Choose the correct method
For a non-ART pregnancy, LMP is the conventional starting point when the date is known and cycles are reasonably regular. The classic result is 280 days from the first day of the LMP.
For IVF, ACOG recommends ART-derived dating using the embryo age and transfer date. Do not mix that result with LMP dating.
Sources: American College of Obstetricians and Gynecologists.
What cycle adjustment does
The LMP mode shifts the 280-day estimate by the difference between the entered cycle length and 28 days. This is a practical calendar convention, not a measurement of ovulation.
If cycles vary, the bleeding date is uncertain, or the bleeding may not have been a true period, speak with a clinician rather than forcing an average into the tool.
Sources: American College of Obstetricians and Gynecologists, NHS.
The IVF formulas
A day-5 embryo transfer is dated by adding 261 days to the transfer date; a day-3 transfer uses 263 days. The clinic record is the best source for embryo age and transfer date.
The calculator cannot interpret treatment variations or change the EDD documented by the fertility clinic.
Sources: American College of Obstetricians and Gynecologists.
How to read the result
The displayed day is an estimated due date rather than a predicted birth date. The 37+0 and 42+0 markers give calendar context; ACOG defines 42+0 as the start of postterm.
ACOG says first-trimester ultrasound is the most accurate method to establish or confirm gestational age. Once an EDD is documented, later changes should be rare and clinician-led.
Use the EDD established by your maternity or fertility care team for decisions about care.
Sources: American College of Obstetricians and Gynecologists, American College of Obstetricians and Gynecologists.
Sources
- 01Methods for Estimating the Due DateAmerican College of Obstetricians and Gynecologists · May 2017; reaffirmed 2025
- 02Pregnancy due date calculatorNHS
- 03When Pregnancy Goes Past Your Due DateAmerican College of Obstetricians and Gynecologists