Pregnancy Due Date Calculator
Estimate your estimated due date (EDD) from your last menstrual period, conception date, or IVF transfer — and see how many weeks pregnant you are today.
Estimated Due Date
How Your Due Date Is Calculated
The standard method, known as Naegele's rule, starts from the first day of your last menstrual period (LMP) and adds 280 days — 40 weeks. Because Naegele's rule assumes ovulation on day 14 of a 28-day cycle, this calculator adjusts for your actual cycle length: with a 32-day cycle, four days are added; with a 26-day cycle, two are subtracted. If you know your conception or ovulation date, the estimate improves — pregnancy lasts about 266 days from conception. IVF pregnancies are the most precise of all, because the transfer date is known exactly: a day-5 embryo transfer adds 261 days and a day-3 transfer adds 263. Whichever method you choose, the calculator also shows how many weeks pregnant you are today and when each trimester begins.
How Accurate Are Due Date Estimates?
Only about 4 to 5 percent of babies arrive on their exact due date — most are born within a two-week window either side of it. Think of the estimated due date (EDD) as the midpoint of a range rather than an appointment. Health services on both sides of the Atlantic, including the NHS in the UK and the American College of Obstetricians and Gynecologists in the US, use the same underlying arithmetic — 280 days from LMP — but your first-trimester ultrasound may shift the official date, because a scan measurement of the embryo between roughly 8 and 13 weeks is more accurate than period-based math. If your scan date and your LMP date disagree by more than a few days, clinicians generally go with the scan.
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Frequently Asked Questions
The most common method adds 280 days (40 weeks) to the first day of your last menstrual period, adjusted for your average cycle length. If you enter a conception date instead, the calculator adds 266 days. For IVF, it adds 261 days to a day-5 transfer or 263 days to a day-3 transfer, since the embryo's age at transfer is known exactly.
It gives a reliable estimate, but only around 1 in 20 babies is born on the exact due date — most arrive within about two weeks either side. A first-trimester ultrasound is the most accurate dating method, and if it differs meaningfully from your period-based date, your midwife or doctor will usually adopt the scan date.
Yes. Choose the conception/ovulation option and the calculator adds 266 days to that date. This can be more accurate than the LMP method if you tracked ovulation, because it removes the assumption about when in your cycle you ovulated.
IVF dating is the most precise. For a day-5 blastocyst transfer, the due date is the transfer date plus 261 days; for a day-3 embryo transfer, it's the transfer date plus 263 days. The difference accounts for the embryo's age on the day it was transferred.
LMP dating assumes ovulation happened on day 14 of your cycle, which isn't true for everyone, so it can be off by several days. Conception-based dating removes that assumption, and ultrasound dating measures the embryo directly. In practice, clinicians treat an early scan as the gold standard and use LMP math as the starting estimate.
Work backwards: your pregnancy is dated from a point 280 days before your due date, so subtract that date from today and divide by seven. This calculator does it automatically — after computing your EDD, it shows your current gestational age in weeks and days along with your trimester.
Yes — both are based on the same 40-weeks-from-LMP convention, refined by early ultrasound. You may see tiny differences in how cycle-length adjustments or rounding are handled, but the underlying arithmetic is identical to what this calculator uses.