Due Date Calculator
Calculate your pregnancy due date based on your last menstrual period.
Pregnancy Information
Please enter the first day of your last menstrual period
Pregnancy Timeline
Enter your information to calculate your due date
About This Calculator
The Due Date Calculator helps you estimate your baby's due date based on the first day of your last menstrual period (LMP). This is the most common method used by healthcare providers to estimate due dates.
Pregnancy typically lasts about 40 weeks (280 days) from the first day of your last menstrual period. The calculator adjusts for your menstrual cycle length to provide a more accurate estimate. Keep in mind that only about 5% of babies are born exactly on their due date, with most arriving within two weeks before or after.
Formula & Calculation Method
Due Date Calculation:
Standard Calculation:
Due Date = LMP + 280 days (40 weeks)
Adjusted for Cycle Length:
Due Date = LMP + 280 days - (Cycle Length - 28)
Weeks Pregnant = (Current Date - LMP) ÷ 7
Days Pregnant = (Current Date - LMP) % 7
Trimester:
First: Weeks 0-12
Second: Weeks 13-26
Third: Weeks 27-40+
The calculation assumes ovulation occurs approximately 14 days before the next period. For cycles different from 28 days, the due date is adjusted accordingly. An ultrasound scan can provide a more accurate due date, especially in early pregnancy.
How to Use This Calculator
- Enter Last Menstrual Period: Input the first day of your last menstrual period (LMP).
- Enter Cycle Length: Input your average menstrual cycle length in days (typically 21-35 days).
- View Results: See your estimated due date, current weeks/days pregnant, and trimester.
Frequently Asked Questions
How accurate is the due date calculator?
The LMP method is accurate for about 80% of pregnancies. However, an early ultrasound (around 8-12 weeks) provides the most accurate due date. Only about 5% of babies are born exactly on their due date.
What if I don't know my last period date?
If you're unsure of your LMP, an early ultrasound can determine gestational age and due date. Your healthcare provider can help calculate this based on fetal measurements.
Why does cycle length matter?
Cycle length affects when ovulation occurs. Longer cycles typically mean later ovulation, which shifts the due date. The calculator adjusts for this to provide a more accurate estimate.
Can the due date change?
Yes, your healthcare provider may adjust your due date based on early ultrasound measurements, especially if there's a significant difference from the LMP calculation. First-trimester ultrasounds are most accurate for dating.
Expert Reviewed
This calculator was reviewed by NumCalculators Editorial Team, Multi-disciplinary Expert Team