The estimated due date is the anchor of every pregnancy timeline — scans, screenings, and milestones are all scheduled around it. This calculator applies Naegele's rule, the standard method used by clinicians worldwide, from the first day of your last menstrual period.
Cómo funciona
Enter the first day of your last period. The calculator adds 280 days (40 weeks), the standard length of pregnancy counted from the LMP, and also shows how many weeks pregnant you are today.
La fórmula
The count starts from the last period, not conception — which typically occurs about two weeks later. That's why 'weeks pregnant' runs about two weeks ahead of embryonic age.
Ejemplo resuelto
With a last period starting 1 January, the estimated due date is 8 October — and on 19 July you would be about 28 weeks pregnant, entering the third trimester.
Preguntas frecuentes
How accurate is the due date?
It's an estimate: only about 4–5% of babies arrive on the exact date, while roughly 90% arrive within two weeks either side. A first-trimester ultrasound often refines the date and takes precedence when it differs.
What if my cycles aren't 28 days?
Naegele's rule assumes a 28-day cycle with ovulation on day 14. For longer or shorter regular cycles, clinicians adjust by the difference (e.g., +4 days for a 32-day cycle); for irregular cycles, dating by ultrasound is more reliable.
How are trimesters divided?
Commonly: first trimester through week 13, second through week 27, third from week 28 to birth. Full term is 39–40 weeks; before 37 weeks is considered preterm.
For scheduling and information only — your prenatal care provider's dating and guidance take precedence.