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The detailed guide below is currently available in English.
How your due date is estimated
The classic method counts 280 days from the first day of the last menstrual period, with a small adjustment when your cycle runs longer or shorter than 28 days: each extra day of cycle length pushes the estimate one day later. If you know your conception date, counting 266 days from it is inherently more precise. For IVF pregnancies the transfer date is known exactly, so a 3-day embryo counts 263 days forward and a 5-day blastocyst 261 days.
Reading the trimester dates
The second trimester conventionally begins at 13 completed weeks and the third at 27 completed weeks; this calculator shows those dates using the equivalent LMP implied by your method. Keep in mind that only around 4–5% of babies arrive on their exact due date and roughly 80% arrive within two weeks of it, so plan for a range rather than a single day. These are estimates, not medical advice — your prenatal care team may adjust the date after an ultrasound.
Częste pytania
Should I adjust for an irregular cycle?
If your cycles are longer than 28 days, you probably ovulated later than the standard model assumes, so the adjusted date (LMP + 280 + cycle − 28) is more realistic. With very irregular cycles, a first-trimester ultrasound is a more reliable basis for dating.
Why do IVF transfers use 263 and 261 days?
A due date sits 266 days after conception. A 3-day embryo is three days old at transfer and a 5-day blastocyst five days old, so the remaining count to the due date is 266 − 3 = 263 and 266 − 5 = 261 days respectively.
Can my due date change during pregnancy?
Yes. Providers often revise the estimate after a first-trimester ultrasound if the measured size differs from the LMP-based date by more than about five to seven days. Later revisions are less common because size differences become less diagnostic.