The educational health content on What To Expect is reviewed by our team of experts to be up-to-date and in line with the latest evidence-based medical information and accepted health guidelines, including the medically reviewed What to Expect books by Heidi Murkoff. First day of last period Most pregnancies last around 40 weeks or 38 weeks from conceptionso typically the best way to estimate your due date is to count 40 weeks, or days, from the first day of your last menstrual period LMP. To know your pregnancy weeks with this calculation, remember the first day of your last period. Ultrasound The best way to determine your conception date is with a pregnancy confirmation ultrasound.
The size of your uterus, which will be noted when your initial internal pregnancy exam is performed, can also be a factor in pinpointing the EDD.