Use our free online Pregnancy Calculator to calculate your expected due date, gestational age, and trimester progress. Tracking the progression of your pregnancy is essential for coordinating prenatal appointments, lifestyle adjustments, and nursery preparations.
This comprehensive calculator supports multiple standard clinical dating methods—including last menstrual period (LMP), conception date, known due date, and early ultrasound dating scans. It maps out your complete 40-week timeline, key prenatal screening windows, and weekly baby size updates.
How to Use the Pregnancy Calculator
- Select your calculation method: Choose between Last Menstrual Period, Conception Date, Known Due Date, or Ultrasound scan data.
- Enter the matching date: Select the calendar date for your chosen method. If you select LMP, provide your average cycle length (typically 28 days).
- Provide ultrasound metrics (if applicable): Input the gestational weeks and additional days estimated in your early scan report. Click Calculate to review your results.
Formula & Calculation Method
Calculations are based on standard clinical assumptions of a 280-day (40-week) gestational timeline:
Estimated Due Date (LMP) = LMP Date + 280 Days + (Cycle Length - 28 Days) Estimated Due Date (Conception) = Conception Date + 266 Days Gestational Age (Days) = 280 - Days Remaining to Due Date Gestational Weeks = floor(Gestational Age ÷ 7)
For ultrasound dating, the calculator works backward from the scan date and gestational age to compute the estimated LMP, which is then used as the baseline for all milestone calculations.
Example Calculation
If your Last Menstrual Period began on April 1, 2026, and you have a standard 28-day cycle:
- Due Date Estimate: April 1 + 280 days = January 6, 2027.
- Conception Date: April 1 + 14 days (ovulation) = April 15, 2026.
- Trimester Transition: First trimester ends at week 12 (June 24, 2026), and second trimester ends at week 27 (October 7, 2026).
Reference Data: Pregnancy Trimester Timeline & Milestones
| Trimester | Weeks Range | Fetal Development Stage | Typical Care / Scans |
|---|---|---|---|
| First Trimester | Weeks 1 – 12 | Organogenesis, heartbeat begins, major systems form | LMP confirmation, dating ultrasound, NT scan |
| Second Trimester | Weeks 13 – 27 | Fetal movement, hair/nails, active swallowing | Anatomy scan (W18-22), Glucose test (W24-28) |
| Third Trimester | Weeks 28 – 40 | Rapid brain growth, fat storage, lung maturation | GBS test, weekly pressure checks, growth scans |
What the Result Means
Your calculated Gestational Age indicates how far along you are in weeks and days. The Due Date represents a projected delivery window; only about 5% of babies are born on their exact due date. The Pregnancy Progress bar shows what percentage of the standard 40-week timeline has passed.
Common Mistakes to Avoid
- Confusing conception date with LMP: Gestational age is counted from the first day of your last period, which is typically 2 weeks before actual fertilization occurs.
- Overlooking irregular cycle lengths: If your menstrual cycles are regularly shorter than 26 days or longer than 32 days, standard LMP estimates will be inaccurate. In these cases, early ultrasound dating is the gold standard.
- Expecting birth on the exact due date: A normal pregnancy can safely deliver anywhere from week 37 to week 42.
When This Calculator Is Useful
Obstetric Appointment Prep
Calculate your exact gestation age to understand which scans and diagnostic screens are due at your next visit.
Trimester Stage Awareness
Track when you transition between trimesters to anticipate physical changes and developmental stages.
Maternity Leave Planning
Establish a clear birth timeline to plan maternity and paternity leave schedules with your employer.
Family Milestone Countdown
Share a calculated weekly progress card and days remaining countdown with family and friends.
This tool provides estimates based on standard obstetric methods. Always consult your OB-GYN or midwife for accurate due date confirmation and prenatal guidance.