Due Date Calculator by Conception Date (Not LMP)
Calculate your due date from the exact conception date rather than last menstrual period, for IVF, tracked ovulation, or known timing.
Reviewed by Abdullah, Technical Content Specialist
Formula
Due Date = Conception Date + 266 days (38 weeks)
Where the conception date is the known or estimated date of fertilization. This is equivalent to adding 280 days (40 weeks) from the last menstrual period, since conception typically occurs 14 days after the LMP in a standard 28-day cycle.
Worked Examples
Example 1: Natural Conception Due Date
Problem:A woman knows she conceived on January 15, 2026 based on ovulation tracking. What is her due date?
Solution:Conception date: January 15, 2026\nDue date = Conception + 266 days\nJanuary: 16 remaining days + February: 28 + March: 31 + April: 30 + May: 31 + June: 30 + July: 31 + August: 31 + September: 30 + October: 8 = 266 days\nEquivalent LMP = January 1, 2026 (conception - 14 days)\nGestational age at due date = 40 weeks 0 days
Result:Due date: October 8, 2026 | Equivalent LMP: January 1, 2026
Example 2: IVF Day-5 Blastocyst Transfer
Problem:An IVF patient had a day-5 blastocyst transfer on March 10, 2026. Calculate the due date.
Solution:Transfer date: March 10, 2026\nConception equivalent = March 10 - 5 = March 5, 2026\nDue date = March 5 + 266 days = November 26, 2026\nEquivalent LMP = March 5 - 14 = February 19, 2026\nVerification: Feb 19 + 280 days = November 26, 2026
Result:Due date: November 26, 2026 | Equivalent LMP: February 19, 2026
Frequently Asked Questions
How is a due date calculated from conception date?
When you know the exact conception date, the due date is calculated by adding 266 days (38 weeks) to that date. This differs from the standard medical calculation that adds 280 days (40 weeks) from the last menstrual period (LMP), because conception typically occurs about 14 days after the LMP during ovulation. Both methods arrive at the same estimated due date when the cycle is a standard 28 days. The conception-based method is more accurate when you know the exact day of conception through IVF, ovulation tracking, or other fertility monitoring. Only about 5% of babies are born on their exact due date, with most arriving within two weeks before or after.
How accurate is a due date based on conception date?
A due date calculated from a known conception date is more accurate than one based on last menstrual period, especially for women with irregular cycles. However, even with an exact conception date, only about 4 to 5% of babies arrive on their predicted due date. About 80% of babies are born within 10 days of the estimated due date, and 90% arrive within two weeks. The variability exists because pregnancy duration naturally varies between 37 and 42 weeks, influenced by factors like genetics, the mother's age, whether it is a first pregnancy, and the individual baby's development rate. First-time mothers tend to deliver an average of 5 days past their due date.
Can conception date be determined if I only know my LMP?
You can estimate your conception date from your LMP by adding 14 days, assuming a standard 28-day menstrual cycle where ovulation occurs around day 14. However, this estimate becomes less reliable with irregular cycles because ovulation can occur anywhere from day 10 to day 21 or later depending on cycle length. For a 35-day cycle, ovulation more likely occurs around day 21, not day 14, shifting the estimated conception date by a full week. Ovulation predictor kits, basal body temperature charting, and cervical mucus monitoring can help pinpoint the actual ovulation date more precisely. An early ultrasound between 8 and 12 weeks remains the gold standard for dating a pregnancy when the conception date is uncertain.
How does IVF affect due date calculation?
IVF provides the most precise due date calculation because the exact date of embryo transfer is known, eliminating the guesswork involved with natural conception dating. For a day-5 blastocyst transfer, the due date is calculated by adding 261 days (266 minus 5 days of embryo development) to the transfer date. For a day-3 embryo transfer, add 263 days to the transfer date. Alternatively, you can calculate the equivalent LMP by subtracting 14 days from the conception date (which is the transfer date minus the embryo age) and then adding 280 days. IVF due dates are considered the most accurate of any dating method, though natural variation in pregnancy length still means the actual delivery date will vary by plus or minus two weeks in most cases.
References
Reviewed by Abdullah, Technical Content Specialist · Editorial policy