Last menstrual period ultrasound dating
This is because the fetus is growing so quickly that there is a big difference in size from week to week.Also the fetus cannot bend and twist too much yet, so the length of the body is fairly constant.Determining the Estimated Due Date The estimated due date (EDD or EDC) is the date that spontaneous onset of labor is expected to occur. Conceptional age, menstrual age, and ultrasound age: a second-trimester comparison of pregnancies of known conception date with pregnancies dated from the last menstrual period.
This is presuming that the pregnancy is actually at this stage of development.Ultrasounds performed after 22 weeks gestation cannot be used to estimate the due date of the baby because the size no longer reflects the age very well.Even average babies can differ by up to 2 to 3 "weeks of growth";.The EDD from the early dating scan is used - if the last menstrual period is not known or is unreliable, or the dating scan differs from the last menstrual period dating by more than 5 days.As the baby gets bigger, it starts to express its individual growth potential.
However, the accuracy of the ultrasound examination is always dependent on the skill of the sonographer and the quality of the equipment.