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If you know the exact date of conception, you can more precisely estimate your baby’s expected delivery date. Understanding how to figure out due date by conception date provides a clearer timeline for pregnancy milestones and prenatal planning.
This Pregnancy Due Date Calculator determines your estimated due date by adding the standard gestational length to your conception date. It offers a straightforward, medically grounded way to project your delivery window.
Enter your estimated conception date to calculate your due date, LMP, and complete pregnancy timeline
Enter the date you believe conception occurred. This is typically the day of ovulation, or the day of intercourse closest to ovulation. The estimated due date is calculated by adding 266 days (38 weeks of fetal age) to the conception date — the standard fetal age timeline used by ACOG and Naegele's Rule (ACOG; Stanford Medicine).
If you only know your Last Menstrual Period (LMP), use the LMP option in Step 2. Conception typically occurs around 14 days after the first day of LMP in a standard 28-day cycle (Wilcox et al., BMJ, 2000).
The accuracy of your due date depends on how the conception date was established. IVF and known ovulation dates are the most precise. Estimated dates based on cycle tracking carry a wider margin of error (Mama Natural; ACOG).
Milestone weeks are counted from the estimated Last Menstrual Period (LMP = conception minus 14 days for a standard 28-day cycle), in accordance with ACOG gestational dating conventions.
From conception to EDD: Conception + 266 days (38 weeks) = Estimated Due Date. This is the direct inverse of Naegele's Rule: EDD = LMP + 280 days, where LMP = conception − 14 days (in a standard 28-day cycle). Fetal age counts from fertilisation; gestational age counts from LMP and adds 2 weeks.
LMP estimate: Conception − 14 days = estimated LMP (assumes ovulation on day 14 of a 28-day cycle). This is used to calculate all gestational-age milestones throughout the pregnancy.
IVF note: For a fresh IVF cycle, the egg retrieval date is the equivalent of the conception date. For a frozen embryo transfer (FET) of a Day-5 blastocyst, subtract 5 days from the transfer date to get the equivalent conception date, then add 266 days for the EDD (pregnancychancecalculator.com; ACOG).
Margin of error: Even with a known conception date, individual pregnancies vary. A study found that actual birth dates have a standard deviation of 14 days around the EDD (Wikipedia, Estimated date of delivery). Only about 4–5% of babies are born on their exact due date.
Sperm survival: Sperm can survive up to 5 days in the female reproductive tract, and the ovum remains viable for up to 24 hours after ovulation. If the conception date is based on an intercourse date rather than known ovulation, the actual conception may have occurred up to 5 days later (Wilcox et al., BMJ, 2000).
First-trimester ultrasound (before 13 weeks 6 days) remains the gold standard for due date confirmation. ACOG recommends that when LMP/conception-based dating and ultrasound dating differ by more than 7 days, the ultrasound date takes precedence.
Naegele's Rule bias: Modern studies suggest Naegele's Rule places the due date approximately 2–4 days too early on average (Mama Natural; Mittendorf-Williams rule, 1990). This applies equally to the conception-based calculation.
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Pregnancy typically lasts about 38 weeks (266 days) from the date of conception. This differs from the commonly cited 40-week timeline, which is calculated from the first day of the last menstrual period (LMP). When conception date is known—such as in assisted reproductive procedures or carefully tracked cycles—the estimate can be more precise.
To figure out due date by conception date, simply add 266 days to the conception date. For example, if conception occurred on March 1, adding 266 days would project an estimated due date in late November. This method assumes a standard gestational duration and average fetal development.
It is important to recognize that the estimated due date (EDD) is not an exact prediction of delivery. Only a small percentage of births occur precisely on the due date. Most healthy pregnancies deliver within a window of approximately two weeks before or after the estimated date.
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Understanding this calculator can raise questions. Here you’ll find clear, evidence-based answers to help you interpret your results and understand the science behind it.
If the conception date is known with certainty, it can provide a more precise estimate than LMP-based calculations, especially for individuals with irregular cycles.
The 40-week model starts from the first day of the last menstrual period because conception dates are often unknown. It standardizes pregnancy tracking across clinical practice.
An estimated due date predicts the midpoint of a normal delivery window. Most births occur between 37 and 42 weeks of gestation.
Yes. Early pregnancy ultrasounds measure fetal size and may adjust the estimated due date if there is a significant difference between calculated and observed gestational age.
Learning how to figure out due date by conception date provides a clear and scientifically grounded method for estimating your delivery timeline. By adding 266 days to the known conception date, you can calculate your projected due date with reasonable accuracy.
While no prediction guarantees the exact day of birth, this Pregnancy Due Date Calculator offers a reliable reference point for prenatal care, milestone planning, and ongoing medical monitoring throughout pregnancy.
