Calculation Method
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How the Due Date Calculator Works
Naegele's Rule (LMP Method)
The most common method for estimating a due date. Add 280 days (40 weeks) to the first day of your last menstrual period. For cycle lengths other than 28 days, the difference is added or subtracted from the due date.
Conception Date Method
If you know your exact ovulation or conception date, add 266 days (38 weeks) to estimate your due date. This method is often more accurate when ovulation timing is confirmed.
IVF Transfer Method
For IVF pregnancies, the due date is calculated from the embryo transfer date. Day 5 blastocyst transfers add 261 days. Day 3 transfers add 263 days. Your fertility clinic's dating takes priority.
Ovulation & Fertile Window
Ovulation typically occurs 14 days before your next period. The fertile window spans the 6 days ending on ovulation day. Sperm can survive up to 5 days in fertile cervical mucus, while the egg is viable for 12-24 hours after release.
Frequently Asked Questions
How accurate is a due date calculator?
Due date calculators using Naegele's Rule provide estimates within about 1-2 weeks of the actual delivery date. Only about 1 in 20 babies is born on their exact due date. First-trimester ultrasound dating is generally more accurate, especially when menstrual cycles are irregular. Always confirm your official due date with your healthcare provider.
How is my due date calculated from LMP?
The standard formula (Naegele's Rule) adds 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation on day 14. For longer cycles, the due date is shifted forward by the difference. For shorter cycles, it is shifted backward. The 280-day figure represents the average full-term pregnancy duration measured from LMP.
Can my due date change?
Yes. Your healthcare provider may adjust your due date based on early ultrasound measurements (especially the dating scan at 10-13 weeks). If the ultrasound measurement differs significantly from your LMP-based date, your provider may revise the due date. First-trimester ultrasound is the most accurate method for establishing gestational age.
What is the fertile window?
The fertile window is the 6-day period each cycle when conception is most likely: the 5 days before ovulation plus ovulation day itself. Sperm can survive in the reproductive tract for up to 5 days, while the egg remains viable for only 12-24 hours after ovulation. Having intercourse during this window maximizes the chance of pregnancy.
How is ovulation calculated?
Ovulation is estimated by subtracting your luteal phase length from your cycle length, counting from the first day of your LMP. For a standard 28-day cycle with a 14-day luteal phase, ovulation occurs on day 14. For a 32-day cycle with a 14-day luteal phase, ovulation occurs on day 18. This method works best for regular cycles.
Is my data private?
Yes. All calculations happen entirely in your browser. The dates and information you enter never leave your device. No data is stored on any server, no tracking cookies are used, and no signup or account is required. Your sensitive health information stays completely private.
What if my cycles are irregular?
If your cycle lengths vary by more than 7 days between cycles, LMP-based calculations become less reliable. For irregular cycles, we recommend using the conception date method if you tracked ovulation, or relying on early ultrasound dating from your healthcare provider. Consider tracking your cycles for 3-6 months to find your average cycle length.
How is IVF due date different?
IVF pregnancies have precisely known transfer dates and embryo ages, making the due date calculation more accurate than LMP-based estimates. For a day-5 blastocyst transfer, add 261 days to the transfer date. For a day-3 transfer, add 263 days. Your fertility clinic's official dating should always be your primary reference. ACOG recommends using ART-derived gestational age for IVF pregnancies.
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