Enter pregnancy dating details
Sample pregnancy week calculations
| Method | Input Value | Cycle Length | Reference Date | Estimated Week | Estimated Due Date |
|---|---|---|---|---|---|
| LMP | Jan 15, 2026 | 28 days | Mar 22, 2026 | Week 10 + 3 days | Oct 22, 2026 |
| Due Date | Nov 02, 2026 | 30 days | Jul 01, 2026 | Week 23 + 4 days | Nov 02, 2026 |
| Conception | Feb 05, 2026 | 28 days | Apr 16, 2026 | Week 13 + 0 days | Oct 29, 2026 |
Pregnancy week counter formulas
1. Gestational age in days
Gestational Age Days = Reference Date − Calculated LMP Date
2. Current pregnancy week and day
Current Week = floor(Gestational Age Days ÷ 7) + 1
Extra Days = Gestational Age Days mod 7
3. Due date from LMP
Due Date = LMP + 280 days + (Cycle Length − 28 days)
4. LMP from due date
LMP = Due Date − [280 days + (Cycle Length − 28 days)]
5. Conception dating conversion
Estimated LMP = Conception Date − (Cycle Length − 14 days)
Estimated Due Date ≈ Conception Date + 266 days
6. Ultrasound dating conversion
Estimated LMP = Ultrasound Date − Reported Gestational Age in Days
7. Progress percent
Progress Through 40 Weeks = (Gestational Age Days ÷ 280) × 100
How to use this calculator
- Select the dating method that best matches your information.
- Enter the relevant date fields for that method.
- Set a reference date to check current or future timing.
- Enter your average cycle length when using LMP, due, or conception dating.
- Press the calculate button to view week, trimester, milestones, and graph.
- Use CSV or PDF export buttons to save the output summary.
- Compare the result with your clinical records when available.
Pregnancy week counter FAQs
1. How is pregnancy week counting usually started?
Pregnancy weeks are commonly counted from the first day of the last menstrual period, not from conception. This obstetric method is the standard used for most clinical schedules and milestones.
2. Why can due dates differ between calculators and scans?
Different inputs produce different estimates. LMP depends on cycle timing, while ultrasound dating estimates gestational development. Providers may revise the due date when scan findings fit a different timeline better.
3. What does Week 10 + 3 days mean?
It means ten pregnancy weeks have started and three additional days have passed within that week. This format helps track development more precisely than whole weeks alone.
4. When should I use ultrasound dating?
Use ultrasound dating when a scan already reported gestational age or when cycle timing is uncertain. Early scans are often used clinically to refine pregnancy dating.
5. Does a longer cycle change the estimated due date?
Yes. A longer cycle can shift ovulation later, so the calculator adjusts LMP-based due dates by the difference from a typical 28-day cycle.
6. Can I use a future reference date?
Yes. A future reference date helps you forecast the pregnancy week, trimester, and milestone timing for appointments, travel planning, or personal preparation.
7. Is the progress percentage a medical measurement?
No. It is a planning indicator showing how far along a 40-week pregnancy timeline has progressed. It does not measure fetal growth, health, or readiness for delivery.
8. Should this calculator replace professional advice?
No. This tool is for education and planning. Clinical guidance should come from a qualified healthcare professional, especially for symptoms, scan interpretation, bleeding, pain, or dating questions.