Pregnancy Due Date Calculator
Quick answer: Enter the first day of your last menstrual period (LMP) to estimate your due date using Naegele's rule (LMP + 280 days), the standard method used in obstetrics.
Pregnancy Due Date Calculator
How the calculation works
Naegele's rule estimates a 280-day pregnancy from the first day of the LMP, assuming a 28-day menstrual cycle and ovulation on day 14. If your cycle is longer or shorter, the due date adjusts accordingly. Most healthcare providers use this method for the initial estimate, then refine it with first-trimester ultrasound dating.
Important context
Only about 4% of births occur exactly on the calculated due date. The full-term window is 37–42 weeks. Ultrasound dating before 13 weeks is more accurate than LMP-based dating, particularly when cycles are irregular or LMP is uncertain. Discuss your individual due date with your prenatal care provider.
Frequently asked questions
How accurate is the due date from LMP?
LMP-based due dates have a margin of about ±1 week even with regular cycles. First-trimester ultrasound (before 13 weeks) is more accurate (±5 days) and is used to confirm or revise the LMP estimate, especially when cycles are irregular.
What is the difference between gestational age and conception age?
Gestational age is counted from the first day of the LMP (the standard used in obstetrics). Conception age is counted from estimated ovulation (~2 weeks later). When clinicians say '12 weeks pregnant', they mean 12 weeks gestational age, which is approximately 10 weeks since conception.
How does cycle length affect the due date?
The standard 280-day calculation assumes a 28-day cycle with ovulation on day 14. If your cycle is longer (e.g., 32 days), ovulation is later, and the due date shifts later by the difference. Our calculator adjusts automatically based on cycle length.
When is ultrasound dating preferred over LMP?
When LMP is uncertain, cycles are irregular, the difference between LMP and ultrasound dating is more than 7 days in the first trimester, or pregnancy was conceived through assisted reproductive technology with known transfer date.
Last reviewed: by iMedic Medical Editorial Team. Our editorial process.