Due Date
Learn how to calculate your due date using various methods including last menstrual period, conception date, IVF transfer, and ultrasound measurements. Understand why due dates are estimates rather than guarantees.
What Is a Due Date?
A due date, or estimated due date (EDD), is the projected date when a baby is expected to arrive. It's calculated as 280 days (40 weeks) from the first day of the mother's last menstrual period (LMP), assuming a regular 28-day cycle. This calculation method assumes conception occurs around day 14 of the menstrual cycle.
How to Calculate Your Due Date
Last Menstrual Period (LMP) Method
The LMP method is the most common way to calculate a due date. Healthcare providers add 280 days to the first day of the last period. For example, if a woman's last period started on January 15, her due date would be approximately October 22 (plus or minus a few days depending on the actual cycle length).
Conception Date Method
If the exact conception date is known, 266 days (38 weeks) are added to that date. This method is most accurate when ovulation was tracked using ovulation predictor kits, basal body temperature charting, or fertility monitoring devices. For instance, if a woman knows she conceived on February 15, her due date would be November 10.
IVF Transfer Date Method
For IVF pregnancies, the due date is calculated based on the embryo transfer date and the age of the embryo at transfer:
- Day 3 transfer: Due date = transfer date + 263 days
- Day 5 transfer: Due date = transfer date + 261 days
- Day 6 transfer: Due date = transfer date + 260 days
Ultrasound Measurement Method
Early ultrasound examinations provide another method for calculating due dates. Measuring the fetus during the first trimester (especially between weeks 6-9) provides one of the most accurate indicators of gestational age. The due date is calculated by determining the gestational age at the time of the ultrasound and extrapolating to the standard 280-day pregnancy duration.
How Accurate Are Due Date Calculations?
The accuracy of due date calculations varies by method:
- LMP method: Has a margin of error of approximately ±2 weeks
- Conception date: Most accurate when the exact date is known
- IVF method: Highly accurate as the conception date is precisely known
- Ultrasound method: Generally accurate within 3-5 days in the first trimester and 1-2 weeks later in pregnancy
According to the American College of Obstetricians and Gynecologists (ACOG), for women with regular 28-day cycles who have early ultrasound confirmation, due dates are typically accurate within a few days. However, only about 5% of babies are born exactly on their calculated due date.
Why Due Dates Can Change
Due dates are not set in stone. Healthcare providers may adjust them based on:
- Irregular menstrual cycles: When cycles are consistently longer or shorter than 28 days
- Early ultrasound findings: When measurements differ significantly from expected dates
- Fetal growth patterns: Discrepancies between expected and actual growth rates
- First-trimester screening results: Abnormal results may prompt earlier delivery
- Prenatal testing outcomes: Certain test results may indicate the need for adjusted timing
Most adjustments occur before the second trimester, after which point due dates typically remain stable. Healthcare providers typically only change a due date when there's compelling evidence that the original estimate was incorrect.
Key Takeaways
- Due dates are estimates, with only about 5% of babies born on their exact due date
- Multiple methods exist for calculating due dates, each with varying levels of accuracy
- The LMP method is the most common but assumes regular 28-day cycles
- Ultrasound dating is often more accurate, particularly in the first trimester
- IVF pregnancies use specific calculations based on embryo transfer date and age
- Due dates may change as healthcare providers gather more information
- Pregnancy milestones and prenatal care are scheduled based on the estimated due date