.
Which Mode to Use
- From last period — the standard method for a natural conception when you know your LMP and typical cycle length.
- From conception date — useful when ovulation timing is known more precisely than the last period, for instance from ovulation tracking.
- From IVF embryo transfer — the standard for IVF pregnancies, since the embryo's exact age is known at transfer, removing the guesswork inherent in LMP dating.
How to Use This Calculator
- Select a mode: From Last Period Date, From Conception Date, or From IVF Embryo Transfer Date.
- For last period mode, enter the Last Period Start Date and Average Cycle Length (days) (default 28).
- For conception mode, enter the Conception Date.
- For IVF mode, enter the Embryo Transfer Date and select Day 5 (Blastocyst) or Day 3 (Cleavage-stage).
- Select Calculate for the estimated due date.
Related Calculations
For an IVF-specific version with additional dating detail, see the IVF Due Date Calculator. To track gestational age and trimester once you have a due date, use the Pregnancy Week Calculator.
Obstetrical Principles of Estimated Due Date (EDD) Calculations
An obstetrical pregnancy due date calculator estimates the anticipated calendar delivery date (Estimated Date of Delivery, EDD) based on clinical reproductive milestones. In obstetrics and maternal-fetal medicine, calculating an accurate EDD is essential for scheduling prenatal screenings (such as non-invasive prenatal testing NIPT, anatomy ultrasounds, and gestational diabetes screening) and assessing fetal growth trajectories.
Naegele's Rule: The Standard Clinical Formula
Formulated by German obstetrician Franz Karl Naegele in the 19th century, Naegele's Rule calculates the EDD based on the first day of the mother's Last Menstrual Period (LMP), assuming a regular 28-day cycle with ovulation occurring on Day 14:
Cycle Length Adjustments and Parikh's Formula
For women whose menstrual cycles deviate from the standard 28-day cycle, Naegele's rule is adjusted using Parikh's Formula by accounting for the true luteal phase duration:
- 32-Day Cycle: Ovulation occurs on Day 18 (4 days later); add 4 days to standard EDD.
- 24-Day Cycle: Ovulation occurs on Day 10 (4 days earlier); subtract 4 days from standard EDD.
Gestational Age vs. Conceptional (Embryonic) Age
| Dating Metric | Reference Starting Point | Full-Term Duration |
|---|---|---|
| Gestational Age (Menstrual Age) | First day of Last Menstrual Period (LMP) | 40 weeks (280 days) |
| Conceptional / Embryonic Age | Actual day of fertilization / ovulation | 38 weeks (266 days) |
Ultrasound Dating: Crown-Rump Length (CRL)
According to American College of Obstetricians and Gynecologists (ACOG) clinical guidelines, if a first-trimester ultrasound measurement of the fetal Crown-Rump Length (CRL) differs from the LMP-calculated due date by more than 5 to 7 days, the ultrasound CRL measurement becomes the official clinical EDD, providing the gold standard for gestational precision (±3 to 5 days accuracy).
Step-by-Step Worked Calculation Example
Example: Calculating EDD for a 31-Day Menstrual Cycle
Problem: A pregnant patient reports that the first day of her Last Menstrual Period was May 12, 2024. She has a consistent 31-day menstrual cycle. Calculate her Estimated Date of Delivery using adjusted Naegele's rule.
Step 1: Apply standard Naegele's Rule:
LMP Date = May 12, 2024
Subtract 3 months = February 12, 2024
Add 1 year = February 12, 2025
Add 7 days = February 19, 2025 (Standard 28-day cycle EDD)
Step 2: Adjust for 31-day cycle (Cycle Length - 28 = 31 - 28 = +3 days):
Adjusted EDD = February 19, 2025 + 3 days = February 22, 2025
Conclusion: The patient's adjusted clinical due date is February 22, 2025.
Statistical Delivery Distribution
- Only 4% to 5% of babies are born on their exact EDD. Approximately 80% of spontaneous births occur within the full-term window between 37 weeks 0 days and 41 weeks 6 days.
Prenatal Screening Windows and Trimester Scheduling
An accurate Estimated Due Date governs the precise clinical timing for standard prenatal genetic and diagnostic screenings:
- First-Trimester Combined Screen & NIPT (Weeks 10 to 13): Cell-free fetal DNA screening for trisomies 21, 18, and 13.
- Comprehensive Anatomy Ultrasound (Weeks 18 to 22): Detailed anatomical survey of fetal organ development, placental placement, and amniotic fluid index (AFI).
- Gestational Diabetes Screening (Weeks 24 to 28): 1-hour 50-gram oral glucose tolerance challenge test.
- Group B Streptococcus (GBS) Screening (Weeks 36 to 37): Vaginal-rectal culture swab prior to labor onset.
In Vitro Fertilization (IVF) Due Date Dating
In assisted reproductive technology (ART), due dates for IVF pregnancies are calculated with exact day precision based on embryo developmental age at transfer:
Day 5 Blastocyst Transfer: EDD = Transfer Date + 261 Days
Bishop Score and Cervical Readiness
As pregnancies reach full-term gestation (39 to 41 weeks), obstetricians evaluate the clinical Bishop Score (assessing cervical dilation, effacement, consistency, position, and fetal station) to determine medical readiness for spontaneous labor or planned induction.