Due Date Calculator
Your estimated due date is 280 days from the first day of your last menstrual period, adjusted if your cycle is longer or shorter than 28 days.
Example: For a last period starting 1 January 2026 with a 28-day cycle, the estimated due date is 8 October 2026. On 1 March 2026 that pregnancy would be 8 weeks 3 days along, in trimester 1.
Enter a date to see your estimate.
Estimates only. Around 4% of babies are born on their estimated due date. Ultrasound dating supersedes last-period dating.
Find an OB-GYN near youHow this is calculated
Naegele's rule adds 280 days to the first day of the last menstrual period. Where your cycle differs from 28 days, the anchor date shifts by the difference. If you supply ultrasound dating, that supersedes last-period dating, as it does in clinical practice. Only about 4% of babies are born on their estimated due date.
This tool is for information only. It does not diagnose, treat, or replace advice from a licensed clinician. Reviewed against ACOG guidance.
Frequently Asked Questions
How is my due date calculated?
Your due date is the first day of your last menstrual period plus 280 days, which is Naegele's rule. If your cycle is longer or shorter than 28 days the starting point shifts by that difference, because ovulation happened later or earlier than the textbook assumption. Ultrasound dating can move the date again.
Will my baby actually be born on the due date?
Probably not. Only about 4% of babies are born on their estimated due date. Treat it as the middle of a range rather than an appointment, and expect labour to start somewhere on either side of it.
Which is more accurate, my last period or an ultrasound?
Ultrasound dating, and in most cases it supersedes last-period dating in clinical practice when the two disagree. Dating from your last period assumes you ovulated on schedule, which is an assumption rather than a measurement. If your OB-GYN has given you a scan-based due date, use that one.
What if I do not remember when my last period started?
Use one of the other dating methods: a due date your OB-GYN already gave you, a conception date you are confident about, or your ultrasound measurement. This calculator accepts any of those. If you have none of them, book a dating scan â that is the standard way to settle it.
Does a longer cycle push my due date later?
Yes. A longer cycle means you ovulated later, so conception happened later and the due date moves later by the same difference. The calculator applies that shift automatically once you enter your cycle length. It only affects last-period dating, not a scan-based or known due date.
What happens if I go past my due date?
Your OB-GYN monitors you more closely and talks through the options with you. Going a little past the estimated date is common; a pregnancy is only considered post-term at 42 weeks 0 days. Keep every prenatal appointment as you approach and pass the date so any decision about induction is made on current information.