Pregnancy Due Date & Weeks Calculator
Use this tool to estimate your due date, how many weeks pregnant you are, and your trimester. You can calculate from your last menstrual period (LMP) or from an existing due date.
1) Calculate from Last Menstrual Period (LMP)
2) Calculate Weeks Pregnant from Due Date
How this due date and weeks calculator works
This pregnancy calculator uses standard obstetric dating methods to estimate your expected due date and gestational age. Most due date calculators begin with the first day of your last menstrual period (LMP), then add 280 days (40 weeks). If your cycle length is longer or shorter than 28 days, the estimate is adjusted accordingly.
The second calculator mode works in reverse. If you already have a due date from your care provider, it estimates your LMP and tells you how many weeks and days pregnant you are on any selected date.
- Estimate due date from LMP
- Adjust estimate based on cycle length
- Calculate current pregnancy week and day
- Identify your trimester
- Show days remaining until due date
Understanding due date estimates
Naegele's rule (the common method)
The classic formula adds 1 year, subtracts 3 months, and adds 7 days to the first day of your LMP. In modern tools, this is usually represented by adding 280 days. This gives a practical estimate used in prenatal care scheduling, screening timelines, and growth checks.
Why cycle length matters
A 28-day cycle assumes ovulation around day 14. If your average cycle is 32 days, ovulation may occur later, so your estimated due date shifts forward. If your cycle is shorter, the due date may shift earlier. This is why the cycle-length field can improve accuracy for people with predictable but non-28-day cycles.
Pregnancy weeks and trimesters
Pregnancy is measured from LMP, not conception date. This can feel confusing at first, but it is the standard used by healthcare professionals.
- First trimester: 0 weeks through 12 weeks + 6 days
- Second trimester: 13 weeks through 26 weeks + 6 days
- Third trimester: 27 weeks to delivery
Only a small percentage of babies are born exactly on the due date. Most births occur between 37 and 42 weeks, depending on individual pregnancy factors and medical guidance.
When due dates may be updated
Early ultrasound measurements are often used to refine dating, especially when period dates are uncertain or cycles are irregular. Your provider may update the official due date if scan findings differ meaningfully from the LMP-based estimate.
- Irregular cycles or uncertain LMP date
- Recent birth control use affecting cycle timing
- Early ultrasound suggesting different gestational age
- Assisted reproductive technology (IUI/IVF timing)
Tips for using this calculator
Use the most reliable date available
If you are certain of your LMP and cycles are regular, the LMP method is a good starting point. If your provider has given you a due date from ultrasound or fertility treatment timing, use the due-date mode for week-by-week tracking.
Recalculate as needed
You can change the as-of date to check your pregnancy week on any day, including appointment dates, travel plans, or maternity leave planning.
Frequently asked questions
Is this calculator a medical diagnosis?
No. It provides an estimate for planning and education. It does not replace prenatal care, ultrasound, or professional medical advice.
Can I be “more than 40 weeks” pregnant?
Yes. Some pregnancies continue beyond 40 weeks. If your selected date is after your due date, the calculator will show how many days past due you are.
Why does conception date differ from gestational age?
Gestational age is counted from LMP. Conception usually happens about 2 weeks after LMP in a 28-day cycle, so gestational age is typically about two weeks ahead of embryonic/fetal age.
Medical disclaimer
This due date and weeks calculator is for informational use only. For personalized care, pregnancy confirmation, and decisions about symptoms or treatment, consult a qualified healthcare professional.