Pregnancy Due Date & Week Calculator
Calculates your estimated due date, current pregnancy week, and trimester from your last menstrual period or conception date. Ideal for first-trimester planning and tracking gestational milestones.
Last updated: September 2026
Formula below · 3 sources (CDC, acog.org, mayoclinic.org) · Updated Sep 2026
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About this calculator
Naegele's rule is the clinical standard for estimating a due date from the last menstrual period (LMP): add 280 days (40 weeks) to the first day of the LMP. This assumes a standard 28-day cycle, so the calculator adjusts by adding (cycleLength − 28) extra days for longer cycles. The formula is: EDD = LMP + 280 days + (cycleLength − 28) days. If you know the conception date instead, the calculator adds 266 days (38 weeks), because conception typically occurs 14 days after the LMP. The result shows the due date as a calendar date and, while you are pregnant, today's gestational age in weeks and days (counted from the cycle-adjusted LMP) with the trimester, using the ACOG definition: first trimester up to 13 weeks 6 days, second 14w0d–27w6d, third from 28 weeks. Only about 4% of babies arrive exactly on the EDD, so the result is best understood as the midpoint of a normal delivery window of ±2 weeks.
How to use
Suppose your LMP was 1 March 2024 and your average cycle length is 30 days. Step 1 — apply Naegele's rule: 1 March + 280 days = 6 December 2024. Step 2 — adjust for cycle length: 30 − 28 = 2 extra days, so EDD = 8 December 2024. Step 3 — current week: if today were 1 May 2024, that is 61 days since LMP; minus the 2-day cycle adjustment gives 59 days, which is 8 weeks 3 days, first trimester. The calculator would show 'Due December 8, 2024 (today: 8w 3d, first trimester)'.
Frequently asked questions
How is a pregnancy due date calculated from the last menstrual period?
Naegele's rule adds 280 days (40 weeks) to the first day of your last menstrual period. This figure covers approximately two weeks before ovulation, two weeks to fertilisation, and 38 weeks of fetal development. The calculator also corrects for cycle length: if your cycle is longer than 28 days, ovulation occurs later, so the due date shifts forward by the same number of extra days. Your healthcare provider may refine this estimate after a first-trimester ultrasound, which measures crown-rump length with greater precision.
What is the difference between gestational age and fetal age during pregnancy?
Gestational age is counted from the first day of your last menstrual period and is the standard used by obstetricians — at conception you are already considered about 2 weeks pregnant under this system. Fetal age (also called embryonic age) counts from the actual moment of fertilisation and is therefore roughly 2 weeks less than gestational age. Ultrasound measurements and clinical milestones (e.g. viability at 24 weeks) all refer to gestational age, so it is the more practically useful number for tracking your pregnancy.
When does each trimester of pregnancy start and end?
By the ACOG definition used here, the first trimester runs to 13 weeks 6 days of gestational age and covers the period of most rapid organ formation (organogenesis). The second trimester runs from 14 weeks 0 days to 27 weeks 6 days — often called the most comfortable phase, as morning sickness typically eases and energy returns. The third trimester begins at 28 weeks and continues to delivery around week 40. (Some sources split at weeks 13 and 27 instead.) Preterm birth is defined as delivery before 37 completed weeks, while post-term is beyond 42 weeks.