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.
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. Gestational age is then the number of full weeks elapsed since the LMP, split into three trimesters: weeks 1–12 (first), 13–26 (second), and 27–40 (third). 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 = 5 December 2024. Step 2 — adjust for cycle length: 30 − 28 = 2 extra days, so EDD = 7 December 2024. Step 3 — find current week: if today is 1 May 2024, that is 61 days since LMP, which equals 8 weeks and 5 days (early second month, first trimester). Your estimated due date is 7 December 2024.
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?
The first trimester spans weeks 1 through 12 of gestational age and covers the period of most rapid organ formation (organogenesis). The second trimester runs from week 13 to week 26 — this is often called the most comfortable phase, as morning sickness typically eases and energy returns. The third trimester begins at week 27 and continues to delivery around week 40. Preterm birth is defined as delivery before 37 completed weeks, while post-term is beyond 42 weeks.