Occasionally there is a big discrepancy between the gestation as estimated by abdominal examination and that estimated by ultrasound scan. Which one should be trusted?
This problem is uncommon but by no means rare. In such a case, the scan is the one which should be taken to be accurate. This is because the scan measures the fetus itself, while abdominal examination depends on the size of the womb (fundal height). The latter may be influenced by several unrelated factors such as a full bladder, amniotic fluid volume, pelvic tumour or maternal obesity.
Scan accuracy (for dating) holds true only in the first half of the pregnancy. This is why it is essential to establish the accurate date in this phase of the pregnancy. Both the scan and abdominal examination could be grossly inaccurate and wildly off the mark in late pregnancy.
For late-bookers, there are occasions when it is actually impossible to accurately date a pregnancy.
How much of the scan findings at an ultrasound scan are documented?
Normally the person performing the scan will produce a report that highlights all the significant findings. Whenever it is seen fit, still pictures may be taken for future reference. Sometimes, a video recording is made sometimes to allow for consultation with colleagues who may not be immediately available during the test but also for training purposes. A verbal or written consent is usually obtained from the mother for this.
A full bladder is not required for a vaginal scan; the mother will be asked to empty her bladder before the procedure.
The expectant mother at a routine eighteen to twenty weeks scan should be assured that she will lie on a comfortable couch or bed, well propped up to see the screen, and she won't have to take her clothes off. The test lasts twenty to thirty minutes only, unless there is an unforeseen problem.