An ultrasound picture is formed using high frequency sound waves, which are passed through the uterus and reflected back as an image on a screen. Ultrasound scans in pregnancy may be routine or they may be offered because of pain or bleeding or because of problems in a previous pregnancy.
There are two ways of doing an ultrasound scan: a transabdominal scan (TAS) where the ultrasound probe is placed on your lower abdomen (below your belly button and above the line of pubic hair); a transvaginal scan (TVS) which is an internal examination when the ultrasound probe is placed in the vagina.
In early pregnancy, especially before 11 weeks, it is usual to have a TV scan, where a probe is placed in the vagina. This gives the clearest and most accurate picture in early pregnancy. It may also be offered after 11 or 12 weeks if a TAS does not give a clear enough picture. From 11 or 12 weeks, including at the routine booking-in scan, it is more common to have a TAS.
There are advantages and disadvantages to both types of examination and certain indications for each type of scan; the person doing the scan may need to use both types of ultrasound to give them a complete picture. The table below should help you to understand the advantages and disadvantages to the type of scan.
- Does not require a full bladder
- Allows closer examination of the pelvic organs – the womb, tubes and ovaries, which is an advantage if it is a very early pregnancy or an ectopic pregnancy is suspected
- Allows confirmation of a pregnancy at an earlier stage, around 1 week earlier than TA scan, which could reduce the need to return for further scans to confirm a diagnosis
- Requires the ultrasound probe to be placed inside the vagina
- No internal examination is needed
- Allows examination of the pregnancy at later gestations
- Allows examination of a very enlarged womb, for example with multiple fibroids
- Requires a full bladder and external pressure of the abdomen, which can be uncomfortable
- The picture may be effected by abdominal fat
- May miss a very small early pregnancy or ectopic pregnancy
What if I don’t want an internal scan?
If you do not want a transvaginal scan, you can ask for a transabdominal scan. That may give some information about your pregnancy, but it could be limited and possibly delay diagnosis. Please read the further information on safety of ultrasound in early pregnancy if this is a concern.