How accurate is 12 week dating scan
Sometimes a trans-vaginal examination shows that your pregnancy is less advanced than expected.
The earlier the ultrasound is done, the more accurate it is at estimating the baby's due date.
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.
Ultrasound examinations from 12 to 22 weeks are regarded as being within 10 days of accuracy (or up to 10 days earlier or 10 days later than the woman's calculated due date).
However, the accuracy of the ultrasound examination is always dependent on the skill of the sonographer and the quality of the equipment.
Usually the expected date of delivery (EDD or EDC) is calculated from your last menstrual period - if the early dating scan calculates the EDD to be within 5 days of the EDD from your last menstrual period.
A scan is performed at 18 to 20 weeks when the fetus is large enough for an accurate survey of the fetal anatomy, and when dates and growth can also be assessed.
After your baby's heartbeat is checked, the main purpose of this scan is to check your baby's anatomy for normality.
This is a routine ultrasound examination performed at 18 to 20 weeks of gestation.