How Pregnancy Due Dates Are Calculated
While babies do have a mind of their own when it comes to arrival time, doctors and researchers have developed reliable methods to predict when yours will arrive. The most common—and the one we use here—is called Naegele's Rule. It's been around for centuries and still works remarkably well.
Three Ways to Measure
Depending on your journey to pregnancy, there are different starting points for the clock. Choose the one that matches your known history best:
First Day of Last Period (LMP)
Best for regular cycles. It adds 280 days (40 weeks) to the first day of your last period. Since ovulation usually happens 14 days into a cycle, the first 2 weeks of "pregnancy" you technically aren't pregnant yet!
Conception Date
Best if you track ovulation. If you know exactly when you ovulated or conceived, we add 266 days (38 weeks) to that date. This removes the guesswork of the follicular phase.
IVF Transfer Date
Most precise. For IVF, biology is timed perfectly. We add 266 days to the fertilization date, adjusting for whether you had a Day 3 or Day 5 embryo transfer.
The 40-Week Timeline
Pregnancy is divided into three distinct stages called trimesters. Here is what you can expect during each window:
Rapid development. Your baby grows from a single cell to a moving fetus with fingerprints. Common symptoms: fatigue, morning sickness, and mood swings.
Often called the "sweet spot." Energy returns, morning sickness fades, and you'll feel first kicks. This is when most people feel their best physically.
Your baby gains weight rapidly and prepares for birth. Fatigue returns, and doctor visits increase. The final stretch often brings both excitement and discomfort.
Can Your Due Date Change?
Yes, it is practically routine for due dates to shift.
- Irregular Cycles: If you ovulate later than day 14, your LMP date might be "off" by a week or more.
- Dating Scans: An ultrasound in the first trimester is the most accurate way to date a pregnancy. If the scan shows the baby is significantly larger or smaller than expected, your doctor will update your EDD (Estimated Due Date).
Note: Only 1 in 20 babies is born on their exact due date. Consider it a "Target Week" rather than a deadline!