Due Date Calculator
Estimate your baby's arrival date based on your last menstrual period. Calculate your estimated due date, gestational age, and current trimester using Naegele's Rule.
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Enter your last menstrual period date and click calculate to see your estimated due date.
How to Calculate Your Due Date
The estimated due date (EDD) is calculated using Naegele's Rule, the standard method used by healthcare providers worldwide. This formula adds 280 days (40 weeks) to the first day of your last menstrual period.
The calculation assumes a 28-day menstrual cycle with ovulation occurring on day 14. For cycles longer or shorter than 28 days, adjustments can be made based on your average cycle length.
Naegele's Rule Formula
Due Date = First Day of LMP + 280 days
Or: First Day of LMP + 7 days - 3 months + 1 year
Example: If your last period started on March 15, 2025:
- • Add 7 days: March 22, 2025
- • Subtract 3 months: December 22, 2024
- • Add 1 year: December 22, 2025
- • Estimated Due Date: December 22, 2025
For irregular cycles, your healthcare provider may adjust the due date based on ultrasound measurements taken in early pregnancy, which are most accurate during the first trimester.
Understanding Your Due Date
Your estimated due date serves as a guide, but it's important to remember that only about 4% of babies are born on their exact due date. Most deliveries occur within a two-week window before or after the estimated date.
Full-Term Pregnancy
A pregnancy is considered full-term between 37 and 42 weeks. Babies born during this period typically have the best outcomes.
Gestational Age
Pregnancy is calculated from the first day of your last menstrual period, not from conception. This is why gestational age is typically 2 weeks more than fetal age.
Trimesters
Pregnancy is divided into three trimesters: first (weeks 1-12), second (weeks 13-27), and third (weeks 28-40+). Each trimester has distinct developmental milestones.
Variability
Factors like cycle length, ovulation timing, and individual variations can affect due date accuracy. Early ultrasound dating can help refine the estimate.
Factors Affecting Due Date Accuracy
Several factors can influence the accuracy of your estimated due date:
Menstrual Cycle Length
If your cycle is consistently longer or shorter than 28 days, ovulation may occur earlier or later, affecting conception timing. Our calculator allows you to adjust for your average cycle length.
Irregular Periods
Women with irregular cycles may need ultrasound dating for more accurate due date estimation. First-trimester ultrasounds (before 13 weeks) provide the most reliable dating.
Early Ultrasound Dating
Ultrasound measurements taken in the first trimester can measure fetal size to estimate gestational age within 3-5 days accuracy. This may adjust your due date if there's a significant difference from LMP calculation.
Multiple Pregnancies
Twin or multiple pregnancies typically have shorter gestation periods, with most delivering before 37 weeks. Your healthcare provider will monitor these pregnancies more closely.
Frequently Asked Questions
How accurate is the due date calculator?
The calculator uses Naegele's Rule, which is accurate for women with regular 28-day cycles. However, only about 4% of babies are born on their exact due date. Most births occur within a two-week window before or after the estimated date. For irregular cycles, ultrasound dating provides more accuracy.
What if I don't remember my last period date?
If you don't remember your LMP or have irregular periods, an early ultrasound (typically performed between 6-13 weeks) can provide an accurate due date estimate. Your healthcare provider will use fetal measurements to calculate gestational age.
How does cycle length affect the due date?
If your cycle is consistently longer than 28 days, you likely ovulate later, which means conception occurred later. This would push your due date slightly later. Conversely, shorter cycles may result in an earlier due date. Our calculator allows you to adjust for your average cycle length.
Can my due date change during pregnancy?
Yes, your due date may be adjusted after an early ultrasound if fetal measurements differ significantly from the LMP calculation. Ultrasounds are most accurate for dating in the first trimester. After the first trimester, due dates are rarely changed.
What is a full-term pregnancy?
A full-term pregnancy is between 37 and 42 weeks from the first day of your last menstrual period. Babies born during this window typically have the best health outcomes. Preterm births occur before 37 weeks, while post-term pregnancies extend beyond 42 weeks.
Should I rely solely on this calculator?
While this calculator provides an accurate estimate based on standard medical calculations, it should not replace professional medical care. Always consult with your healthcare provider for pregnancy dating, monitoring, and care. Your provider will use multiple methods including physical exams and ultrasounds for the most accurate assessment.
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