How is the Estimated Due Date (EDD) calculated?
A standard full-term singleton pregnancy typically lasts about 280 days (or 40 weeks). However, doctors calculate this differently depending on what information is available. Our calculator supports the most accurate clinical methods:
1. Ultrasound Scan (Most Accurate)
While period dates can be easily misremembered, early ultrasound scans (usually done between 8 and 12 weeks) measure the baby's Crown-Rump Length (CRL). This gives the doctor a highly accurate gestational age in weeks and days. Entering that exact gestational age into our calculator gives you the most medically precise due date.
2. Last Menstrual Period (LMP)
This is the standard method (known as Naegele's rule). It assumes conception occurred about 14 days after the start of your last period. The formula adds 280 days to the first day of your LMP. If your cycle is consistently longer or shorter than the standard 28 days, our calculator adjusts the math accordingly for higher accuracy.
3. IVF (In Vitro Fertilization)
Because IVF bypasses natural conception timelines, doctors use the exact embryo transfer date to calculate the due date rather than the LMP.
- 3-Day Transfer: Due date is calculated as the transfer date plus 263 days.
- 5-Day Transfer: Due date is calculated as the transfer date plus 261 days.
What if I am expecting Twins?
If you are pregnant with twins or multiples, a full-term pregnancy is generally considered to be 37 weeks rather than 40 weeks to prevent complications. When you toggle the "Expecting Twins" option, our calculator automatically adjusts your due date forward by 21 days while keeping your gestational age tracking accurate.
Clinical Disclaimer
Only about 4-5% of babies are actually born on their exact estimated due date. A normal, healthy full-term delivery can occur anywhere between 37 weeks and 42 weeks. This calculator provides an estimation; always rely on the dates and advice provided by your OB/GYN or midwife.