Due date and gestational age calculator
Choose the first day of the last menstrual period (LMP) and get the estimated due date, the weeks of pregnancy, the trimester and the days left.
Free and private: it runs in your browser, we do not send what you type.
This is an estimate; it does not replace a medical assessment or an ultrasound.
It is today's date. Change it to see the gestational age on another day, such as an appointment.
Choose the date of the first day of the last menstrual period.
- Estimated due date
- Gestational age
- Trimester
- Days until the due date
How we calculate it
- Estimated due date = LMP + 280 days (40 weeks), the PAHO 280-day rule. Naegele's rule is a shortcut for the same calculation and can differ by a few days.
- Gestational age = days between the LMP and the reference date, in completed weeks and days.
- Trimesters: the first runs up to 13 weeks and 6 days, the second from week 14 to 27 weeks and 6 days, and the third starts at week 28.
When it can be wrong
The LMP method assumes regular cycles and a well-remembered date. Ecuador's Ministry of Public Health recommends calculating the due date from the LMP and, if the gestational age is in doubt, confirming it with the first ultrasound.
Formulas reviewed on November 8, 2026 against these sources.
- Ecuador Ministry of Public Health · Prenatal care clinical practice guideline (2015, in Spanish)
- PAHO/CLAP · Perinatal clinical record: completion instructions (publication 1563, in Spanish)
- NLM MeSH · Pregnancy Trimester, First
- NLM MeSH · Pregnancy Trimester, Second
- NLM MeSH · Pregnancy Trimester, Third
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