About Pregnancy Calculator
The pregnancy calculator translates the first day of your last menstrual period into a complete pregnancy timeline: an estimated due date, current gestational age in weeks, and the trimester you are in right now. Healthcare providers use this same 280-day Naegele's rule as a first-line estimate because it is simple, universal, and requires no ultrasound data. For example, if your last period began on May 1, 2026, the calculator would place your due date around February 5, 2027, and if today is June 20, 2026, you would be approximately 7 weeks pregnant and in the first trimester. The tool is especially valuable in the early weeks before an ultrasound confirms dating, but it also helps partners, midwives, and doulas speak the same week-based language. Only about 4% of babies are born exactly on their due date, so the real power of the calculator is framing expectations rather than predicting the delivery day to the hour.
How It Works
You enter the first day of your last menstrual period in YYYY-MM-DD format. The calculator adds 280 days to that date to estimate your due date. It then subtracts the last-period date from today's date, converts the difference into weeks, and assigns a trimester based on standard thresholds: First trimester is weeks 1-12, Second trimester is weeks 13-26, and Third trimester is weeks 27-40.
Formula & Calculation Logic
The calculation uses Naegele's rule: Estimated Due Date = LMP + 280 days. Gestational age in weeks is floor(days since LMP / 7). Trimester assignment uses the conventional breakpoints of 13 and 27 weeks. This model assumes a 28-day menstrual cycle with ovulation around day 14; cycles that are shorter or longer may shift conception and due-date estimates by several days.
Step-by-Step Guide
- Step 1: Locate the first day of your most recent menstrual period on a calendar.
- Step 2: Enter that date into the Last Period field.
- Step 3: Click Calculate to generate your estimated due date.
- Step 4: Read your current weeks pregnant and current trimester from the output.
- Step 5: Compare the result with your first ultrasound dating, which may adjust the timeline.
Example Calculations
- Scenario 1: With a last period of April 10, 2026, the due date is January 15, 2027, and on June 20, 2026 the user is 10 weeks pregnant in the first trimester.
- Scenario 2: With a last period of January 1, 2026, the due date is October 8, 2026, and on June 20, 2026 the user is approximately 24 weeks pregnant in the second trimester.
Common Use Cases
- Estimating due date at the first prenatal appointment or positive pregnancy test.
- Planning maternity leave and childcare around an approximate delivery window.
- Tracking which prenatal tests and screenings are due in each trimester.
- Helping partners and family members understand pregnancy timing.
Pro Tips
- Use the first day of bleeding, not the last day, as the start date.
- If your cycle is longer than 28 days, your provider may add days to the estimate.
- An early first-trimester ultrasound is the most accurate way to confirm dating.
- Remember that a due date is an estimate; full term spans 37 to 42 weeks.
Common Mistakes to Avoid
- Entering the conception date instead of the first day of the last period.
- Forgetting that pregnancy weeks are counted from LMP, not from a missed period.
- Treating the due date as an exact appointment rather than a 5-week window.
- Ignoring cycle length when interpreting early results.
Why Use This Tool?
- Gives an instant due date without needing clinical equipment.
- Translates calendar dates into the week-based language providers use.
- Helps expectant parents prepare for trimester-specific milestones.
- Serves as a baseline to compare with later ultrasound measurements.