Complete Guide: How the Pregnancy Week-by-Week Calculator Works
A pregnancy week-by-week calculator helps you pinpoint exactly how far along you are using either the first day of your last menstrual period (LMP) or a known due date. Instead of manually counting weeks on a calendar, this free tool instantly converts your dates into your current gestational week, trimester, and a fun size comparison for your growing baby.
💡 Quick Summary
This calculator uses Naegele's Rule, the standard clinical method of adding 280 days (40 weeks) to the first day of your LMP, to estimate your due date, current gestational age, and where you fall within each trimester.
Why Track Your Pregnancy Week-by-Week?
- Understand fetal development: Each week brings specific developmental milestones for your baby, from organ formation to viability.
- Plan prenatal appointments: Many scans and screenings (dating scan, NT scan, anatomy scan, glucose test) are scheduled around specific weeks.
- Track trimester transitions: Know exactly when you move from first to second to third trimester.
- 100% Free & Private: No signup and no data stored — everything calculates instantly in your browser.
How Gestational Age Is Calculated
| Method | How It Works | Best For |
|---|---|---|
| LMP-based (Naegele's Rule) | Adds 280 days to the first day of your last period | Most common clinical method, works best with regular cycles |
| Due-date based | Works backward 280 days from a known due date | If your doctor already gave you an estimated due date |
| Ultrasound dating | Measures fetal size in early scans for the most precise estimate | Irregular cycles; always confirm with your provider |
Step-by-Step Instructions
- Select whether you're entering your LMP date or your due date.
- Enter the date using the date picker.
- Click ⚡ Calculate My Pregnancy Week.
- Review your current week, day, trimester, due date, and baby's size comparison.
- Use the 📋 Copy Summary button to save your results to your clipboard.