How is the due date calculated?

The most common method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle. If your cycle is different, the calculation may vary slightly.

What is gestational age?

Gestational age is how far along the pregnancy is, measured from the first day of the last menstrual period — not from conception. At the time of conception, gestational age is already about 2 weeks.

Is this calculator medically accurate?

This calculator uses the standard Naegele's rule which is what most OB-GYNs use. However, ultrasound dating is more accurate — your healthcare provider may adjust your due date based on your first ultrasound.

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Pregnancy Due Date Calculator

Find out your due date, how many weeks pregnant you are, which trimester you're in, and key milestone dates — from your last period or conception date.

First day of last menstrual period

The most common calculation method (Naegele's rule: LMP + 280 days)

Conception date (estimated)

Due date = conception date + 266 days (38 weeks)

Known due date

Enter a due date to calculate how far along you are today

Cycle length (days)

28 days

Adjusts the due date for cycles longer or shorter than 28 days

How due dates are calculated

The standard method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. This formula was developed in the early 19th century and is still the basis for due date calculations worldwide.

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Three methods

Calculate from your last menstrual period (most common), from conception date if you know it, or backwards from a known due date to find how far along you are.

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Cycle length adjustment

The standard calculation assumes a 28-day cycle. If your cycle is longer or shorter, the tool adjusts the due date accordingly — because ovulation and conception happen later or earlier in the cycle.

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Milestone timeline

See the full timeline of key pregnancy dates: first heartbeat, nuchal translucency window, anatomy scan window, viability milestone, full term, and more.

⚕️ Medical disclaimer: This calculator is for informational purposes only. Only 5% of babies are born on their due date. Ultrasound dating by a healthcare provider is more accurate. Always consult your doctor or midwife.

Frequently Asked Questions

How is my due date calculated?

Using Naegele's rule: first day of LMP + 280 days. If your cycle is not 28 days, we add the difference (cycle length − 28) to adjust for when ovulation likely occurred.

Only 5% of babies arrive on their due date — so what's it for?

The due date is a reference point, not a prediction. Doctors use it to track fetal development, schedule screenings, and determine if a pregnancy is overdue. "Full term" is 39–40 weeks; anything from 37–42 weeks is considered normal.

When does each trimester start?

1st trimester: weeks 1–13 (days 1–91). 2nd trimester: weeks 14–26 (days 92–182). 3rd trimester: weeks 27–40+ (day 183+).

My doctor's due date is different — why?

Doctors often adjust the due date after an early ultrasound, which measures the baby's size directly. This is more accurate than date-based calculations, especially if your cycle is irregular.