Complete guide
Conception calculator from LMP or estimated due date
Enter either the first day of the last period or an estimated due date. Calculator24 returns a central conception estimate, the corresponding obstetric dates and a broader intercourse window that could lead to pregnancy.
What this calculator estimates
The result is the calendar center implied by common obstetric conventions. It is useful for timeline orientation but cannot determine the biological moment of fertilization.
Formulas from LMP and due date
ACOG notes that ovulation in an average cycle occurs about 14 days before the next period. The due-date convention places conception about 266 days before EDD.
conception≈LMP+cycle length−14conception≈EDD−266 daysLMP basis≈EDD−280 daysWorked example
With September 1 as LMP and a 28-day cycle, the estimated ovulation date is September 15. The estimated due date is 266 days later, June 8, 2027.
Why the possible window is wider
ACOG states pregnancy can result from sex five days before ovulation through one day afterward because sperm and egg survival overlap. That interval is not proof of conception timing.
Cycle length changes the estimate
The LMP route shifts estimated ovulation with the entered average cycle length. Irregular cycles, recent hormonal contraception, breastfeeding and other factors can reduce calendar reliability.
Clinical dating and paternity
First-trimester ultrasound is more accurate for establishing gestational age than a calendar alone. A conception estimate cannot establish paternity; that requires appropriate testing and professional advice.
Dating relationships
| Relationship | Offset |
|---|---|
| LMP to EDD | 280 days |
| Conception to EDD | 266 days |
| Average ovulation | 14 days before next period |
| Possible intercourse window | 5 days before to 1 day after ovulation |
Frequently asked questions
Can this find the exact conception day?
No. It estimates a calendar center and possible interval.
Why subtract 266 days from the due date?
The standard due-date convention places conception about 38 weeks before EDD.
Does cycle length matter?
Yes for the LMP method because ovulation is estimated relative to the next period.
What if cycles are irregular?
Calendar estimates become less reliable.
Can it prove paternity?
No. A date estimate cannot identify a biological parent.
Which date should guide prenatal care?
Use the best obstetric estimate documented by the clinician.
What to keep in mind
This is calendar estimation. Conception cannot be identified to one exact day from LMP or due date, and cycle timing varies. Use clinician-established gestational dating for care or paternity questions.
Results display up to 8 decimal places; exported numbers preserve calculation precision. This is a calculation summary, not an official certificate.