Last updated: March 2026
How Due Dates Are Calculated
The most widely used method for calculating a due date is Naegele's Rule, developed by German obstetrician Franz Naegele in the early 1800s. The formula is simple: take the first day of your last menstrual period, add 280 days (40 weeks), and that's your estimated due date.
A full-term pregnancy lasts approximately 280 days (40 weeks) from the first day of the last menstrual period. Only about 4% of babies are born on their actual due date.
This method assumes a 28-day menstrual cycle with ovulation occurring on day 14. Since not everyone has a 28-day cycle, adjustments can be made. If your cycle is 35 days, you likely ovulated on day 21 instead of day 14, shifting the due date forward by a week.
The conception date method adds 266 days (38 weeks) from the estimated date of fertilization. This is more accurate when conception timing is known, such as with fertility tracking or assisted reproduction.
For IVF pregnancies, the due date calculation is the most precise because the exact embryo age is known. A Day 5 blastocyst transfer means adding 261 days from the transfer date to reach the estimated due date.
Regardless of the calculation method used, only about 5% of babies arrive on their exact due date. The due date is best thought of as the middle of a window — most births occur between weeks 37 and 42, with the highest probability around weeks 39-40.
Frequently Asked Questions
How is my due date calculated?
The most common method uses Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the calculator adjusts accordingly.
How accurate is the due date?
Only about 5% of babies arrive on their exact due date. Most births occur within a two-week window around the due date (between weeks 38 and 42). Ultrasound dating in the first trimester is the most accurate method, with a margin of error of about 5-7 days.
What's the difference between gestational age and fetal age?
Gestational age counts from the first day of your last period (about 2 weeks before conception). Fetal age counts from conception. So at 10 weeks gestational age, the fetus is actually about 8 weeks old.
Does cycle length affect my due date?
Yes. The standard calculation assumes a 28-day cycle. If your cycle is 35 days, you likely ovulated later, so your due date shifts forward by 7 days. The calculator adjusts automatically when you enter your cycle length.
How is the IVF due date calculated?
For IVF pregnancies, the due date is calculated from the transfer date. For a Day 5 embryo, add 261 days (266 minus 5). This method is more accurate than LMP because the exact conception timing is known.
When should I get my first ultrasound?
Most providers recommend a dating ultrasound between weeks 8-12. First trimester ultrasounds are the most accurate for confirming or adjusting the due date. The anatomy scan typically happens at weeks 18-20.