Three Ways to Calculate Your Due Date
An estimated due date (EDD) is the date when a pregnancy is expected to reach 40 weeks of gestational age. It is always an estimate — only about 5% of babies are born exactly on their due date, with most arriving within two weeks either side. However, knowing the EDD is important for planning prenatal care, monitoring fetal growth, scheduling tests at specific gestational milestones, and determining when a delivery may be considered early, full-term, or post-term. Three main methods are used to establish the EDD.
Method 1: Last Menstrual Period (LMP)
The most common method uses the first day of the last menstrual period. Add 280 days (40 weeks) to get the EDD. This is Naegele's rule and is the standard clinical method. It assumes a 28-day cycle with ovulation on day 14 and may be less accurate for people with irregular cycles. The gestational age at any point is simply the number of days since the LMP divided by 7, expressed in weeks and days.
Method 2: Conception Date
If you know or strongly suspect the date of conception, you can add 266 days (38 weeks) to that date. Since conception occurs approximately 14 days after the LMP in a standard cycle, the EDD will usually be the same as the LMP-based calculation. This method is useful for people who tracked ovulation or used ovulation predictor kits. The corresponding LMP equivalent is simply the conception date minus 14 days.
Method 3: IVF Embryo Transfer Date
For pregnancies conceived through IVF, the transfer date is known precisely, which makes due date calculation more accurate. For a Day-3 embryo transfer, add 263 days to the transfer date (266 days from conception minus 3 days of in-vitro development). For a Day-5 embryo transfer (blastocyst), add 261 days (266 minus 5). Some clinicians use slightly different conventions — always confirm with your fertility clinic for their specific calculation.
First Trimester Ultrasound: The Gold Standard
While all the calculator methods above provide useful estimates, a first trimester ultrasound (ideally at 8–12 weeks) is considered the most accurate method for establishing a due date. The ultrasound measures the fetal crown-rump length (CRL), which correlates closely with gestational age in the first trimester. If the ultrasound-based date differs by more than 7 days from the LMP-based date in early pregnancy (or more than 10 days in the second trimester), the clinician will typically adjust the EDD to the ultrasound estimate. Use the calculator above for planning purposes, and confirm your EDD with your healthcare provider at your first prenatal visit.
This calculator is for informational purposes only and is not medical advice. Always confirm your due date with a qualified healthcare provider.