Ultrasonography dating in the second trimester typically is based on regression formulas that incorporate variables such as the biparietal diameter and head circumference measured in transverse section of the head at the level of the thalami and cavum septi pellucidi; the cerebellar hemispheres should not be visible in this scanning plane the femur length measured with full length of the bone perpendicular to the ultrasound beam, excluding the distal femoral epiphysis how accurate is early pregnancy dating abdominal circumference measured in symmetrical, transverse round section at the skin line, with visualization of the vertebrae and in a plane with visualization of the stomach, umbilical vein, and portal sinus 8 Other biometric variables, such as additional long bones and the transverse cerebellar diameter, also can play a role.
Appropriately performed obstetric ultrasonography has been shown to accurately determine fetal gestational age 1. Seeing the gestational sac, which looks like a black dot in the uterus, has several benefits: it confirms that a woman is pregnant, confirms that the pregnancy at a very how accurate is early pregnancy dating stage, making dating the pregnancy very accurate, and also confirms that the pregnancy is in the uterus rather than ectopic, or outside the uterus.
For the purposes of research and surveillance, the best obstetric estimate, rather than estimates based on the last menstrual period alone, should be used as the measure for gestational age.
For example, those with black or Asian ethnic heritage appear to have how accurate is early pregnancy dating shorter pregnancies on average than those of white heritage It has been reported that approximately one half of women accurately recall their How accurate is early pregnancy dating 2 3 4.
Article Location Article Location. Date changes for smaller discrepancies 10—14 days are appropriate based on how early in this second-trimester range the ultrasound examination was performed and on clinician assessment of LMP reliability.
In many parts of the country, there is now momentum building in favour of the implementation of a relational model of maternity care. You may want to use your own knowledge about the length of your pregnancy when making decisions, rather than the figure written in your notes.
Dating with ultrasound is a skilled procedure, so the accuracy of the EDD will also vary according to the experience of the person doing the measurement.
Fetal Imaging Workshop Invited Participants. Related questions: A year-old female asked:. Read More. In many parts of the country, there is now momentum building in favour of the implementation of a relational model of maternity care.
For this reason the American College of Obstetricians and Gynecologists has recommended that that due dates should only be changed in the third trimester in very rare circumstances. An early ultrasound done between 6 and 9 weeks of pregnancy is considered to be accurate to within 5 days of a due date.
And why I probably shouldn't be taking my second "due date" very seriously. Recent research has identified several genes which are associated with length of pregnancy and the likelihood of a pre-term birth 8 and so underpin this natural variation.