Dating and viability
What is a dating scan?
A dating scan is an ultrasound examination which is performed in order to establish the gestational age of the pregnancy.
Dating scans also reveal other important information such as:
- the number of fetuses and gestation sacs
- the presence of a heart beat
- the size of the fetus, from which the gestational age is estimated
- unusual features of the uterus such as the shape or the presence of fibroids
How is it done?
Most dating scans are done with a trans-abdominal transducer and a fullish bladder. If the pregnancy is very early the gestation sac and fetus will not be big enough to see, so the transvaginal approach will give better pictures.
For transvaginal ultrasound you can have an empty bladder
When is it recommended?
Dating scans are usually recommended if there is doubt about the validity of the last menstrual period.
For example if you
- do not know when the first day of your last period, or the likely day of conception was
- have had irregular periods
- have had an unusual cycle because you recently stopped the oral contraceptive pill, or you recently had a miscarriage, or you are breast-feeding
- had some implantation bleeding which can be been confused with your last period date
At 5 ½ weeks gestation a tiny sac can be seen in the uterus, but the baby and its heart beat may not be detected yet.
5 ½ weeks gestation means 5 ½ weeks from the first day of the last menstrual period, which is usually about 3 ½ weeks from the date of conception (confusing, isn’t it!)
By 6 to 7 weeks gestation the fetus is clearly seen on trans-vaginal ultrasound and the heart beat can be seen at this early stage (90 to 110 beats per minute under 6 to 7 weeks, then 110 to 200 beats per minute as the baby matures).
By 8 weeks gestation the baby and its heart beat can be detected relatively easily with trans-abdominal and trans-vaginal examination.
This is presuming that the pregnancy is actually at this stage of development.
Sometimes a trans-vaginal examination shows that your pregnancy is less advanced than expected.
How accurate is the ultrasound examination?
The earlier the ultrasound is done, the more accurate it is at estimating the baby's due date.
Ultrasounds performed during the first 12 weeks of pregnancy are generally within 3 - 5 days of accuracy.
The most accurate time is between 8 and 11 weeks gestation. 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.
However, the accuracy of the ultrasound examination is always dependent on the skill of the sonographer and the quality of the equipment.
The scan or my dates - which should I believe?
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.
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.
The size of the baby correlates less and less with its age as time goes on.
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).
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";. These scans are only used to estimate the due date of your baby if this is all you have to go on.
If you have more than one ultrasound during your pregnancy, giving you 'multiple dates', then the earliest ultrasound estimate should be used, because it will be more accurate.
If you have any questions about this, please feel free to talk to your sonographer or doctor about it.