Pregnancy Due Date Calculator
Use this calculator to estimate your baby's due date based on the first day of your last menstrual period (LMP) and your average cycle length.
Understanding Your Pregnancy Due Date
Calculating your baby's due date is one of the first and most exciting steps after discovering you're pregnant. While it's often referred to as an "estimated due date" (EDD), it provides a crucial timeline for your pregnancy journey, helping you and your healthcare provider plan for prenatal care and the baby's arrival.
How is the Due Date Calculated?
There are several methods to estimate a due date, with the most common being based on the first day of your Last Menstrual Period (LMP).
1. Naegele's Rule (Based on LMP)
This is the most widely used method. It assumes a typical 28-day menstrual cycle and that ovulation occurs on day 14. The traditional rule states that you add 280 days (40 weeks) to the first day of your last menstrual period. This calculator uses a modified Naegele's Rule to account for varying cycle lengths, providing a more personalized estimate.
Formula: Estimated Due Date = First Day of LMP + 280 days + (Average Cycle Length – 28 days)
For example, if your LMP was January 1, 2024, and you have a standard 28-day cycle:
- January 1, 2024 + 280 days = October 8, 2024
If your LMP was January 1, 2024, and you have a 30-day cycle:
- January 1, 2024 + 280 days + (30 – 28) days = January 1, 2024 + 282 days = October 10, 2024
If your LMP was January 1, 2024, and you have a 26-day cycle:
- January 1, 2024 + 280 days + (26 – 28) days = January 1, 2024 + 278 days = October 6, 2024
2. Conception Date (If Known)
If you know the exact date of conception (e.g., through IVF or very precise tracking), you can add 266 days (38 weeks) to that date. This method is often more accurate if the conception date is certain, as it bypasses assumptions about ovulation timing.
3. Ultrasound Dating
Early ultrasounds (typically between 8 and 13 weeks) are often considered the most accurate way to determine a due date, especially if there's uncertainty about the LMP or if menstrual cycles are irregular. The sonographer measures the baby's size (crown-rump length) to estimate gestational age.
Why is it an Estimate?
It's important to remember that only about 4% of babies are born exactly on their due date. Most babies arrive between 37 and 42 weeks of gestation. Your due date is a guide, not a deadline. Factors like individual variations in ovulation, implantation, and fetal growth can influence the actual birth date.
Understanding Gestational Age and Trimesters
Your pregnancy is typically divided into three trimesters, each with its own developmental milestones and maternal changes:
- First Trimester: Week 1 to Week 13
- Second Trimester: Week 14 to Week 27
- Third Trimester: Week 28 to Week 40 (or until birth)
Gestational age refers to how far along your pregnancy is, measured from the first day of your LMP. This calculator will also provide your current estimated gestational age and which trimester you are in.
When to Consult Your Doctor
While this calculator provides a helpful estimate, always confirm your due date with your healthcare provider. They will use your medical history, physical examination, and potentially ultrasound findings to give you the most accurate information and guide you through your pregnancy journey.