How is the due date calculated?

This calculator uses Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. If your cycle is not 28 days, the result is adjusted by the difference — for example, a 30-day cycle shifts the due date forward by 2 days. Ultrasound dating by a healthcare provider is more accurate, especially after the first trimester.

What is gestational age?

Gestational age counts pregnancy in weeks and days from the first day of the last menstrual period, not from conception. At the due date (40 weeks) the baby is approximately 38 weeks from conception. Week 1–12 is the first trimester, weeks 13–26 the second, and weeks 27–40 the third.

Is my personal data private?

Yes, completely. The date you enter never leaves your browser — all calculations are done locally in JavaScript with no server contact of any kind.

How many weeks pregnant am I right now?

Count the days from the first day of your last period to today and divide by 7. The calculator does this automatically and shows the result as weeks and days, for example 14w 3d. It also tells you which trimester that falls in.

Why adjust the due date for cycle length?

Naegele's rule assumes a 28-day cycle, where ovulation is around day 14. A longer cycle means later ovulation, so the calculator adds the extra days: a 31-day cycle pushes the due date 3 days later, a 26-day cycle 2 days earlier.

What is the viability milestone at 24 weeks?

Around 24 weeks gestation a baby is generally considered to have a chance of survival with intensive neonatal care, which is why the calculator highlights this date. Survival rates and outcomes improve significantly with each additional week.

How accurate is an LMP-based due date?

Only about 4% of babies arrive on the exact estimated date. LMP dating assumes a regular cycle and known last period; a first-trimester ultrasound is more accurate and is used to revise the date if it differs by more than about a week.

Pregnancy Due Date Calculator

Pregnancy due date calculator

Enter the first day of your last menstrual period (LMP) and your average cycle length to estimate your due date and track exactly how far along you are. It is the same starting point obstetricians and midwives use worldwide, making it a quick way to check your dates between appointments.

How it works

The calculator applies Naegele’s rule: it adds 280 days (40 weeks) to your LMP. If your cycle is not 28 days, it adds the difference — 280 + (cycle length − 28) days — because a longer cycle means later ovulation and conception. Gestational age is the number of days from LMP to today, shown as weeks and days, and the milestone dates are anchored to the same adjusted timeline.

It instantly shows:

Estimated due date (Naegele’s rule, cycle-adjusted)
Current gestational age in weeks and days, updated to today
Trimester — first (weeks 1–12), second (13–26) or third (27–40)
Milestone dates — end of first trimester, viability (24 weeks), end of second trimester, and the due date

Example

LMP of 1 January with a 28-day cycle:

1 Jan + 280 days = 8 October (estimated due date)

With a 30-day cycle it shifts 2 days later to 10 October. If today were 9 February, gestational age would be 39 days = 5w 4d, in the first trimester.

Milestone	Gestational age
End of first trimester	12 weeks
Viability threshold	24 weeks
End of second trimester	26 weeks
Estimated due date	40 weeks

All calculations run entirely in your browser — no dates or personal information are uploaded anywhere. Always confirm your due date with a healthcare provider; an ultrasound scan, particularly at 10–13 weeks, is more precise than an LMP estimate.