Naegele's Rule: The most common method for calculating due dates is Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day menstrual cycle with ovulation occurring on day 14.

Ultrasound Dating: Early ultrasounds (before 20 weeks) are the most accurate for dating pregnancy. The ultrasound measures the baby's size and compares it to standard growth charts to estimate gestational age.

IVF Pregnancy: For IVF pregnancies, the calculation is based on the embryo age at transfer. A day 3 embryo transfer is equivalent to 17 days after LMP, while a day 5 transfer is equivalent to 19 days after LMP.