How Due Date Is Calculated
The estimated due date (EDD) is calculated using Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle. If your cycle is longer or shorter, the calculation adjusts accordingly.
Understanding Gestational Age
Gestational age is counted from the first day of your last period, not from conception. This means you're already considered about 2 weeks pregnant at the time of conception. Your doctor will use this dating method for all prenatal care.
Important Prenatal Appointments
- Week 8-12: First prenatal visit, blood tests, dating ultrasound
- Week 11-13: First trimester screening (nuchal translucency)
- Week 15-20: Second trimester screening (AFP, quad screen)
- Week 18-22: Anatomy scan (detailed ultrasound)
- Week 24-28: Glucose screening test
- Week 36+: Weekly visits, Group B Strep test at 36 weeks
How Accurate Is the Due Date?
Only about 5% of babies are born on their exact due date. Most babies arrive between 38 and 42 weeks. The due date is an estimate based on the average pregnancy length. An ultrasound in the first trimester can refine this estimate with greater accuracy.
Frequently Asked Questions
What if I don't know my last period date? Your doctor can estimate the due date using an early ultrasound, which is most accurate in the first trimester. You can also use the conception date method if you know when you conceived.
Can my due date change? Yes. If an ultrasound shows a significant difference from the LMP-based due date (more than 7 days in the first trimester), your doctor may adjust it.
What is full term? Full term is 39-40 weeks. Early term is 37-38 weeks. Late term is 41 weeks, and post-term is 42+ weeks.