Pregnancy & Fertility
Pregnancy Due Date Calculator
Estimate your due date and current stage of pregnancy from your last period or conception date.
Pregnancy Due Date Calculator Tool
How due dates are traditionally estimated
Most due dates are estimated using Naegele's rule, a method that has been used in obstetrics since the 1800s. It takes the first day of your last menstrual period (LMP) and adds 280 days, or 40 weeks, which reflects the average length of a full-term pregnancy counted from the start of the last period rather than from conception itself. This calculator applies that same rule and lets you enter either your last period date or, if you know it, your conception date instead.
Why pregnancy is dated from the last period
It might seem more logical to count from conception, but the exact date of conception is rarely known with certainty, while the start of the last period is usually easy to identify. Because ovulation typically occurs around two weeks after the start of a period, '40 weeks pregnant' actually corresponds to about 38 weeks since conception in a standard 28-day cycle. If your cycle is longer or shorter than 28 days, this calculator adjusts the estimate accordingly, since ovulation timing shifts with cycle length.
Understanding your estimated due date
Only a small percentage of babies are actually born on their exact estimated due date — most arrive within a window of about two weeks before or after it. Think of the date this calculator gives you as the centre of a normal range rather than a firm deadline, which is exactly how obstetric care providers typically frame it too. Your care provider may adjust your due date after an early ultrasound, which is generally considered more precise than LMP-based dating, especially if your cycles are irregular.
Tracking your pregnancy progress
Alongside your estimated due date, this calculator shows your current gestational age in weeks and days, and which trimester you're in — the first trimester runs through week 13, the second through week 27, and the third continues to birth. For a closer look at ovulation timing itself, our Ovulation Calculator is a helpful companion tool if you're currently trying to conceive rather than already expecting.
Frequently Asked Questions
How accurate is a due-date calculator?
Due date calculators provide a solid estimate based on average pregnancy length, but only a small percentage of babies are born exactly on their estimated date — most arrive within about two weeks either side.
What is Naegele's rule?
Naegele's rule is the traditional method of estimating a due date by adding 280 days (40 weeks) to the first day of the last menstrual period. It's still widely used as a first-pass estimate in prenatal care.
Why does my due date change after an ultrasound?
Early ultrasounds measure the embryo or fetus directly and are often considered more accurate than last-period-based dating, especially for people with irregular cycles, so care providers sometimes adjust the estimated date afterward.
Can I calculate my due date from conception date instead of my last period?
Yes, this calculator lets you choose either method. Conception-based dating adds approximately 266 days rather than 280, since it starts roughly two weeks later in the cycle.
Does cycle length affect the due date estimate?
Yes. Naegele's rule assumes a standard 28-day cycle; this calculator adjusts the estimate if you enter a longer or shorter average cycle length.