How Accurate Is My Pregnancy Due Date?

You've calculated your due date or gotten one from your doctor — but how accurate is it really? Will your baby actually arrive on that exact day?

Let’s explore how due dates are calculated, what makes them more or less accurate, and what you can realistically expect.

📅 What Is a Due Date?

A due date is a prediction of when your baby might be born. It’s based on the average pregnancy length of 280 days (40 weeks) from the first day of your last menstrual period (LMP).

However, only about 4%–5% of babies are actually born on their estimated due date. Most come within a two-week window before or after.

🔍 How Is It Calculated?

• Last Menstrual Period (LMP): Adds 280 days to your last period. Most common method.
• Ultrasound (usually around 8–12 weeks): Measures baby’s size to estimate gestational age — often more accurate than LMP.
• Ovulation date or IVF retrieval: Can be extremely precise if known.

🧠 Which Method Is Most Accurate?

The first-trimester ultrasound is considered the most accurate tool for dating pregnancy, especially if your cycles are irregular or you’re unsure of your LMP.

Here’s how accuracy compares:

• 📏 Ultrasound before 13 weeks: accurate within 5–7 days
• 📏 LMP method: less accurate if you ovulate early/late or don’t remember your dates
• 📏 Ultrasound after 20 weeks: less accurate due to varying baby growth rates

📉 Why the Due Date Isn’t Exact

Many factors affect when labor begins:

• Your hormone levels and body signals
• Your baby’s development pace
• Medical conditions like gestational diabetes or high blood pressure
• If it’s your first baby (they often arrive later)

📌 So What Should You Expect?

Most full-term pregnancies last between 37 to 42 weeks. If your baby comes anytime in that range, it’s still considered normal.

Think of your due date as a helpful guide, not a deadline.

🎯 Track Your Due Date Today

Want a personalized estimate? Try our Free Due Date Calculator based on your LMP.