Pregnancy conception dating
Use of the LMP to establish the due date may overestimate the duration of the pregnancy, and can be subject to an error of more than 2 weeks [5-7]. In cases where the date of conception is known precisely, such as with in vitro fertilization, the EDD is calculated by adding 266 days to the date of conception. American College of Obstetricians and Gynecologists. For women who have irregular menstrual periods or women who cannot remember the first day of their last menstrual period, it can be difficult to determine gestational age using this method.
This is just an estimate since only about 5% of babies are born on their estimated due date.Gestational age, or the age of the baby, is calculated from the first day of the mother’s last menstrual period.Since the exact date of conception is almost never known, the first day of the last menstrual period is used to measure how old the baby is.Measuring the baby using ultrasound is most accurate in early pregnancy. The best time to estimate gestational age using ultrasound is between the 8th and 18th weeks of pregnancy.
The most accurate way to determine gestational age is using the first day of the woman’s last menstrual period and confirming this gestational age with the measurement from an ultrasound exam.
For a woman with a regular period, conception typically occurs about 11-21 days after the first day of the last period.