The estimated due date follows Naegele's rule: the first day of the last menstrual period (LMP) plus 280 days (40 weeks), and the current pregnancy week is the number of full weeks since the LMP plus 1. For example, an LMP of 1 May 2026 gives a due date of 5 February 2027, and someone 11 full weeks and 3 days past her LMP is in week 12.
How is it calculated?
The calculation starts from the first day of the last menstrual period (LMP). The estimated due date is that date plus 280 days — exactly 40 weeks. The current pregnancy week is the number of full weeks elapsed since the LMP plus 1: someone 11 full weeks and 3 days along is in week 12. Trimesters split as follows: the first runs through the end of week 13, the second from week 14 to 27, and the third from week 28 until birth. Days remaining are the difference between today and the estimated date.
Example: an LMP of 1 May 2026 gives an estimated due date of 5 February 2027 (280 days later). Keep in mind that only about 4-5% of babies arrive exactly on their due date — most births happen within about two weeks before or after it. Ultrasound dating, particularly in the first trimester, may shift the estimate, and when there is a gap, the doctors' dating is the one that counts. With irregular cycles the LMP-based estimate is less accurate.
Frequently asked questions
What are the chances of giving birth exactly on the due date?
Only about 4-5% of babies are born exactly on their estimated due date. Most births occur within roughly two weeks before or after it, so the due date is a statistical estimate, not a precise forecast.
Why is the ultrasound date different from the LMP calculation?
Ultrasound dating, especially in first-trimester scans, is more accurate than calculating from the last period and may shift the estimate by several days. When the two disagree, the medical dating based on the ultrasound is the one that counts.
How are pregnancy weeks counted, and what if my cycle is irregular?
The current week is the number of full weeks since the first day of the last period plus 1 — for example, 11 weeks and 3 days means week 12. The formula assumes a regular cycle of about 28 days, so with irregular cycles the estimate is less accurate and early ultrasound dating is more reliable.
The information on this site is a general estimate for informational purposes only, is not medical advice and is no substitute for consulting a doctor. Your data is never stored.
