The Pregnancy Conception Calculator estimates a due date and current gestational age starting from a known or estimated conception date, rather than the more commonly used last-menstrual-period starting point. Enter a conception date, and the tool returns an estimated due date, along with current week and trimester when today's date is used as the reference.
Some people know their conception date with more confidence than their last menstrual period date, particularly when conception was tracked through ovulation monitoring, a fertility treatment cycle, or a single recorded date. This page uses that starting point directly rather than requiring an LMP conversion first.
Estimate your due date from a conception date
When a conception date is known with reasonable confidence, gestational length from conception is about 266 days, or 38 weeks, to reach full term.
Due date ≈ conception date + 266 daysWorked example: a conception date of 1 April.
Adding 266 days to 1 April lands on approximately 23 December of the same year, the estimated due date under this method.
Understand why conception dating uses 266 days, not 280
Standard obstetric dating from the last menstrual period (LMP) uses 280 days because "week zero" is counted from the start of the period, roughly two weeks before ovulation and conception actually occur in an average cycle.
Counting instead from the conception date itself removes those first two weeks from the count, which is why conception-based dating uses 266 days rather than 280 for the same underlying 40-week pregnancy.
280 days (from LMP) − 14 days (average time to ovulation) = 266 days (from conception)Both methods should describe the same due date when the underlying dates are accurate and the cycle matched the 14-day average assumption; a meaningful difference between the two calculated dates can indicate that the actual cycle did not follow that average.
Find your current gestational age from conception
Once a conception date is entered along with today's date, current gestational age follows by counting the elapsed days and converting to weeks and days, then adding the standard two-week offset used in clinical practice, since gestational age is conventionally reported from LMP-equivalent dating even when the starting input was a conception date.
Worked example: 90 days have elapsed since a known conception date.
1. Convert elapsed days to weeks and days. 90 ÷ 7 = 12 weeks, remainder 6 days.
2. Add the clinical offset. Add 2 weeks for the LMP-equivalent convention: 12 weeks 6 days + 2 weeks = 14 weeks 6 days gestational age, the figure typically reported by a clinician.
Know when conception dating is most useful
Conception-based dating is particularly relevant for pregnancies conceived through assisted reproduction, where a specific insemination, transfer, or ovulation-tracked date is documented precisely, removing the guesswork inherent in estimating ovulation timing from a period start date alone.
It is also useful for anyone who tracked ovulation directly through predictor kits, temperature charting, or clinical monitoring and has more confidence in that date than in their last period's start date.
Compare conception dating with LMP and ultrasound dating
| Method | Reference point | Days to due date |
|---|---|---|
| Conception date | Known or estimated conception | 266 days |
| LMP (Naegele's rule) | First day of last period | 280 days |
| Ultrasound | Scan-reported gestational age | 280 minus days elapsed |
A first-trimester ultrasound remains the most clinically reliable dating method overall, particularly when there is any uncertainty about the conception date itself; a documented conception date is a strong second option specifically because it removes the cycle-length assumption that LMP dating depends on.
This is general information, not medical advice
This calculator provides a general estimate for planning and information purposes. It does not replace prenatal care, clinical ultrasound dating, or guidance from a midwife or obstetric provider. Any concerns about pregnancy dating, timing or health should be directed to a qualified healthcare provider.
Frequently asked questions
How is a due date calculated from a conception date?
Add 266 days (38 weeks) to the known or estimated conception date, which accounts for full-term gestation counted from actual fertilization rather than from the start of the last menstrual period.
Why is conception dating 266 days instead of 280?
Because LMP-based dating counts from the start of the period, roughly 14 days before ovulation and conception typically occur, conception-based dating removes those 14 days, using 266 days instead of 280 for the same 40-week pregnancy.
Is conception dating more accurate than LMP dating?
It can be, specifically when the conception date is known with genuine confidence, since it removes the assumption that ovulation occurred exactly 14 days after the last period started. If the conception date itself is only an estimate, accuracy is similar to LMP dating.
Why does gestational age from conception need an extra two weeks added?
Clinical practice reports gestational age using the LMP-equivalent convention even for pregnancies dated by conception, so an offset of about two weeks is added to a raw conception-based day count to match how a clinician would report the same pregnancy's gestational age.
Is this useful for pregnancies conceived through fertility treatment?
Yes. Assisted reproduction often provides a precisely documented conception-related date, such as an insemination or transfer date, making conception-based dating especially relevant and accurate for those pregnancies.
Should I trust this over an ultrasound dating result?
No. A clinical ultrasound, particularly in the first trimester, is generally considered the most reliable dating method available. This calculator is a general estimate for information and planning, not a replacement for clinical dating.
What if my LMP-based and conception-based due dates disagree?
A meaningful disagreement between the two methods can suggest the actual cycle did not match the standard 14-day ovulation assumption; a healthcare provider, often using ultrasound, can help clarify which estimate is more accurate for the specific pregnancy.
Summary
Dating a pregnancy from a known conception date adds 266 days to reach an estimated due date, 14 days fewer than the 280-day LMP-based method, since conception dating starts the count closer to actual fertilization.
This approach is especially useful when a conception, insemination or transfer date is documented precisely, though a clinical ultrasound remains the most reliable dating method overall when available.