Use this free pregnancy due date calculator to estimate your expected due date, current pregnancy week, trimester, and key milestone dates. You can calculate using your last menstrual period (LMP), known conception date, or an existing due date.
Pregnancy Due Date & Weeks Calculator
Choose a calculation method, enter your date, and click calculate.
Medical disclaimer: this tool provides estimates and is not a diagnosis. Always follow your OB/GYN or midwife for clinical dating and prenatal care.
How this due date calculator works
Most pregnancy due dates are estimated as 40 weeks (280 days) from the first day of the last menstrual period. This is called gestational age dating and is the standard method used in obstetrics. If your cycle is not exactly 28 days, the estimate can be adjusted based on your average cycle length.
Input options explained
- Last menstrual period (LMP): The most common method. You enter the first day of your last period and your average cycle length.
- Known due date: Useful if your provider already gave you an estimated due date and you want to track your current week.
- Conception date: Helpful when conception timing is known (for example, fertility treatment or clearly tracked ovulation).
Understanding pregnancy weeks and trimesters
Pregnancy is typically measured in weeks rather than months because development changes rapidly and weekly milestones are clinically useful.
- First trimester: Weeks 1 to 13
- Second trimester: Weeks 14 to 27
- Third trimester: Weeks 28 to 40
The calculator shows your week-and-day format (for example, 22 weeks, 3 days), which is the format commonly used in prenatal appointments.
Important milestone dates you can track
Common timeline checkpoints
- 12 weeks: End of early pregnancy and major first-trimester screening period
- 20 weeks: Anatomy scan window in many practices
- 28 weeks: Start of the third trimester
- 37 weeks: Early-term pregnancy range begins
- 40 weeks: Estimated due date
These dates are approximate and can vary based on your provider’s recommendations and your personal medical history.
How accurate is a due date estimate?
Due dates are estimates, not exact predictions. Only a small percentage of babies are born on the exact estimated due date. Labor commonly starts between 37 and 42 weeks. Your provider may refine your dates based on early ultrasound findings, which are often the most accurate clinical method in the first trimester.
Factors that can affect dating
- Irregular menstrual cycles
- Uncertain LMP date
- Late or early ovulation
- Assisted reproductive technology timing
- Ultrasound dating adjustments
Frequently asked questions
Is gestational age the same as fetal age?
No. Gestational age starts from LMP, about two weeks before conception in a typical cycle. Fetal age usually counts from conception.
Can my due date change?
Yes. If early ultrasound measurements differ significantly from LMP-based dating, your provider may update the due date.
Why does cycle length matter?
Longer or shorter cycles can shift ovulation timing. Adjusting cycle length can slightly improve estimates when using LMP-based calculations.
Final note
This calculator is a practical planning tool for tracking pregnancy week by week, but it should not replace prenatal medical care. If you have concerns about symptoms, fetal movement, bleeding, contractions, or your due date accuracy, contact your healthcare professional right away.