Ultrasound scanning is probably most commonly associated with pregnancy. NHS pregnancy scans are performed at approximately 12 and 20 weeks gestation- the nuchal and anomaly scans. Both these scans are performed trans abdominally (TA) or ‘through the tummy’.
In early pregnancy, which is normally before the 10th week of pregnancy, we are often examining pregnancy structures such as the gestational or pregnancy sac, the yolk sac which supplies nutrients to the small embryo and the embryo itself. These are usually very small and would not be visualised clearly if the scan was though the tummy. A transvaginal scan (TV), where a thin ultrasound probe is inserted into the vagina is used, which can assess the pregnancy accurately.
The TV scan leads to a much more detailed clearer image and the position of the probe allows the target of the scan to be much closer in proximity, enabling a better view. It bypasses the problems found with TA including the need for a full bladder (a cause of discomfort) high body mass index (BMI) and image quality. TV scan is considered the first line investigation in early pregnancy and is used to determine pregnancy location (intrauterine, or correctly positioned within the womb, versus ectopic pregnancy, where the pregnancy is incorrectly positioned in the womb or fallopian tube), viability (whether there is a heart beat or not) and detects multiple pregnancy such as twins. Usually a pregnancy, which is correctly positioned within the womb and will continue healthily is first visualised as a small empty gestational sac at 4 weeks and three days gestation, using TV scan. The appearance of the yolk sac occurs at 5 weeks, with visualisation of the embryo at 5 weeks and 3 days with or without a heart beat. The embryo is usually only 2-3mm big and none of these structures would be visible this early if a TA scan were performed. A heart beat is nearly always seen at 6 weeks in a healthy ongoing pregnancy.
In 90% of cases of suspected early pregnancy problems, a single TVS scan can provide all the information required for further management. A TV scan is by its nature a more ‘invasive’ procedure, but studies have shown that it is both acceptable and well tolerated for women undergoing it. TV scans do not cause pregnancy loss or pregnancy problems and are not painful. The scan should always be performed by someone who is qualified to do so and who can make a diagnosis or draw a correct conclusion. A diagnosis cannot in all cases be made on a single scan and the diagnosis can also be dependant on the true gestation. Therefore a final diagnosis may not be made at the first scan or if the operator is unable to interpret what they see.
Dr Sarah Merritt