Pregnancy Week and Due Date Calculator
Calculate your estimated due date (Naegele's rule), how many weeks pregnant you are, the days left and your trimester dates from the first day of your last period.
Things to keep in mind:
- The due date is an estimate; only a small share of babies are born on that exact day, and birth between weeks 37 and 42 is considered normal.
- LMP-based dating can be misleading with irregular cycles, an uncertain LMP, right after stopping the pill or after assisted reproduction (IVF).
- A first-trimester ultrasound dates a pregnancy more reliably than the LMP; the date set by your doctor takes precedence.
- The result is an estimate, not a medical diagnosis or treatment advice; consult your physician or dietitian for decisions about your health.
Calculator
Result
Enter the values and press Calculate to see the result here.
About this tool
Pregnancy is conventionally dated from the first day of the last menstrual period (LMP) and lasts on average 280 days (40 weeks). The calculator finds the estimated due date with Naegele's rule and, for cycles other than 28 days, shifts it by the difference in cycle length.
Formulas
- Adjusted start = LMP + (cycle length − 28) days
- Estimated due date = adjusted start + 280 days
- Gestational age (days) = calculation date − adjusted start; weeks = days ÷ 7 (whole part), extra days = days mod 7
- Estimated conception = LMP + cycle length − 14 days
- Trimesters: first 0–13+6, second 14+0–27+6, third from 28+0 weeks; term begins at 37+0 weeks
The calculation engine does not know today's date, so enter the day you want the gestational age for in "Calculation date (today)".
Example: with an LMP of 1 January 2026 and a 28-day cycle the due date is 8 October 2026. On 15 March 2026 the pregnancy is 73 days, i.e. 10 weeks 3 days; 207 days remain and you are in the first trimester. The second trimester starts on 9 April and the third on 16 July 2026.
How to use
- Select the first day of your last period.
- Enter your average cycle length (28 if unsure).
- Enter today's date as the calculation date.
- Press Calculate.
Notes
Dates are calendar days without time zones. Gestational age is given as completed weeks + days (e.g. 10 weeks 3 days = 10+3, i.e. you are in week 11). If the calculation date is before the start of pregnancy, gestational age and the trimester code are 0; after the due date the days remaining are negative.
Sources
References
- [1]Committee Opinion No 700: Methods for Estimating the Due DateACOG, Obstet Gynecol, 2017
Frequently asked questions
The due date is the first day of the last period plus 280 days (or plus one year, minus three months, plus seven days). For cycles longer or shorter than 28 days the date is shifted by the difference.
Because the date of conception is usually unknown, pregnancy is dated from the first day of the last period by international convention. At conception the pregnancy is therefore already about 2 weeks.
10 weeks and 3 days have been completed, so you are in the 11th week of pregnancy.
The calculation engine does not use a clock; this also lets you calculate the pregnancy week for any date, such as an appointment.
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