Conception Calculator
Calculate Your Pregnancy Due Date With the Method You Know
Choose LMP, conception, ultrasound, or embryo transfer to estimate your due date and pregnancy timeline.
Dates are estimates for general information. They are not a diagnosis and do not replace dating by your prenatal or fertility care team.
How to Calculate a Pregnancy Due Date From Your Last Period
The standard LMP method adds 280 days, or 40 weeks, to the first day of the last menstrual period. It is another way of applying Naegele's rule. Because that rule assumes a 28-day cycle, this calculator also shifts the estimated due date by the difference between your entered cycle length and 28 days.
For example, a consistent 31-day cycle shifts the estimate three days later, while a 25-day cycle shifts it three days earlier. The adjustment is still a calendar model. An uncertain LMP, irregular cycles, recent hormonal contraception, or atypical bleeding can make the estimate less representative.
Pregnancy Due Date From a Known Conception or Ovulation Date
When a conception or closely tracked ovulation date is known, the calculator adds 266 days, or 38 weeks, to estimate the due date. This is two weeks shorter than the LMP interval because gestational age conventionally begins about two weeks before fertilization in a 28-day model.
A remembered intercourse date is not automatically a known conception date. Sperm can remain viable before ovulation, and fertilization may occur later than intercourse. Use this method for a clinically tracked conception or ovulation date, and use the conception-date calculator when you need to work backward from an established due date.
Pregnancy Due Date From Ultrasound Gestational Age
Enter the ultrasound date and the gestational weeks and days reported for that scan. The calculator subtracts the reported age from 40 weeks to find the remaining time and adds that interval to the scan date. It does not calculate gestational age from ultrasound measurements or images.
ACOG identifies first-trimester ultrasound as the most accurate ultrasound method for establishing or confirming gestational age. Whether a scan should change a previously documented due date depends on timing, measurement differences, and clinical guidance, so this tool should not be used to rediagnose pregnancy dating.
IVF Due Date From Day 3, Day 5 or Day 6 Embryo Transfer
IVF pregnancy dating uses the transfer date and embryo age. The calculator adds 263 days after a day-3 transfer, 261 days after a day-5 transfer, or 260 days after a day-6 transfer. These are equivalent to adding 266 days from estimated fertilization.
The same date formula applies to fresh and frozen embryo transfer when the transfer date and embryo age are known. ACOG recommends using assisted-reproduction dating to assign the estimated due date for pregnancies resulting from ART. Your clinic's documented embryo age and due date take priority over an online entry.
How Accurate Is a Pregnancy Due Date and Can It Change?
A due date is the 40-week reference point used to organize gestational age and care, not a prediction of the exact delivery day. The calculator shows a broad 37-to-42-week delivery window to make natural variation visible. That window is informational and is not a recommendation about delivery timing.
A care team may establish or revise the best obstetric estimate after comparing a reliable LMP, an accurate early ultrasound, and assisted-reproduction records. Once a due date is documented, use that clinical date consistently. Contact your care team when records conflict instead of repeatedly switching between online results.
Sources for This Due Date Calculator Method
Our formulas and interpretation notes were reviewed against the following clinical and public-health references on August 2, 2026. These sources explain general dating methods and do not validate an individual result.
Frequently Asked Questions About Due Date Calculations
How many days are added to the last period for a due date?
The standard estimate adds 280 days, or 40 weeks, to the first day of the LMP. This calculator can also adjust for an average cycle longer or shorter than 28 days.
How do I calculate a due date from conception?
Add 266 days, or 38 weeks, to a known conception or ovulation date. A remembered intercourse date may precede conception and should not automatically be used as the same date.
Is an ultrasound due date more accurate than an LMP due date?
First-trimester ultrasound is the most accurate ultrasound method to establish or confirm gestational age. A clinician should decide how it relates to LMP and other records.
How is an IVF due date different from an LMP due date?
IVF dating uses the known embryo transfer date and embryo age instead of estimating ovulation from a menstrual cycle, which gives the care team a documented ART timeline.
Can my prenatal care provider change my estimated due date?
Yes. A clinician may establish or revise the best obstetric estimate after reviewing menstrual dates, early ultrasound, and ART records. Use the date documented for your care.