Calculate Your Estimated Due Date (EDD)
Select your preferred estimation method below and enter your dates.
Methods of Due Date Estimation
Establishing an accurate due date is crucial for monitoring fetal growth milestones and deciding delivery timeframes. Healthcare providers use several methods depending on what dates you recall and what clinical reports are available:
Last Menstrual Period (LMP)
Dating from LMP calculates the due date by adding 280 days (40 weeks) to the first day of your last period. Standard Naegele's rule calculation assumes a regular 28-day menstrual cycle. If your average cycle length is longer or shorter, the date must be adjusted since ovulation changes. For instance, a 35-day cycle delays ovulation by 7 days, so the pregnancy is dated 7 days younger than raw LMP suggests.
Conception Date
If you tracked ovulation or used basal body temperature logs, you may know the exact date of conception. Pregnancy lasts approximately 266 days (38 weeks) from conception. This dating is highly accurate, though sperm can live inside the female reproductive tract for up to 5 days, meaning the date of intercourse is not always the exact conception date.
IVF Embryo Transfer
In vitro fertilization (IVF) dating is the most precise method because the transfer day is clinically recorded. Calculation depends on whether the embryo was transferred on Day 3 (cleavage stage) or Day 5 (blastocyst stage). Day 3 embryos require 263 days post-transfer to reach term, while Day 5 blastocysts require 261 days.
Ultrasound Dating (Dating Scan)
If your period is irregular or you cannot recall dates, a first-trimester ultrasound is the gold standard. The scan measures the crown-rump length (CRL). A first-trimester scan is accurate to within 3 to 5 days. If the ultrasound differs from your period calculation by more than 7 days, the ultrasound date becomes the official clinical due date.
Questions to Ask Your Clinician
- What date did my first-trimester scan suggest, and does it align with my menstrual cycle calculations?
- Should we adjust my clinical due date based on my early ultrasound report?
- If my cycles are irregular, how will this affect our schedule for prenatal tests and screenings?
- If I go beyond my due date, what is your clinical policy regarding monitoring and labor induction?
Frequently Asked Questions
A first-trimester crown-rump length (CRL) ultrasound (usually performed between 8 and 14 weeks of pregnancy) is considered the most clinically accurate method, with a margin of error of 3 to 5 days. It is more reliable than calculations based on the last menstrual period.
Yes. If a dating ultrasound differs from your menstrual cycle calculations by more than 7 days, your clinician will usually update your official due date to match the ultrasound measurements. Later ultrasounds in the second or third trimester have larger margins of error and are rarely used to change due dates.
IVF due dates are highly accurate because the exact transfer date and age of the embryo are known. For a Day 3 transfer, the due date is transfer date + 263 days. For a Day 5 transfer, the due date is transfer date + 261 days.
A full-term pregnancy is traditionally defined as 40 weeks (280 days) from the first day of the last menstrual period, or approximately 38 weeks (266 days) from the date of conception.
Conception usually occurs within 12 to 24 hours of ovulation, as the egg must be fertilized shortly after release. Therefore, your conception date is practically identical to or occurs within 24 hours of your ovulation day.
No. This tool is for educational purposes only. Only a qualified healthcare provider can establish an official clinical due date through physical examination and scan reports.
