How the Calculator Determines Your Due Date
Our free due date calculator uses Naegele's rule, the exact same mathematical formula utilized by obstetricians and midwives globally. It calculates your Estimated Date of Delivery (EDD) by taking the first day of your last menstrual period (LMP), adding seven days, and then adding nine months (or roughly 280 days total).
Unlike generic calculators that assume every woman has a perfect 28-day cycle, our tool allows you to input your average cycle length. Since ovulation typically occurs 14 days before the end of a cycle, adjusting this input ensures a much more scientifically accurate estimate for women with shorter or longer biological cycles.
The Alternate Methods: Conception and Ultrasound
Sometimes, you know more data than just your LMP.
- Conception Date: If you strictly tracked ovulation via temperature or hormone strips, or if you conceived via In Vitro Fertilization (IVF), you enter the exact day of conception. The calculator simply adds 266 days (38 weeks) from that moment.
- Ultrasound Date: Early dating ultrasounds (usually between 7 and 12 weeks) are the most empirically accurate way to determine an EDD because fetal growth in the first trimester is extremely consistent. You can enter the exact measurement the technician gave you on the day of the scan.
Frequently Asked Questions
How accurate is a due date calculator?
A due date calculator provides an 'estimated date of delivery' (EDD). Only about 4% to 5% of babies are born precisely on their due date. It is perfectly normal for a pregnancy to last anywhere from 37 weeks (early term) to 42 weeks (post-term).
Why use the First Day of my Last Period (LMP)?
Medical professionals use LMP because tracking the exact date of conception is extremely difficult for most women. Gestational age actually counts from the first day of your last period, meaning you are technically considered 'two weeks pregnant' on the day you conceive.
What if my periods are irregular?
If your cycle is much longer or shorter than the standard 28 days, Naegele's rule (LMP + 280 days) becomes incredibly inaccurate. Our calculator lets you adjust your average cycle length to compensate. However, an early ultrasound is considered the most accurate method for dating irregular pregnancies.
When are the three trimesters?
The First Trimester runs from Week 1 to the end of Week 13. The Second Trimester runs from Week 14 to the end of Week 27. The Third Trimester runs from Week 28 until birth (usually Week 40). Our milestone timeline chart visually breaks these down for you.