CalcPregnancy Due Date Calculator
Estimate delivery date from the last menstrual period.
Due Date Calculator: Estimate Your Pregnancy Timeline
The due date calculator estimates your expected delivery date using Naegele’s rule, one of the most widely used methods in obstetrics for predicting pregnancy duration. While only about 5% of babies arrive on their exact due date, this calculation provides a crucial reference point for prenatal care planning and fetal development monitoring.
Due Date Calculation (Naegele’s Rule)
Due Date = First Day of Last Menstrual Period (LMP) + 280 days
Naegele’s rule adds 280 days (40 weeks) to the first day of your last menstrual period. A practical shortcut: subtract 3 months from the LMP date, then add 7 days. For example, if your LMP was March 15, 2025: subtract 3 months = December 15, 2024; add 7 days = December 22, 2025. This calculation assumes a regular 28-day cycle with ovulation occurring on day 14. If your cycle is longer or shorter, adjustments may be needed.
Worked Examples
Example 1: Regular 28-Day Cycle
A woman with regular 28-day cycles had her last menstrual period begin on January 10, 2025.
LMP: January 10, 2025
Subtract 3 months: October 10, 2024
Add 7 days: October 17, 2025
Her estimated due date is October 17, 2025. This corresponds to 40 weeks gestation. Her first-trimester ultrasound may refine this date if the measured fetal size differs by more than 7 days from the LMP-based estimate.
Example 2: Longer 32-Day Cycle
A woman with regular 32-day cycles had her LMP on June 5, 2025. Her cycle is 4 days longer than the standard 28-day cycle.
Standard calculation: June 5 − 3 months = March 5; + 7 days = March 12, 2026
Adjustment for longer cycle: March 12 + 4 days = March 16, 2026
Because she ovulates approximately 4 days later than the standard day-14 assumption, her due date shifts forward by 4 days. This adjustment is important for accurate gestational age assessment and timing of prenatal screenings.
Common Uses
- Establishing estimated date of delivery (EDD) for prenatal care scheduling and milestone tracking
- Timing critical prenatal screening tests such as nuchal translucency (11–14 weeks) and anatomy scan (18–22 weeks)
- Determining gestational age at any point during pregnancy for fetal growth assessment
- Planning maternity leave and preparing for the baby’s arrival with a target timeframe
- Identifying post-term pregnancies (beyond 42 weeks) that may require induction of labor
- Coordinating care among obstetricians, midwives, and specialists using a shared reference date
Common Mistakes
- Using the last day of menstruation instead of the first day as the starting point, which shifts the due date forward by 3–7 days incorrectly
- Not adjusting for cycles that differ from 28 days, leading to inaccurate dating in women with naturally longer or shorter cycles
- Ignoring early ultrasound dating, which is more accurate than LMP-based calculation when the discrepancy exceeds 7 days in the first trimester
- Treating the due date as an exact delivery date rather than an estimate, when only 4% of babies are born on their calculated due date
Pro Tip
If you know your exact ovulation date (through ovulation predictor kits, basal body temperature tracking, or fertility monitoring), calculate your due date as ovulation date + 266 days (38 weeks). This method is more accurate than Naegele’s rule for women with irregular cycles or those who conceived through assisted reproductive technology. Additionally, a first-trimester ultrasound measurement of crown-rump length (CRL) between 8–13 weeks provides the most accurate dating, with a margin of error of only ±3–5 days.
Frequently Asked Questions
The due date is an estimate with a normal delivery window of 37–42 weeks. Only about 4–5% of babies are born on their exact due date. First-trimester ultrasound dating is accurate to ±5 days, while LMP-based dating is accurate to ±7–10 days in women with regular cycles.
Yes. If an early ultrasound shows a significant discrepancy (more than 7 days in the first trimester, or more than 10–14 days in the second trimester) from the LMP-based date, your healthcare provider may adjust the due date. Once set based on early ultrasound, the due date should not be changed later in pregnancy.
If you are unsure of your LMP date, an early ultrasound is the best alternative. A crown-rump length measurement between 8–13 weeks gestation provides the most accurate dating. If you conceived through IVF, the due date is calculated from the embryo transfer date (transfer date + 266 days for a day-3 embryo, or + 263 days for a day-6 blastocyst).
Pregnancy is counted from the first day of the last menstrual period, which is approximately 2 weeks before conception actually occurs. So while fetal development takes about 38 weeks from conception, the clinical pregnancy duration is 40 weeks from LMP. This is why the first “week” of pregnancy occurs before you are actually pregnant.