Due Date Calculator

1. Choose your calculation method. Most people use "Last Period (LMP)" because that date is easy to know. If you tracked ovulation or did IVF, use those options for a more precise result.
2. Enter the date. For LMP, enter the first day of your last menstrual period, not the last day. For conception, enter the estimated date of ovulation or intercourse.
3. Adjust cycle length if needed. The default is 28 days. If your cycles are consistently longer or shorter (say, 32 or 24 days), update this field. It shifts your due date accordingly.
4. Read your results. The calculator shows your due date, current pregnancy week, trimester, trimester start dates, and key milestone dates.
5. Confirm with your provider. An early ultrasound (before 13 weeks) gives the most accurate due date and may revise the estimate by a few days.

Naegele's rule is the standard method used by doctors worldwide. It adds 280 days (40 weeks) to the first day of the last menstrual period:

Due Date = LMP + 280 days

Equivalently:
Due Date = LMP + 9 months + 7 days

The 280-day figure assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the calculator adjusts the effective LMP to compensate:

Adjusted LMP = LMP date - (cycle length - 28 days)

Conception method: Conception date + 266 days = due date. The 266-day figure counts from fertilization (conception) rather than last period, and produces the same result as adding 280 days to LMP when ovulation was on day 14.

IVF method: For a day-5 blastocyst transfer, the calculator adds 261 days to the transfer date (equivalent to LMP + 280 days, where LMP is estimated as transfer date minus 19 days). For a day-3 embryo, it uses transfer date minus 17 days as the LMP equivalent.