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Pregnancy Due Date Calculator

Estimate a due date from the first day of your last period, adjusted for your own cycle length.

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How to use Due Date Calculator — Pregnancy Due Date by LMP

  1. 1Enter the first day of your last period.
  2. 2Adjust the options if you need to.
  3. 3Read the result — it updates immediately, and nothing is sent anywhere.

About Due Date Calculator

Naegele's rule has been the standard way of estimating a due date for two centuries, and it is simple enough to do on a calendar: take the first day of the last menstrual period and add 280 days. What the classic version glosses over is cycle length. The 280 days assumes a 28-day cycle with ovulation around day 14, so a person with a consistent 34-day cycle is likely to have conceived later than the rule supposes, and their due date sits roughly six days further out. This calculator applies that adjustment, which is why the cycle field is there rather than being fixed at 28.

Alongside the due date, the results show the pieces that come up in appointments and forms: the estimated conception date, the start of the second and third trimesters, and your current gestational age in the weeks-plus-days form that maternity notes use. It is worth understanding that gestational age counts from the last period, not from conception — so it runs about two weeks ahead of the time since conception, and the trimester boundaries shown here (day 91 and day 189) are conventions that other sources place a week later.

The honest framing matters more than the arithmetic. A calculated due date is a planning tool for scans, leave and appointments, not a forecast of a birthday: term covers 37 to 42 weeks, and births spread across that range. Dating from an early ultrasound is more accurate than dating from a remembered date, IVF pregnancies should use the clinic's own dating, and nothing here is a substitute for a midwife or doctor. The calculation runs entirely in your browser — no dates and no health information are uploaded, logged or stored.

Frequently asked questions

How is a due date calculated?
By Naegele's rule: 280 days, or 40 weeks, added to the first day of your last menstrual period. This calculator then shifts the result by the difference between your cycle length and 28 days, because the classic rule assumes a 28-day cycle and a longer or shorter cycle moves ovulation.
Why does cycle length change the answer?
The 280-day rule assumes ovulation around day 14. If your cycle is typically 35 days, ovulation is roughly a week later, so the due date moves about seven days later too; a 24-day cycle moves it about four days earlier. Leave the field at 28 if you are not sure — that is the figure the standard rule uses.
How accurate is a calculated due date?
It is a planning date, not a prediction. A full-term birth is defined as 37 to 42 weeks, so most babies arrive in the weeks either side of the estimate, and only a minority are born on the date itself. Dating from an early ultrasound is more reliable than dating from a remembered period, and your midwife or doctor may revise the date after a scan.
What does "week 12+3" mean?
Twelve completed weeks and three days since the first day of your last period. Pregnancy is dated from that day clinically, which is roughly two weeks before conception — so gestational age is about two weeks more than the time since conception, and a "12 week scan" is measured this way.
When do the second and third trimesters start?
This tool puts the second trimester at day 91 from your last period, which is week 13+0, and the third at day 189, which is week 27+0. These boundaries are conventions rather than biological switches, and other sources place them at week 14 and week 28, so expect a week of variation between calculators and leaflets.
Can I use this after IVF, or if I don't know my last period?
No — and it is better to say so than to produce a number. IVF dating works from the transfer date and the age of the embryo, not from a period, so your clinic's date is the one to use. If your cycles are very irregular or you cannot recall the date, an early dating scan is the right way to establish it.