The LMP is used because most women know when their last period was, but may not know when they actually conceived.
If you do know the conception date, this is classed as day 14 and not day one.
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.
Ultrasound uses the size of the fetus to determine the gestational age (the time elapsed since the the first day of the last menstrual period).
The due date may be estimated by adding 280 days ( 9 months and 7 days) to the first day of the last menstrual period (LMP). The accuracy of the EDD derived by this method depends on accurate recall by the mother, assumes regular 28 day cycles, and that ovulation and conception occurs on day 14 of the cycle.
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].
The date of the first documented positive pregnancy test and the beta-human chorionic gonadotropin (b HCG) level may help ascertain the minimum gestational age.
Your baby's head, limbs, hands and feet, and some organs can be seen, and - though looking for abnormalities isn't the purpose of the ultrasound - some major problems can be seen at this early stage.
The dating scan confirms the pregnancy, checks whether it is one child or more and calculates the date when the baby is due.
It is especially important to have this done if bleeding or pain has occurred, or to make sure that the pregnancy is in your womb and not in your fallopian tube (ectopic pregnancy.) Many other tests/scans undertaken during pregnancy take place at a certain gestational age and knowing this accurately ensures that you do not miss the recommended timeframe.
But ultrasounds can also lift the lid on a few worries you might be having. The main reason for the ultrasound is to work out how many weeks pregnant you are, and to estimate your due date.
The length of your pregnancy is calculated from the first day of your last menstrual period (LMP), even though you don't actually conceive until 14 days after your LMP, and later than this if your cycle is longer than 28 days.