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:
How do we do the scan?
Most dating scans are done with a trans-abdominal ultrasound and a fullish bladder. If the pregnancy is very early the gestation sac and embryo will not be big enough to see, so the transvaginal approach will give better pictures. For transvaginal ultrasound you can have an empty bladder.
Why and when is a dating scan recommended?
Dating scans may be recommended for women for many reasons including:
What can be seen?
At 5 ½ weeks gestation a tiny sac can be seen in the uterus, but the baby and its heartbeat 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 developing baby is clearly seen on trans-vaginal ultrasound and the heartbeat 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 heartbeat 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?
Ultrasounds performed during the first 12 weeks of pregnancy are generally within 5 days of accuracy. The most accurate time is between 8 and 11 weeks gestation. This is because the baby is growing so quickly that there is a big difference in size from week to week.
Also, the baby cannot bend and twist too much yet, so the length of the body is fairly constant at these gestations.
However, the accuracy of the ultrasound examination is always dependent on the position of the uterus, how much tissue the probe needs to penetrate, the size of the baby, the skill of the sonographer and the quality of the equipment.
This is why at Ultrasound Care we use the latest equipment, and all our sonographers are highly skilled.
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 then the ‘expected date of delivery’ (EDD or EDC) is calculated from your last menstrual period. 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 the EDD from the early dating scan is used. Bear in mind, that even women with regular 28 day menstrual cycles do not always ovulate on day 14 of the cycle, so your due date may change according to your date of conception.
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. For example, a baby with 190cm tall parents is likely to be longer than a baby with 160 cm tall parents.
Ultrasound examinations more than 12 weeks and less than 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 are much less reliable in estimating the due date of the baby because the size no longer reflects the gestational 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 in which your baby was seen 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 this.
To assist you in estimating the due date of the baby, please use the Ultrasound Care pregnancy calculator on our website.
Accurate dating scans are important in preparing for the delivery of your baby. Ultrasound Care has the most up to date ultrasound technology and our sonographers are highly skilled enabling us to provide reliable and accurate dating scans.