Understanding Your Due Date
It's the date everyone asks about. But is it accurate? Here's the science behind how doctors calculate when you'll meet your baby.
How is it Calculated? (Naegele's Rule)
Most due date calculators, including the one your doctor uses, rely on a simple formula called Naegele's Rule.
The Formula:
1. Take the first day of your Last Menstrual Period (LMP).
2. Add 7 days.
3. Subtract 3 months.
4. Add 1 year.
This method assumes a standard 28-day cycle with ovulation occurring exactly on day 14. If your cycle is longer or shorter, your actual date of conception might differ!
LMP vs. Ultrasound Date
Often, your "dating scan" (usually between 8-13 weeks) will give you a different date than your LMP calculation. Which one is right?
Doctors generally trust the ultrasound date if there's a discrepancy of more than 5-7 days. In the first trimester, all babies grow at nearly the exact same rate, making crown-to-rump measurement a highly accurate predictor of gestational age.
Is the "Due Date" Accurate?
Here's a secret: Only about 4-5% of babies are born on their exact due date.
Before 37 Weeks
Considered "Preterm."
37 to 42 Weeks
The normal "Term" window.
Most first-time moms are actually more likely to go over their due date than under it.
Trimester Breakdown
Pregnancy is divided into three trimesters, each with its own unique milestones:
- First Trimester (Weeks 1-13): The most rapid development. Heartbeat, organ formation, and often... morning sickness!
- Second Trimester (Weeks 14-27): The "Golden Period." Energy returns, you feel the baby move, and you can find out the gender (around week 20).
- Third Trimester (Weeks 28-40+): The home stretch. Rapid weight gain for baby, and preparation for labor.
🗓 What to do next?
Once you have your date, you can start planning!
Week 12: Nuchal Translucency Scan (and first chance to see baby!)
Week 20: Anatomy Scan (Gender Reveal)