About Pregnancy Week Calculator
The pregnancy week calculator converts your last menstrual period into a precise gestational age in weeks and days, plus an estimated due date and current trimester. While a full-term pregnancy is commonly described as 40 weeks, daily variation matters when scheduling screenings, interpreting fetal size, or discussing symptoms with a provider. This tool splits gestation into weeks plus remaining days, so instead of simply saying 'about 18 weeks,' it can say '18 weeks and 3 days.' That level of detail is useful when a blood test needs to happen between 10 weeks 0 days and 13 weeks 6 days, or when an anatomy scan is ideally performed around 20 weeks. By grounding every output in the LMP, the calculator gives parents and clinicians a shared reference point before or between ultrasounds.
How It Works
Enter the first day of your last menstrual period. The calculator determines the number of full days between that date and today, then divides by 7 to get whole weeks and a remainder for days. It labels the trimester using standard boundaries, and it adds 280 days to the LMP to produce an estimated due date.
Formula & Calculation Logic
Days since LMP = today - LMP in days. Weeks pregnant = floor(days / 7). Days = days modulo 7. Trimester follows weeks: 0-12 first, 13-26 second, 27-40 third. Due date = LMP + 280 days. Like all LMP models, it assumes ovulation occurred around day 14 of a 28-day cycle.
Step-by-Step Guide
- Step 1: Confirm the first day of your last menstrual period.
- Step 2: Type that date into the Last Menstrual Period field.
- Step 3: Run the calculation to get weeks, days, trimester, and due date.
- Step 4: Use the week-and-day result to schedule time-sensitive prenatal tests.
- Step 5: Cross-check the output with an ultrasound or your provider's dating.
Example Calculations
- Scenario 1: An LMP of March 15, 2026, on June 20, 2026, yields 14 weeks and 0 days, placing the user at the start of the second trimester with a due date around December 20, 2026.
- Scenario 2: An LMP of February 1, 2026, on June 20, 2026, gives 19 weeks and 5 days and a due date around November 8, 2026.
Common Use Cases
- Pinpointing gestational age for nuchal translucency or first-trimester screening windows.
- Communicating exact pregnancy progress to providers, doulas, or support groups.
- Calculating how many weeks remain until estimated due date.
- Determining trimester for app notifications, registry planning, or maternity leave discussions.
Pro Tips
- Use the first day of actual bleeding, not spotting or the heaviest flow day.
- If you know your conception date, subtract 14 days to approximate the LMP.
- Double-check dates if your cycle is typically shorter than 28 or longer than 35 days.
- Combine the calculator with an early dating scan for the most reliable timeline.
Common Mistakes to Avoid
- Entering the date of conception instead of the LMP.
- Confusing weeks pregnant with fetal age, which is about two weeks less.
- Using the calculator after a provider has adjusted dating based on ultrasound.
- Forgetting that days count just as much as weeks for some screening windows.
Why Use This Tool?
- Breaks pregnancy down into weeks and days for precise tracking.
- Helps parents understand which trimester they are currently in.
- Supports timing of prenatal screenings and milestone appointments.
- Offers a quick, no-equipment way to estimate gestational age.