You’ve calculated your due date, started planning... and then your doctor says it might be different. Sound familiar? Don’t panic — it’s actually common for due dates to shift slightly during pregnancy.
Most due dates are estimated using Naegele’s Rule: count 280 days (40 weeks) from the first day of your last menstrual period (LMP). This assumes a regular 28-day cycle, with ovulation on day 14.
But every body is different — and that’s where adjustments come in.
If your early ultrasound (usually before 13 weeks) shows a baby that’s measuring ahead or behind the expected size, your provider may adjust your due date based on that measurement.
Ultrasounds are considered very accurate in the first trimester — often more than using your LMP.
If you have irregular or long cycles, ovulation may have happened earlier or later than expected. Doctors may rely more on ultrasound data than your reported LMP.
If you conceived through IVF or fertility tracking, your due date might be based on egg retrieval or embryo transfer — not your LMP.
It’s perfectly normal for due dates to shift slightly during pregnancy. Your healthcare provider uses a mix of science and experience to estimate the best date for your baby’s arrival.
✅ Want to double-check your current estimate? Use our Free Due Date Calculator.